Ultrasound during pregnancy is one of the most common tools your care provider uses to monitor your baby’s health and development. Whether it is your first scan in the early weeks or the anatomy scan in your second trimester, ultrasounds provide valuable information about how your baby is growing. However, many expecting parents have questions about how many ultrasounds are necessary, how accurate the measurements are, and whether there are any risks.

This episode walks you through what to expect at each stage of pregnancy, from the first-trimester scan to third-trimester growth checks. Hear about the research on ultrasound safety, what professional medical organizations recommend, and how accurate ultrasounds are for estimating your baby’s size. Plus, 3D and 4D ultrasounds, home Dopplers, and what happens when an ultrasound reveals something unexpected. If you want to feel informed and confident heading into your next ultrasound appointment, this episode will help you get there.

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What is an Ultrasound and How Does it Work?

Ultrasound technology was invented in Glasgow in the mid-1950s and was adopted by British and American hospitals in the 1970s. An ultrasound exam uses high-frequency sound waves to create images of your baby and placenta. The handheld part of the machine is a transducer. It emits sound waves at a specific frequency and captures the returning echoes. Different tissues, fluids, and structures reflect waves at varying speeds depending on their density. A computer converts the reflected sound wave patterns into a visual image called a sonogram. The terms ultrasound and sonogram are technically different. But, many people use them interchangeably, and they refer to the same exam.

For the ultrasound to work, the technician applies a water-based gel to your belly. This gel creates a tight bond between your skin and the transducer, allowing sound waves to transmit directly to the tissues beneath your skin. Without the gel, all you would see is a black screen. The gel is usually cold, although some offices keep it in a warmer.

Once the technician or your care provider applys the gel they place the transducer on your belly. The sound waves bounce off bones and tissue. Then return to the transducer to generate a black-and-white image of your baby on a screen. When the ultrasound is over, you can wipe the gel off, although your belly may still be a little sticky. You should receive a picture of your baby to take home. Depending on the purpose of the ultrasound and who performs it, you may need to wait for complete results once your doctor or midwife has reviewed the scans.

Types of Ultrasounds You May Encounter

There are several types of ultrasound exams, and you may encounter more than one during your pregnancy. A transvaginal ultrasound uses a specially designed probe inserted into the vagina. This type is most common in early pregnancy because your uterus is still deep in the pelvis, and a transvaginal approach provides a closer, clearer image. A standard transabdominal ultrasound is the traditional exam where the technician moves a transducer across your abdomen to generate 2D images. This is the most common type and what most people picture when they think of a pregnancy ultrasound.

A Doppler ultrasound measures slight changes in the frequency of sound waves as they bounce off moving objects, such as blood cells. This allows your care provider to assess blood flow in the umbilical cord, placenta, or your baby’s brain. Doppler ultrasound is common in higher-risk situations. A fetal echocardiography uses ultrasound waves to assess your baby’s heart anatomy and function. It also helps evaluate suspected congenital heart defects. Your care provider may also use a 3D or 4D ultrasound to examine suspected fetal anomalies in greater detail.

First-Trimester Ultrasound

In the first trimester, your first appointment around week eight usually includes a transvaginal ultrasound. This scan confirms cardiac activity to verify that you are pregnant and measures the crown-rump length to date your pregnancy. It is worth noting that the Society of Radiologists in Ultrasound published a consensus statement in 2024 recommending the term “cardiac activity” rather than “heartbeat” for first-trimester scans. This is because cardiac development is a gradual process, and the heart chambers are not fully formed in the first trimester. You may hear your care provider use either term.

A transvaginal scan can also diagnose ectopic pregnancies. This is when the baby is implanted outside the uterus. It can also diagnose molar pregnancies. Those occur when tissue that normally becomes a fetus instead becomes an abnormal growth. If you have an ultrasound before 8 weeks and your doctor does not detect cardiac activity, do not panic. Placental placement or your baby’s position can make it challenging to pick up a signal this early. This is one of the reasons care providers generally do not schedule your first appointment until about week eight.

The accuracy of your due date is crucial. It drives the timing of everything in your prenatal care, from when you take tests to recommendations to induce labor. Your doctor or midwife will primarily use the date of your last period to determine your due date. If your care provider has already established a due date based on your last period, they usually will not change that date based on an early ultrasound.

