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You can expect to have at least one, or 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 your baby is healthy and diagnose potential issues. While these exams can be helpful when medically indicated, many expecting mothers may be getting additional ultrasounds that are unnecessary. There are a lot of studies on ultrasounds and this episode breaks down the evidence on ultrasounds in each trimester and the research on their safety. This article is a must-read if you are considering a 3D or 4D ultrasound of your baby.

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What is an Ultrasound Exam?

An ultrasound exam is a procedure that uses high-frequency sound waves to scan your abdomen and pelvic cavity. An ultrasound creates a picture, called a sonogram, of your baby and placenta. The handheld part of an ultrasound machine is called a transducer. A transducer is a fancy term for a device that converts one type of energy or signal into another. As applied to an ultrasound, a transducer converts sound waves into an image you can see. It emits sound waves, or echoes, at a specific frequency and captures the returning echoes at frequencies dependent on the tissues through which the waves travel. The sound wave is returned to the transducer and is digitized as echoes or dots on the screen. Although the terms ultrasound and sonogram are technically different, they are used interchangeably and refer to the same exam.

Types of Ultrasounds 

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

  • Transvaginal scans are specially designed probe transducers that are used inside the vagina to generate sonogram images. Transvaginal scans are most often used during the early stages of pregnancy.
  • Standard ultrasound is the traditional ultrasound exam that uses a transducer over the abdomen to generate 2-D images of the developing fetus.
  • Advanced ultrasound is similar to the standard ultrasound, but the exam targets a suspected problem and uses more sophisticated equipment.
  • Doppler ultrasound measures slight changes in the frequency of the ultrasound waves as they bounce off moving objects, such as blood cells.
  • 3-D ultrasound uses specially designed probes and software to generate 3-D images of the developing fetus.
  • 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. Fetal echocardiography is used to help assess suspected congenital heart defects.

Why Do They Use Gel?

Ultrasound gel is a type of conductive medium that enables a tight bond between your skin and the transducer. The traditional ultrasound procedure involves placing a water-based gel on your abdomen to work as a conductor for the sound waves. The gel allows waves to transmit directly to tissues beneath your skin and the parts being imaged. 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, but for some lucky ladies out there, it may be kept in a warmer, so it isn’t a chilly shock when they put it on. When the ultrasound is over, they will wipe it off, although your belly still may be a little sticky. 

How an Ultrasound Exam Works

Once the gel is on your abdomen, the handheld transducer is put on your belly to produce sound waves into your uterus. The sound waves bounce off bones and tissue returning to the transducer to generate black and white images of your baby. Seeing an ultrasound is a pretty cool experience. It gives you a real picture of your little one, and for a lot of expecting parents is a big sigh of relief to see that there is, in fact, a tiny human growing inside of you. 

When and Why Ultrasounds are Done

Ultrasounds can take place at any point during pregnancy. Unlike many tests where you have to wait for results, you see the results of an ultrasound on a monitor during the procedure. The American College of Obstetricians and Gynecologists recommends that expecting mothers should have at least one standard exam during your pregnancy, which usually is performed at 18–22 weeks.

Many healthy pregnancies technically do not require an ultrasound because they should only be used when medically indicated. With that being said, 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 and additional ultrasounds may be suggested if your care provider suspects a complication or problem related to your pregnancy. 

Often ultrasounds are combined with other tests, such as triple tests, amniocentesis, or a CVS, to validate a diagnosis. You can check out these episodes for more details on non-invasive and invasive prenatal testing.

First Trimester Ultrasounds

In the first trimester an early ultrasound is usually done with a transvaginal scan and accomplishes two things. First, it confirms that you are pregnant. Second, it also dates your pregnancy, so you know your baby’s age. Your care provider will use it to confirm a heartbeat, measure the crown-rump (from the top of the head to their bottom) length or gestational age, confirm 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 at 6 to 7 weeks and a heartbeat is not detected, don’t panic. The heartbeat may not be detected because you have a tipped uterus, a larger abdomen, or because of inaccurate dating with your last menstrual period. This is one of the reasons care providers generally do not have you in until about week 8.

The accuracy of your due date is crucial because it can have a big impact on what your care provider recommends as you get close to your due date. 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.

Second Trimester Ultrasounds

Today, it is routine for expecting moms to get an ultrasound anatomy scan in the second trimester. Generally, you can expect to have this done at 18-20 weeks. From your practitioner’s point of view, this is a great way to see how a baby is developing and offer reassurance that everything is developing exactly the way it should be.

The second-trimester anatomy scan ultrasound diagnoses fetal malformation, structural abnormalities, confirms a multiples (twins) pregnancy, verifies dates and growth, confirms a miscarriage, 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 also around their head. Since this is an anatomy scan, the ultrasound technician will be looking specifically at the four chambers of the heart, the 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 be looking at your amniotic fluid levels, the location of the placenta, and the fetal heart rate. It is so cool to get a sneak peek of your baby and you also get a photo to take home, blast on social media, or put in your baby book.

This scan is especially exciting because most people are able to find out the sex of their little one if you opt to not be surprised. If you choose not to find out the sex make sure to tell the ultrasound technician upfront. 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 the birth.

While an anatomy scan ultrasound has become common practice, it is not mandatory. The choice as to whether you want to have it done is ultimately up to you.

