Overview

You can expect at least one, and likely multiple, ultrasounds during your pregnancy. This technology does more than give you a sonogram picture for your baby book. The information from an ultrasound can confirm that your baby is healthy and diagnose potential issues. While these exams can be helpful when medically indicated, many expecting mothers may get additional ultrasounds that are unnecessary. This article breaks down the evidence on ultrasounds and the research on their accuracy and safety. This episode is a must-listen if you are considering a 3D or 4D ultrasound of your baby.

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What is an Ultrasound Exam? 

Ultrasound technology was invented in Glasgow in the mid-1950s. British and American hospitals adopted this technology in the 1970s. An ultrasound exam is a procedure that uses high-frequency sound waves to scan your abdomen and pelvic cavity. An ultrasound creates a picture of your baby and placenta called a sonogram.

The handheld part of an ultrasound machine is called a transducer which converts sound waves into an image you can see. It emits sound waves at a specific frequency and captures the returning echoes at frequencies dependent on the tissues through which the waves travel. Although the terms ultrasound and sonogram are technically different, they are often used interchangeably and refer to the same exam.  

Types of Ultrasounds  

There are seven different ultrasound exams, and the main process is the same for all of them.

Transvaginal scans are specially designed probe transducers used inside the vagina. Transvaginal scans take place during the early stages of pregnancy.

A standard ultrasound is the traditional ultrasound exam that uses a transducer over the abdomen to generate 2-D images of your baby.

An advanced ultrasound is similar to a standard ultrasound, but the exam targets a suspected problem and uses more sophisticated equipment.

A doppler ultrasound measures slight changes in the frequency of the ultrasound waves as they bounce off moving objects, such as blood cells.

A 3-D ultrasound uses specially designed probes and software to generate 3-D images of the developing fetus.

A 4-D or dynamic 3-D ultrasound uses specially designed scanners to look at the face and movements of the baby before birth.

Fetal echocardiography uses ultrasound waves to assess the baby’s heart anatomy and function and help assess suspected congenital heart defects.  

When and Why Ultrasounds are Done 

Ultrasounds can take place at any point during your pregnancy. Unlike many tests where you have to wait for results, you see the results of an ultrasound during the procedure. In some cases, the ultrasound images may be sent to your doctor or another specialist for review before you know more information. The American College of Obstetricians and Gynecologists recommends that expecting mothers have at least one standard exam between 18–22 weeks during their pregnancy. It is common to have two ultrasounds, one early on to date your pregnancy and the second at 18-22 weeks.

Many healthy pregnancies technically do not require an ultrasound because they should only be used when medically indicated. I have yet to meet anyone who did not have at least one ultrasound during their pregnancy. The average number of ultrasounds varies with each healthcare provider. Your doctor or midwife may suggest additional ultrasounds if they suspect a complication or problem related to your pregnancy.  

Often ultrasounds are combined with other tests, such as amniocentesis or a CVS, to validate a diagnosis. See Your Guide to Prenatal Testing for more in-depth information on prenatal tests.

How an Ultrasound Exam Works 

For ultrasound to work, a water-based gel is applied to your belly. This gel is a conductive medium that enables a tight bond between your skin and the transducer. The gel allows waves to transmit directly to tissues beneath your skin. Without the gel, all you would see is a black screen. Your care provider or an ultrasound technician will apply gel liberally before performing an ultrasound. It is usually pretty cold. If you are lucky, it may be kept in a warmer, so it isn’t a cold shock when they put it on.

Once the gel is applied, the technician or your care provider places the handheld transducer on your belly to produce sound waves into your uterus. The sound waves bounce off bones and tissue returning to the transducer to generate a black-and-white image of your baby. You can wipe the gel off when the ultrasound is over, although your belly may still be a little sticky. You should get a physical picture of your baby as a keepsake to take home. For many expecting parents is a big sigh of relief to see that there is, in fact, a tiny human growing inside of you. Depending on the purpose of the ultrasound and who performs it, you may need to wait for complete results from your doctor or midwife once they have reviewed the scans.