Keep in mind that before 14 weeks, the accuracy of gestational age from measurement of the crown to rump length is ±5–7 days. Research shows that early ultrasound estimation of gestational age tends to increase the reported preterm delivery rate compared with estimation by the date of the last menstrual period. A study of over 44,000 women found that early ultrasound measurements tended to skew gestational age earlier, resulting in more preterm birth diagnoses. If your care provider uses ultrasound to date your pregnancy, earlier measurements tend to be more accurate.

A first-trimester screen combines an ultrasound with a blood test. This takes place between 10 weeks and 13 weeks, six days. The screen can identify risk for specific chromosomal abnormalities, including Trisomy 21 (Down syndrome). It is important to remember that a positive screening test result does not mean your baby has an abnormality. A screening test gives you the likelihood of a condition. You would need a diagnostic test to diagnose anything. See Your Guide to Prenatal Testing for in-depth information.

Some diagnostic tests, including CVS and amniocentesis, use ultrasound guidance during the procedure. In a CVS, an ultrasound guides a thin catheter or needle to the placenta to collect a tissue sample. In an amniocentesis, an ultrasound guides a needle through your abdomen into the amniotic sac. Both of these procedures require ultrasound guidance. The ultrasound allows your care provider to visualize where the needle is going in real time. This is essential for the safety of these tests.

Second-Trimester Ultrasound, The Anatomy Scan

The anatomy scan is one of the most anticipated ultrasound appointments. It typically takes place between 18 and 22 weeks. This is one exam that the American College of Obstetricians and Gynecologists recommends for all pregnancies. Typically, even mothers who are opting for a pregnancy with no or little intervention opt into this exam. This is a thorough scan where the ultrasound technician (sonographer) examines your baby’s major organs and structures in detail.

During the anatomy scan, the sonographer will measure your baby from crown to rump and around their middle and head. They will look specifically at the heart’s four chambers, kidneys, bladder, stomach, brain, spine, and sex organs. They will also evaluate the umbilical cord, amniotic fluid levels, placental location, and fetal heart rate. This ultrasound can diagnose fetal malformations and structural abnormalities. It can verify dates and growth, identify issues with amniotic fluid, and evaluate your baby’s overall well-being.

Here are a few things to know ahead of time. The anatomy scan can take 30 to 45 minutes or longer. The time depends on your baby’s position and whether the sonographer can get all the images they need. If your baby is not cooperating, the technician may ask you to walk around or drink water. Or even come back for a follow-up scan. This does not necessarily mean something is wrong. It simply means the sonographer could not capture all the required views. If you have an anterior placenta, meaning it is attached to the front wall of your uterus, it can make imaging a little more challenging and can also affect when and how strongly you feel your baby move.

The anatomy scan is especially exciting because most parents can find out their baby’s sex. If you choose not to find out, make sure to tell the ultrasound technician up front. A systematic review and meta-analysis found that ultrasound accuracy for determining sex rises from about 87% in the first trimester to 99% in the second and third trimesters. By the time of your anatomy scan, the accuracy is very high. That said, factors like your baby’s position can occasionally make it difficult to get a clear view. You might want to ask how confident the technician is. The only way to be 100% sure is to wait until birth.

I always recommend that partners attend every prenatal appointment. It is a fantastic way to involve your partner, for them to hear everything firsthand,and ask their own questions. It would be amazing if your partner could make every appointment. This is one of the most important for them to attend. The anatomy scan is exciting, and it is also a time when you may receive new information or have questions. Having your partner there means you are processing that together.

What Happens When an Ultrasound Finds Something Unexpected

Most ultrasounds come back normal. However, ultrasounds sometimes reveal findings that require further investigation. It helps to understand what these findings can look like, so you are prepared if it happens to you.

Some findings are clearly significant and prompt immediate follow-up, such as a structural abnormality in the heart or spine. In these cases, your care provider may refer you to a maternal-fetal medicine specialist for a more detailed scan and to discuss your options.

