Do low-risk pregnancies need a third trimester ultrasound?

More patients are asking for a third trimester ultrasound, but do they have benefit in patients with a low-risk pregnancy? With no guidelines about offering them, what should physicians do? Ultrasounds in the third trimester may detect small-for-gestational-age fetuses and help identify a risk of intrauterine fetal demise (stillbirth), but they can also increase false positives and lead to unwarranted investigations. Experts debated the utility of third trimester ultrasounds at the annual conference of the Society of Obstetricians and Gynecologists of Canada, held in Ottawa in early June. While they discussed pros and cons, the conclusion was that patients would ask for third trimester ultrasounds regardless of benefit. What are some benefits? The main advantage of third trimester ultrasounds is they detect small-for-gestational-age and large-for-gestational-age babies, said Dr. Anne-Maude Morency, assistant professor in the department of obstetrics and gynecology at McGill University and a maternal-fetal medicine specialist at the Royal Victoria Hospital. This helps parents and physicians prepare for morbidities related to babies born in the higher or lower growth percentiles. For example, the prenatal care team can contact pediatric specialists prior to the birth, decide on the best mode of delivery or the timing of the birth. A third trimester ultrasound can also detect intrauterine growth restriction—a risk factor for intrauterine fetal demise. A randomized trial in the Netherlands found that small-for-gestational-age fetuses were detected significantly more often in the routine third trimester ultrasound group compared to usual care. The third trimester ultrasounds can also detect fetal abnormalities that only develop in the third trimester, such as ovarian or abdominal cysts, ventriculomegaly or achondroplasia, Dr. Morency said. More patients with low-risk pregnancy are requesting third trimester ultrasounds, Dr. Morency said. “It is reassuring to patients. They can confirm the gender, which is not important to us, but is important for them.” “At the end of the day, there are no clear guidelines, no consensus, so why not?” Read: Keepin' it regular: Talk to patients about periods and other tips from new clinical review on endometriosis What about drawbacks? Third trimester ultrasounds in low-risk pregnancies can lead to a cascade of follow-up testing and evaluations that may not be needed, said Dr. Venu Jain, associate professor maternal fetal medicine at the University of Alberta. They can also lead to parental anxiety and a further “medicalization of normal pregnancies,” he added. A number of reviews and trials, including the trial from the Netherlands, have found that while third trimester ultrasounds detect abnormalities and small for gestational age fetuses, there is no difference in outcomes compared to usual care, Dr. Jain said. Another concern is that third trimester ultrasounds in low-risk pregnancies are not a good use of healthcare resources. Providing this to all patients might mean imaging resources are limited for other patients, such as for patients with endometriosis. “I would argue this is not justifiable,” Dr. Jain said. Do it right Regardless of the benefits and the drawbacks, patients with low-risk pregnancies are still asking for third trimester ultrasounds. “Fortunately, or unfortunately, this is already happening” Dr. Jain said. “If this is going to keep happening, maybe they can happen better.” If prenatal care providers choose to offer their low-risk patients third trimester ultrasounds, they should provide them properly, Dr. Jain said. All in the timing Ultrasounds should be timed at 34 to 37 weeks, because 32 weeks is too early to detect small-for-gestational-age fetuses or late-onset intrauterine growth restriction, said Dr. Morency. An ultrasound at 37 weeks or later would be too late to investigate abnormalities or delivery planning. What assessments to do in low-risk pregnancies An ultrasound should include measurements of growth, amniotic fluid volume, the fetal presentation, and basic anatomy (e.g., brain, heart, diaphragm, stomach, kidneys, bladder), Dr. Morency said. Choosing Wisely Canada notes that for third trimester ultrasounds in low-risk pregnancies, physicians should not request a biophysical profile or a fetal Doppler assessment. Despite this recommendation, some physicians are still ordering biophysical profiles and fetal Doppler assessments. Data presented by Dr. Jain suggested 41% of obstetricians and gynecologists would order a third trimester ultrasounds in low-risk pregnancies, compared to 12% of family physicians. Of these, more than 25% would ask for a biophysical profile score and more than 10% would order a fetal Doppler study even if there was no clear indication. The data came from a survey 223 obstetricians and gynecologists and 283 family physicians. The fact that a fetal Doppler assessment and biophysical profile are not needed has to be communicated to radiology colleagues, Drs. Jain and Morency said. Read: Ontario to expand list of tests midwives can order; midwives say it's a first step How many ultrasounds in the third trimester If physicians choose to offer an ultrasound in the third trimester to patients with low-risk pregnancies, only one assessment is needed. Multiple ultrasounds have no further benefit, according to a 2020 review of third trimester ultrasounds in low-risk pregnancies published in the American Journal of Obstetrics and Gynecology Maternal-Fetal Medicine. Counsel patients Dr. Morency noted that explaining the reasons not to order a third trimester ultrasound actually takes longer than performing the actual ultrasound. But if an ultrasound is offered, patients need to be told why it is being done and what the findings mean. “It requires counselling, like everything we do,” Dr. Morency said. “Not everyone understands what we do and why we do it.” Some patients think it is simply to confirm gender, she said. A patient handout about the pros and cons of a third trimester ultrasound in low-risk pregnancies would help prime patients for the conversation, Dr. Jain said. Read: Perle Feldman drops the baby

Do low-risk pregnancies need a third trimester ultrasound?

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