First Trimester Ultrasounds 

In the first trimester, your first appointment in week eight usually includes a transvaginal scan ultrasound. This ultrasound confirms a heartbeat to verify that you are pregnant and measures the crown-rump (from the top of the head to the bottom) length to date your pregnancy. A transvaginal scan can also diagnose molar or ectopic pregnancies and assess abnormal gestation. Ectopic means the baby is attached outside the uterus, and a molar pregnancy happens when tissue that normally becomes a fetus instead becomes an abnormal growth in your uterus.

If you have an ultrasound done before eight weeks and a heartbeat is not detected, don’t panic. Placental placement or your baby’s position can make it challenging to pick up a heartbeat. This is one of the reasons care providers generally do not have you in until about week eight. 

The accuracy of your due date is crucial because 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 and hormone levels in your blood to determine your due date. If you and your doctor or midwife have determined a due date based on your last period, they usually will not change that date based on the results of an early ultrasound. Keep in mind that ultrasound dating of conception can be off by at least 5-7 days in your early pregnancy. 

A first-trimester screen test combines an ultrasound evaluation of your baby with a blood test for you. This test is between 10 weeks and 13 weeks, six days, and can identify risk for specific chromosomal abnormalities, including Trisomy-21 (Down syndrome). It is crucial to remember that a positive result on a screening test does not equate to having an abnormality; it indicates that you may want to discuss further diagnostic testing with your care provider. See Your Guide to Prenatal Testing for more in-depth information on the first-trimester screen and other prenatal tests.

A CVS (chorionic villus sampling) is a diagnostic test to determine if a baby has chromosome abnormalities like Down syndrome and genetic disorders like cystic fibrosis. A CVS is available between 10 to 13 weeks. The most common method is transcervical, where an ultrasound guides a thin catheter through the cervix to your placenta. The other way is a transabdominal approach, where an ultrasound guides a long thin needle through the abdomen to your placenta.

Second Trimester Ultrasounds 

An amniocentesis is a diagnostic test for chromosome abnormalities, neural tube defects, and genetic disorders. Your care provider will only offer this test if a screening test indicates your baby is at high risk for one of these conditions. The test is available between 15 and 20 weeks. An ultrasound-guided needle is inserted through your belly into the amniotic sac to collect a sample of amniotic fluid.

If you did not have an ultrasound in your first trimester and there are questions about your baby’s gestational age, your care provider may recommend an ultrasound to estimate your due date. Research shows that early ultrasound estimation of gestational age is known to increase the reported preterm delivery rate (<37 weeks) compared with estimation by the date of the last menstrual period. It is important to try and nail down your due date as accurately as possible in the beginning. This isn’t always easy, especially if your pregnancy was a surprise. A study of over 44,000 women found that early ultrasound (16-18 weeks) measurements of gestational age tended to skew the age to earlier, resulting in more preterm births. If your care provider uses ultrasound measurements to date your pregnancy, it tends to be more accurate earlier. 

Anatomy Scan

It is routine for expecting moms to get an ultrasound anatomy scan in the second trimester. Generally, you can expect this between 18-20 weeks. From your practitioner’s point of view, this is a great way to reassure you that everything is developing exactly how it should be. Ideally, your partner attends all of your prenatal appointments. One of the more significant appointments for your partner to attend is the anatomy scan ultrasound.

The second-trimester anatomy scan ultrasound diagnoses fetal malformation and structural abnormalities, verifies dates and growth, identifies excessive or reduced levels of amniotic fluid, and overall evaluates your baby’s well-being. During this ultrasound appointment, your baby will be measured from crown to rump, around their middle and head. Since this is an anatomy scan, the ultrasound technician will look specifically at the heart’s four chambers, kidneys, bladder, stomach, brain, spine, and sex organs. This ultrasound survey of the organs, including the umbilical cord, ensures that they are developing normally or may identify any potential problems as soon as possible. The sonographer will also look at the location of the placenta, and the fetal heart rate.