Other findings fall into a gray area. These are soft markers, which are minor variations that may be seen on ultrasound. Examples include an echogenic bowel, which means a section of the intestine appears brighter than expected on the ultrasound. A choroid plexus cyst, which is a small fluid-filled space in the brain that is almost always harmless. Or an echogenic intracardiac focus, which is a small bright spot on the heart that typically does not affect heart function. In most cases, isolated soft markers are normal variants that resolve on their own and have no impact on your baby’s health. However, they can be unsettling to hear about.

A study that looked at anxiety following ultrasound exams found that indeterminate findings, especially incomplete exams, were associated with significantly higher anxiety despite an overall good prognosis. Of 286 ultrasounds in this study, 40.5% were indeterminate, meaning they were either incomplete or showed minor findings of uncertain significance. Indeterminate results were more common with obesity and increasing gestational age, with the majority occurring in the third trimester. Research also shows that even when soft markers ultimately resolve and the baby is healthy, the experience of receiving an uncertain finding can increase maternal anxiety and even affect early bonding.

If your ultrasound comes back with an unexpected finding, try not to jump to the worst-case scenario. Ask your care provider to explain what the finding means, what the next steps are, and whether they recommend further testing. In many cases, a follow-up ultrasound or referral to a specialist will provide clarity. If you find yourself in that waiting period, it may help to know that the vast majority of these findings ultimately have no impact on your baby’s health.

Third-Trimester Ultrasounds

In the third trimester, an ultrasound can evaluate the placental location, observe your baby’s position, monitor fetal movements, assess amniotic fluid levels, and estimate your baby’s size. Not all pregnancies will include a third-trimester ultrasound. Whether your care provider recommends or offers one depends on their practice and your specific circumstances.

There is an ongoing debate about whether routine third-trimester ultrasounds should be standard for low-risk pregnancies. A Cochrane review assessed the effects of routine late-pregnancy ultrasounds after 24 weeks. It concluded that routine late-pregnancy ultrasound in low-risk or unselected populations does not confer a benefit to the mother or baby. However, critics of this review have pointed out that many of the included studies were decades old and do not reflect current ultrasound technology.

A rebuttal published in the British Journal of Obstetrics and Gynaecology argued that in developed countries with advanced ultrasound technology, there is no reason not to offer this exam routinely. The authors cited examples of fetal anomalies, such as congenital heart defects or microcephaly, that may only be detectable in the third trimester. The bottom line is that this remains an unresolved question in obstetric care. Your care provider’s approach depends on the evidence they find most compelling and the specifics of your pregnancy.

Regardless of where the debate lands, if your care provider does recommend a third-trimester growth scan, the timing matters. The International Society of Ultrasound in Obstetrics and Gynecology published updated guidelines in 2024 for third-trimester ultrasound. These guidelines recommend that screening for small or large for gestational age babies in the general population should be at 36 weeks rather than 32 weeks, because screening at 36 weeks is more accurate. This is a helpful reference point if your care provider recommends a growth scan and you are wondering about the timing.

Your care provider may also recommend a third-trimester ultrasound if you have risk factors or complications. This could include gestational diabetes, high blood pressure, concerns about your baby’s growth, or questions about amniotic fluid levels or placental position.

How Accurate Are Ultrasounds for Measuring Your Baby’s Size?

One of the most common questions about ultrasound during pregnancy is how accurate the size measurements are. This comes up especially in the third trimester, when your care provider may estimate your baby’s weight using an ultrasound. Understanding how these estimates work and their limitations can help you put the numbers in context.

The only way to estimate your baby’s weight before birth is by plugging measurements from an ultrasound into a formula. There are four key measurements: head circumference, biparietal diameter (which is the width of your baby’s head), abdominal circumference, and femur length. These measurements go into algorithms that estimate a weight. The accuracy of that estimate depends on the formula used, the skill of the person performing the scan, and your baby’s position during the exam.

One study found that ultrasounds were accurate within 10% of the actual birth weight about 84% of the time. Another study found that mothers predicted their baby’s weight within 10% in 59% of cases. Ultrasounds were accurate within 10% in 65% of cases. This means mothers were almost as accurate as ultrasounds at estimating their babies’ weights. These studies are consistent with broader research showing that ultrasound weight estimates have a considerable margin of error.