If you did not have an ultrasound in your first trimester and there are questions about your due date and the gestational age of your baby your care provider may want to use 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 date of the last normal menstrual period. It is really 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 done with over 44,000 women found that early ultrasound (16-18 weeks) measurements of gestational age tended to skew the age to earlier and this resulted in more preterm births. If your care provider is using ultrasound to date your pregnancy, it tends to be more accurate earlier on.

Third Trimester Ultrasounds

In the third trimester and ultrasound may be used to identify uterine and pelvic abnormalities, identify the location of the placenta, confirm a miscarriage, observe how your baby is positioned in your uterus (fetal presentation), observe fetal movements, and try to determine the size of your baby.

The most common way to measure your baby and how far along you are in your pregnancy is by measuring fundal height. To measure fundal height a measuring tape measures the length from the top of your uterus to your pubic bone in centimeters. After 16 weeks the number of centimeters usually corresponds to which week you are in your pregnancy. If the measurement of your fundus is off it may prompt your care provider to recommend an ultrasound to make sure everything is fine.

While fundal height is helpful, we have no way to accurately measure the weight or height of your baby before they are born. The methods used usually combine ultrasound with some calculations to come up with the size of your baby. There are a lot of questions about how accurate ultrasounds are to measure your baby. One study found ultrasounds to be 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 just as accurate at estimating the size of their baby as ultrasounds were. These two studies are consistent with other studies on this subject, in that ultrasounds have a big 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 not one universal calculation to take measurements from ultrasound and apply them to a height and weight. One study looked at several formulas used to estimate fetal weight and found that all of them either over or underestimate the fetal weight. The researchers concluded that the interpretation of ultrasound estimated fetal weight should be done cautiously, especially in small for gestational age babies. 

Another use of third-trimester ultrasounds is evaluating a baby who is small for gestational age. This means that they are suspected to be below the 10th 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 that 10th percentile that actually are not. This is an issue we see when interventions that may not apply to everyone, including mothers who are low risk, become routine care.

A Cochrane review published in 2015 looked at third-trimester scans and included 13 trials recruiting 34,980 women and concluded that routine late pregnancy ultrasound in low‐risk or unselected populations does not confer a benefit on mother or baby. This is a very short summary, from a very long research paper. A rebuttal to these conclusions was published in the British Journal of Obstetrics and Gynaecology that criticized some of the shortcomings of the Cochrane Review. Namely 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 women who are low risk.

Ultrasound Safety

Ultrasounds have an excellent safety record. They have been used in obstetrics for five decades with no proven harmful effects. The general consensus is that an ultrasound is a noninvasive procedure which, 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 it is recommended that ultrasound only be used if medically indicated.

However, some questions have been raised about ultrasound exposure in 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 3 weeks after their birth. The ultrasound-exposed pups were significantly less interested in social interaction compared to male pups who did not undergo an ultrasound. The pups who had an ultrasound also demonstrated significantly more activity relative to the pups who were 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 big differences between the exposure of diagnostic ultrasound to mice versus humans that would require much more exploration before this work would be used to change any clinical practice. The bottom line of this study is that more studies should be done.

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. For this study they exposed the brains of chicks on day 19 of a 21 day incubation period to 5 or 10 min of ultrasound waves, or to 1, 2, 3, 4 or 5 minutes of pulsed Doppler ultrasound before they were hatched. After the chicks had hatched learning and memory function were assessed on day 2. The results showed that ultrasound exposure did not affect memory function, but 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 and that energy is converted to heat depending on the frequency and intensity of the ultrasound wave. From a heat-absorption point of view, caution is perhaps even more important during Doppler studies in a fetus close to term. 

Of course, mice and chicks compared to humans have a lot of differences. We have talked before about how they are not human studies on so 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 that have been done on animals. It may not be ideal, but it is better than no research at all. 

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

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, which usually is performed at 18–22 weeks of pregnancy.

The only thing we 100% know about ultrasound safety is that we don’t know everything and more studies need to be done to determine the safety of ultrasounds. This is becoming more important as their use is increasing. Ultimately you need to decide whether you want to opt into having one or more ultrasounds done.

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, and this 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 general consensus from all professional medical organizations is don’t do it.

According to the American College of Obstetricians and Gynecologists it is recommended that ultrasound exams be performed only for medical reasons by qualified health care providers.

According to the FDA 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 World Health Organization states that there are potential complications related to cavitation, the formation of a bubble within a biological structure. The risk of cavitation is very low at the ultrasound intensities used for medical diagnosis.

If you choose to get a 3D or 4D ultrasound you will likely be paying the cost out of pocket, since this is not for a medical reason. I would also talk about this to your care provider for their professional opinion.

Should you get an ultrasound done during your pregnancy or should you avoid them? Well, ultimately it is your decision. I do urge you to weigh the pros of an ultrasound with the potential risks or unknown risks. Perhaps the most important question you want to ask is what are you going to do with these results. If the end goal is to evaluate a concern you may weigh that differently than if the goal is to get a picture of your baby’s adorable face. Ultrasounds, like so many procedures during your pregnancy, are a choice. It is so easy to go along with routine practices and if you are comfortable doing that then great. It can be challenging to question the common practices of your care provider. I want to make sure that you know you can. You are the captain of this ship. You do have a choice and if you have questions this is something you should be discussing with your doctor or midwife.


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