The anatomy scan is especially exciting because most parents can find out their baby’s sex. You do have the option to be surprised. If you choose not to find out the sex, make sure to tell the ultrasound technician up front. The technician should be able to tell, with about 95% accuracy, whether you are having a boy or a girl. You might want to ask them how certain they are, and of course, the only way to be 100% sure is to wait until birth. 

Third Trimester Ultrasounds 

In the third trimester, an ultrasound can identify uterine and pelvic abnormalities, identify the placental location, observe your baby’s position (fetal presentation), observe fetal movements, and try to determine the size of your baby.

Routine Third-Trimester Ultrasounds

A Cochrane review assessed the effects of routine late pregnancy ultrasounds (after 24 weeks). The review concluded that routine late pregnancy ultrasound in low-risk or unselected populations does not confer a benefit on the mother or baby. The British Journal of Obstetrics and Gynaecology published a rebuttal to these conclusions. One criticism was that many of the studies included were decades old. Their opinion was that in developed countries, where advanced ultrasound technology is widely available, there is no reason not to offer this examination routinely to all women. The authors also cited some instances where fetal anomalies may be detected that could not be picked up before the third trimester. This would include things like congenital heart defects or microcephaly. You can see that there is some disagreement in the medical community as to whether third-trimester scans should be routine for low-risk women. 

Measuring Your Baby’s Size

There are a lot of questions about how accurate ultrasounds are for measuring your baby. One study found ultrasounds accurate (within 10%) 84% of the time. Another study found that mothers predicted weight within 10% in 59% of cases and ultrasounds in 65% of cases. This means that mothers were almost as accurate at estimating the size of their babies as ultrasounds were. These two studies are consistent with other research in that ultrasounds have a considerable margin of error. Although, in the absence of any other accurate way to predict the size of a baby before birth, we often rely on these measurements.  

To make things even more confusing, there is no universal calculation to take measurements from ultrasound and apply them to height and weight. One study looked at several formulas used to estimate fetal weight and found that all of them either over or under estimate the fetal weight. The researchers concluded that the interpretation of ultrasound estimated fetal weight should be made cautiously, especially in small for gestational age babies.  

Small for gestational age means they are suspected to be below the 10^th percentile for height and weight. A study that looked at third-trimester ultrasounds to evaluate small for gestational-age babies found that they were not very accurate. The researchers note that screening for this misses the diagnosis of a large number of SGA newborns. They go on to warn that the consequences of routine screening in a low‐risk population may lead to unnecessary planned preterm delivery and elective Cesarean sections for babies suspected to be below the 10^th percentile that is not. We see this issue when interventions that may not apply to everyone, including low-risk mothers, become routine care. 

Biophysical Profile

If a question about fetal health and well-being comes from an examination, symptoms, or your pregnancy is a high risk, your care provider may recommend a biophysical profile. This is a test to determine fetal health that can take place after 32 weeks. This combines an ultrasound with a non-stress test where belts around your belly measure fetal heart rate and contractions.

Ultrasound Safety 

Ultrasounds have an excellent safety record. They have been used in obstetrics for decades with no proven harmful effects. The consensus is that an ultrasound is a non-invasive procedure that, when used properly, has not demonstrated fetal harm. However, the long-term effects of repeated ultrasound exposure on the fetus are not fully known, and professional medical organizations recommend that ultrasound only be used if medically indicated. 

Animal Studies

There are in vivo (living animal) studies linking large but clinically relevant doses of ultrasound to altered learning, memory, and abnormalities of the anatomy of the nervous system. There is also a well-documented significant increase in the likelihood of non-right-handedness in boys exposed to diagnostic ultrasound in utero. This is potentially relevant given the increased prevalence of autism in males and reports of excess non-right-handedness in this population.