To make things even more challenging, there is no universal formula to convert ultrasound measurements into weight. One study examined several commonly used formulas and found that all overestimated or underestimated fetal weight. The researchers concluded that ultrasound-estimated fetal weight should be interpreted with caution. Especially for babies suspected of being small for gestational age.

Fetal Growth Percentiles

Fetal growth percentiles compare your baby’s measurements to a reference population of babies at the same gestational age. The 50th percentile represents the median, where half the babies measure above and half below. ACOG considers newborns with a birth weight below the 10th percentile to be small for gestational age. Large for gestational age is generally a birth weight at or above the 90th percentile. However, there is no universal agreement on these definitions. Some researchers have proposed using the 15th and 97th percentiles as more clinically meaningful cutoffs.

A study that examined third-trimester ultrasounds to evaluate small-for-gestational-age babies found they were not very accurate. Ultrasounds missed the diagnosis of many small-for-gestational-age newborns. The researchers warned that routine screening in a low-risk population may lead to unnecessary planned preterm delivery and elective cesarean sections for babies who are not actually below the 10th percentile.

On the other end of the spectrum, a study examining over 26,000 pregnancies with an ultrasound between 35 and 38 weeks found that 8.6% of the time, the ultrasound falsely identified a baby as large for gestational age when they were not. Ultrasounds were more accurate at predicting large babies when a doctor specifically ordered the scan because they suspected a large baby.

This is a theme we see with many interventions. When screening tools designed for specific indications become routine for everyone, they can generate results that are not always accurate and may lead to unnecessary interventions. If your care provider tells you your baby is measuring large or small, it is reasonable to ask about the margin of error, what the next steps are, and whether any recommended interventions are necessary based solely on the measurement.

Biophysical Profile

If a question about fetal health and well-being arises during your pregnancy, or if your pregnancy is high-risk, your care provider may recommend a biophysical profile. This test can take place after 32 weeks and combines an ultrasound with a non-stress test. During the non-stress test, belts around your belly measure your baby’s heart rate and any contractions.

The ultrasound portion of the biophysical profile evaluates four things: your baby’s body movements, muscle tone, breathing movements, and the volume of amniotic fluid surrounding your baby. Each of these four areas is scored on a scale of 0 to 2, and the non-stress test adds another 0 to 2 points, for a total possible score of 10. A score of 8 or 10 is generally reassuring. A lower score may prompt additional monitoring or, in some cases, a discussion about delivery timing. The biophysical profile gives your care provider a more detailed picture of your baby’s well-being beyond what a standard ultrasound alone can provide.

How Many Ultrasounds Should You Have?

According to ACOG, you should have at least one standard ultrasound during your pregnancy, typically at 18 to 22 weeks. It is common to have two, one early on to date your pregnancy and the anatomy scan in the second trimester. However, the actual number of ultrasounds varies widely depending on your care provider’s practice and your individual circumstances.

A study published in the Journal of Ultrasound in Medicine analyzed over 1.7 million pregnancies with commercial insurance from 2016 to 2022 and found an average of 5.3 ultrasounds per pregnancy. Utilization increased over the study period and was higher among older mothers and those with additional risk factors. Many healthy pregnancies do not technically require that many, because professional organizations recommend an ultrasound only when medically indicated. However, in practice, very few expecting parents go through pregnancy without at least a couple of scans.

More ultrasounds are not necessarily better. While each individual scan provides information, the more ultrasounds you have, the more likely you are to receive inconclusive or indeterminate results. As we discussed earlier, indeterminate findings can cause significant anxiety, even when the prognosis is ultimately good. If your care provider recommends an additional ultrasound, it is always reasonable to ask why they recommend it and what information it will provide.

Ultrasound Safety

Overall, ultrasounds have an excellent safety record. They have been used in obstetrics for decades without proven harmful effects in humans. The American College of Obstetricians and Gynecologists states that currently, there is no evidence that ultrasound is harmful to a developing fetus. No links have been found between ultrasound and birth defects, childhood cancer, or developmental problems later in life. However, ACOG also notes that it is possible that effects could be identified in the future, and for this reason, ultrasound should be performed only for medical reasons by qualified healthcare professionals.

The FDA cautions that ultrasound can heat tissues slightly, and in some cases, it can also produce very small bubbles, called cavitation, in some tissues. The long-term effects of tissue heating and cavitation are not fully known. For this reason, ultrasound should be performed only when there is a medical need, and by appropriately trained operators.