A study motivated by these observations 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 their birth. The ultrasound-exposed pups were significantly less interested in social interaction than male pups who did not undergo an ultrasound. The pups who had an ultrasound also demonstrated significantly more activity than those not exposed to an ultrasound but only in the presence of an unfamiliar mouse. What does all of this mean? In simple terms, these results suggest that fetal exposure to diagnostic ultrasound can alter typical social behaviors in young mice that may be relevant for autism. There are, of course, significant differences between mice versus humans that would require much more exploration before any change in clinical practice. The bottom line of this study is that we need more research.

There is also an animal study that has shown that ultrasound can cause damage to developing fetal and neonatal tissues. In this study, researchers investigated whether an ultrasound of the brain of a chick can lead to learning and memory impairment after they are hatched. Researchers exposed the brains of chicks on day 19 of a 21-day incubation period to 5 or 10 min of ultrasound waves or 1-5 minutes of pulsed Doppler ultrasound. After the chicks had hatched, learning and memory functions were assessed on day 2. The results showed that ultrasound exposure did not affect memory function. Still, the pulsed doppler exposure did cause significant memory impairment for the chicks that underwent 4 and 5 minutes of pulsed Doppler exposure.

The short outcome of this study is that extended exposure to pulsed Doppler ultrasound can adversely affect cognitive function in the chick when exposure occurs close to the time of hatch. When ultrasound waves travel through tissue, energy is absorbed by the tissue components, which converts to heat depending on the frequency and intensity of the ultrasound wave. From a heat-absorption point of view, caution is perhaps even more critical during Doppler studies in a fetus close to term.  

Of course, mice and chicks are not humans. Human studies are lacking in many things related to pregnancy because no one would sign up for a study that could potentially cause some harm to their unborn baby. Without any human studies, the only scientifically significant studies are those done on animals. It may not be ideal, but it is better than no research. 

Guidance from Professional Organizations

The American Institute of Ultrasound in Medicine states that obstetric ultrasound examinations should be performed only when there is a valid medical reason, and the lowest possible ultrasonic exposure settings should be used to gain the necessary diagnostic information.

The FDA recommends that patients talk to their healthcare provider to understand the reason for the ultrasound examination, the medical information that will be obtained, the potential risks, and how the results will be used to manage the medical condition or pregnancy.

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, it is possible that effects could be identified in the future. For this reason, it is recommended that ultrasound exams be performed only for medical reasons by qualified healthcare professionals.  

How Many Ultrasounds Should You Have?

Ultrasounds enable your care provider to evaluate your baby’s well-being and diagnose potential problems. For mothers with an uncomplicated pregnancy, an ultrasound may not be a necessary part of prenatal care. According to ACOG, you should have at least one standard exam during your pregnancy, at 18–22 weeks. It is estimated that expecting mothers have an average of five ultrasounds during pregnancy.

Ideally, the findings of an ultrasound come back normal. In reality, many ultrasounds result in indeterminate findings with limited clinical utility. The more often ultrasounds are performed, the more frequently expecting parents get inconclusive results. A study that looked at anxiety following ultrasound exams found indeterminate, especially incomplete, findings associated with significantly higher anxiety despite their overall good prognosis. Of 286 ultrasounds, 40.5% were indeterminate. Indeterminate finds were more common with obesity and increasing gestational age, with the majority of indeterminate findings in the third trimester.

3D and 4D Ultrasounds 

Your care provider may recommend a 3D or 4D ultrasound to examine suspected fetal anomalies, such as cleft lip and spinal cord issues, which would be a medical reason. On the other hand, you may be interested in a 3D or 4D ultrasound for keepsake baby photos and videos. The consensus from all professional medical organizations is don’t do it. 

According to the American College of Obstetricians and Gynecologists, ultrasound imaging should be used prudently and only when expected to answer a relevant clinical question or otherwise provide medical benefit to the patient.

The FDA recommends that expecting mothers avoid fetal “keepsake” images from 3D or 4D ultrasounds. The FDA states that ultrasound can heat tissues slightly, and in some cases, it can also produce very small bubbles (cavitation) in some tissues. The long-term effects of tissue heating and cavitation are not known. Therefore, ultrasound scans should be done only when there is a medical need, based on a prescription, and performed by appropriately-trained operators. The FDA extends this caution to Doppler fetal ultrasound heartbeat monitors. These handheld ultrasound devices let you listen to your baby’s heartbeat. Before purchasing a doppler heartbeat monitor, listen to this episode, and please discuss it with your doctor or midwife.

Many companies offer 3D and 4D ultrasounds to expecting parents and feature a variety of packages with photo and video keepsakes. While these are relatively affordable, they are not covered by health insurance since they are not deemed medically necessary. If you are considering a 3D or 4D ultrasound, please consult your care provider for their professional recommendation.

Weighing the Risks and Benefits

Ultrasounds, like so many procedures during your pregnancy, are a choice that should come with informed consent. A key to informed consent is that you always have the option to opt in or opt out. Whether you choose to have any ultrasounds during your pregnancy is up to you. I urge you to talk to your prenatal care provider and weigh the pros of an ultrasound and the potential risks or unknown risks. The B.R.A.I.N. acronym is one of the most versatile tools to evaluate any procedure. Each word in this acronym is the key to a question you can ask to help you figure out whether any intervention or procedure is the right choice for you. 

Benefits: What are the benefits? An ultrasound can give you and your doctor valuable information about your baby’s health. The benefit of getting a keepsake picture of your baby may not be as critical as the benefit of evaluating a concern. If you opt for a diagnostic test like amniocentesis or a CVS, the benefit is a reduced risk of miscarriage, and no doctor would perform those tests without an ultrasound to guide the needle.

Risks: What are the Risks? The only thing we 100% know about ultrasound safety is that we don’t know everything, and more studies are needed to determine the safety of ultrasounds. We know that ultrasound can heat tissues slightly, and in some cases, it can also produce very small bubbles (cavitation) in some tissues. The long-term effects of tissue heating and cavitation are not known. Based on animal studies, it is reasonable to assume that higher exposure may increase your risk. Some expecting mothers choose fewer ultrasounds to mitigate the risk.

Alternatives: What are the alternatives? While ultrasound data can be valuable, you have alternatives to ultrasound in many cases.

For the purposes of dating your pregnancy, if you know when conception occurred or when your last period was, your care provider should be able to estimate your due date without using ultrasound measurements.

If you are considering purchasing a home doppler to hear your baby’s heartbeat for reassurance that your baby is okay, you could consider monitoring movement or kicks. By about week 16, you may feel flutters, as if a butterfly was flapping its wings. Around week 20, you should be able to feel more substantial movements like pokes and kicks. By week 23, your care provider should be able to hear your baby’s heartbeat with a standard stethoscope rather than using a doppler.

A first-trimester screen also uses a blood sample and combines that with an ultrasound to measure nuchal translucency, which is the amount of fluid behind your baby’s neck. There are other options for prenatal testing. A cell-free DNA or NIPT test uses a blood sample from you with no ultrasound required to detect chromosomal abnormalities. There is a lot of nuance on what prenatal tests are the most appropriate for you, and you should discuss your options with your doctor or midwife.

Intuition: What does your intuition tell you? If you are having a tough time sifting through the pros and cons of making a decision, it can be easy to overlook your intuition. Of course, you listen to your care provider’s advice and ask questions, but what does your gut say? While this may not be an evidence-based method, it may be helpful if you are having difficulty making a decision.

Nothing: What happens if you do nothing? This is my favorite question to ask for any intervention. If your care provider recommends an ultrasound, what happens if you decline it? The answer could be that they would need to use an alternative you are more comfortable with. The answer could also be that you would not have a diagnosis that would be helpful in your prenatal care.

Talk to Your Doctor or Midwife

You can utilize the BRAIN acronym to ask questions to understand the reason for an ultrasound and evaluate your options. As always, you should discuss any questions you have with your doctor or midwife. They are your trusted partner in navigating your pregnancy and have a lot of expertise and knowledge in prenatal care.

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