The American Institute of Ultrasound in Medicine advocates a conservative approach that obtains necessary diagnostic information at minimal exposure. They strongly discourage the nonmedical use of ultrasound. It states that using ultrasound without a medical indication to view the fetus, obtain images, or identify the sex is inappropriate and contrary to responsible medical practice. The AIUM also recommends that all fetal ultrasound exams be performed by appropriately trained and credentialed medical professionals who have specialized training in fetal imaging. This is important because these individuals are trained to recognize medically important patterns and to use techniques that minimize unnecessary exposure.

It is also worth understanding the ALARA principle, which stands for As Low As Reasonably Achievable. This principle guides how ultrasound operators approach each exam. The goal is to use the lowest possible ultrasonic exposure settings necessary to get the necessary diagnostic information. Modern ultrasound machines have built-in safety features and display thermal and mechanical indices that help operators monitor exposure levels in real time.

What We Can Learn From Animal Studies

Because it would be unethical to expose pregnant women or their unborn babies to potential harm in controlled studies, direct human research on ultrasound risks is limited. Animal studies provide the best available evidence to explore possible effects. One study applied 30 minutes of diagnostic ultrasound to pregnant mice at embryonic day 14.5 and studied the social behavior of their male pups three weeks after birth. Ultrasound-exposed pups were significantly less interested in social interaction than pups that did not undergo ultrasound exposure. In simple terms, these results suggest that fetal exposure to diagnostic ultrasound may alter typical social behaviors in young mice. There are significant differences between mice and humans that would require much more exploration before any change in clinical practice. The primary takeaway from this study is that we need more research.

There is also an animal study that investigated whether ultrasound exposure of a chick’s brain can lead to learning and memory impairment after hatching. Researchers exposed the brains of chicks on day 19 of a 21-day incubation period to 5 or 10 minutes of ultrasound, or to 1 to 5 minutes of pulsed Doppler ultrasound. Standard ultrasound exposure did not affect memory function. However, pulsed Doppler exposure caused significant memory impairment in chicks exposed for 4 or 5 minutes. This is relevant because Doppler ultrasound uses a higher acoustic output than standard imaging. When ultrasound waves travel through tissue, the tissue absorbs energy and converts it to heat depending on the frequency and intensity. This study reinforces the importance of the ALARA principle, particularly for Doppler.

Of course, animal studies do not directly translate to human outcomes. These studies are the best available evidence we have, and while they do not prove harm to human babies, they do support the recommendation that ultrasound should be used prudently and only when medically indicated.

3D, 4D, and 5D Ultrasounds

A standard ultrasound generates a flat, 2D black-and-white image. A 3D ultrasound combines multiple images from different angles to create a still, three-dimensional image that shows more depth and detail. A 4D ultrasound is similar to 3D but includes real-time motion, so you see a live video of your baby moving in 3D. Some elective ultrasound clinics now offer 5D ultrasounds. Despite the name, 5D does not add another dimension beyond 4D. These are simply higher-quality and more realistic images produced with updated software.

Your doctor or midwife may recommend a 3D ultrasound for medical reasons, such as further evaluating a suspected fetal anomaly like a cleft lip or spinal cord issue that was flagged on a standard 2D scan. If you get a medically indicated 3D ultrasound, you can request pictures.

Most of the time a doctor does not prescribe 3D and 4D ultrasounds. They are elected by parents who want to see their baby and get keepsake photos or videos. The appeal is understandable. You get to see a somewhat realistic view of your baby’s appearance before birth. It can be fun to see whose facial features they have, and for many parents, seeing their baby provides reassurance and helps them bond. The best timing for keepsake 3D or 4D images is generally between 26 and 30 weeks, when your baby has developed enough fat under their skin for detailed features but still has enough room to move.

However, the consensus from professional medical organizations is against elective ultrasounds for keepsake purposes. As outlined in the safety section above, the AIUM, FDA, and ACOG all recommend that ultrasound should be used only when there is a medical indication. The concern is not that a single elective ultrasound has been proven harmful. The concern is that the long-term effects are not fully known, and exposing your baby to ultrasound energy without a medical reason does not align with the precautionary approach these organizations recommend.

If you decide to move forward with an elective 3D or 4D ultrasound, choose a reputable business. Look for a company whose sonographers are registered by the American Registry for Diagnostic Medical Sonographers or accredited by the AIUM. Health insurance does not cover elective ultrasounds since they are not deemed medically necessary. Packages generally range between $100 and $200. Prioritize a company with a good reputation over saving money. You may also want to opt for a shorter session to limit exposure. For a more in-depth look at this topic, check out Are 3D Ultrasounds Safe and Worth It?.

Home Dopplers

A home Doppler is a handheld device for individual use that uses ultrasound to detect and amplify your baby’s heartbeat. It is natural to want reassurance that your baby is okay between prenatal appointments, especially in early pregnancy when you cannot yet feel movement. However, there are some important considerations to keep in mind before purchasing one.

Home Dopplers can be difficult to use, especially before 14 to 16 weeks, when your baby is still very small. If you cannot find the heartbeat, this can create more anxiety than relief. There is also a risk of false security. Dopplers can pick up sounds that are not your baby’s heartbeat. This includes your own heartbeat, placental blood flow, or movement noises. Mistaking one of these for your baby’s heartbeat could provide false reassurance if something is actually wrong. The FDA extends the same cautions about tissue heating and cavitation to home Doppler devices. It recommends that ultrasound be used only when there is a medical need and performed by trained operators.

If you are looking for reassurance between appointments, there are other ways to feel connected to your baby that do not involve an ultrasound. As your pregnancy progresses, you will have more and more natural cues that your baby is doing well.

By about week 16, you may begin to feel flutters from your baby. These are often described as if a butterfly were flapping its wings. By week 20, you should be able to feel more substantial movements, such as pokes and kicks. These movements become a regular and natural form of reassurance as your pregnancy continues. By about week 23, your care provider should be able to hear your baby’s heartbeat with a standard stethoscope. Unlike a Doppler, a stethoscope does not emit ultrasound energy. That means it does not carry the same concerns about tissue heating. This is also something you could potentially do at home if you had a stethoscope. Although it does take some practice to locate the heartbeat on your own.

If you are considering a home Doppler, discuss it with your doctor or midwife. A home Doppler should never be a substitute for medical care. You should always contact your care provider if you have concerns about your baby’s well-being. Regardless of what you hear on a Doppler. For a deeper dive into this topic, including safer use guidelines and shopping tips if you do choose to buy one, check out What to Know Before Buying a Home Doppler to Hear Your Baby’s Heartbeat.

Weighing the Risks and Benefits

Ultrasounds, like all procedures during pregnancy, are a choice that should come with informed consent. A key part of informed consent is that you always have the option to opt in or opt out. If your care provider recommends an ultrasound, you can ask questions. What information will it provide? What will happen if you decline? Are there any alternatives? For dating your pregnancy, if you know when your last period was, your care provider should be able to estimate your due date without an ultrasound. For prenatal screening, a cell-free DNA (cfDNA) or NIPT test uses a blood sample, with no ultrasound required, to detect chromosomal abnormalities. There is nuance in which prenatal tests are most appropriate for you, and your care provider can help you navigate those options.

The takeaway on ultrasound safety is straightforward. When used for medical purposes by trained professionals, ultrasound has an excellent safety record with no proven harmful effects. Professional organizations recommend using ultrasound only when medically indicated and with the lowest possible exposure. You can choose to have every recommended ultrasound or fewer scans. The important thing is to make that decision in partnership with your care provider and based on your individual circumstances.

Talking to Your Doctor or Midwife

Your doctor or midwife is your best resource for understanding which ultrasounds they recommend for your pregnancy and why. If you have questions about the purpose of any ultrasound, the expected findings, or your options, bring them up at your next prenatal appointment. If your ultrasound reveals an unexpected finding, do not hesitate to ask for a clear explanation of the findings, what it means, and what the next steps are. You can also ask for a referral to a specialist if you would like a more detailed evaluation.

As always, you and your care provider share the same goa. That is to keep you and your baby safe while supporting the birth experience you want. Use what you have learned in this episode to have informed conversations about your preferences and how different ultrasound options may fit your situation.

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