Why does gestational age matter so much?
Gestational age, or how far along a pregnancy is, guides all of prenatal care. It tells your care team whether the baby is growing well, when to schedule tests, and, if needed, when to step in. The date of the last menstrual period is not always reliable, because many women's cycles vary. That is why best practice is to confirm the due date with an ultrasound early in pregnancy.
In wealthy countries, this dating ultrasound is almost always done. Elsewhere, it is rare: machines are expensive, need steady power, and require a trained sonographer to take and read the measurements. In the public sector of Lusaka, Zambia, women typically first come in for prenatal care around 23 weeks of pregnancy.
What did the researchers do?
From 2018 to 2021, the team enrolled 4,695 pregnant volunteers in North Carolina and Zambia. Along with the usual ultrasound, they recorded "blind sweeps": the probe glides across the belly for about ten seconds along a set path, without trying to capture a specific view of the baby.
A neural network, a type of AI that learns from examples, was trained on more than 21 million image frames from these sweeps. It was then tested on three groups: 716 randomly chosen women, 47 women who conceived through in vitro fertilization (IVF), whose conception date is known exactly, and 249 women in Zambia whose sweeps were done by 8 nurse midwives with no ultrasound training, using a small battery-powered probe that connects to a phone or tablet.
What were the results?
In the main group, the AI was off by 3.9 days on average, compared with 4.7 days for a trained sonographer measuring the baby's head, belly, and thigh bone (fetal biometry). It landed within 7 days in 86.0% of scans, vs 77.0% for the sonographer. Results were similar in the US and in Zambia.
In women who conceived through IVF, the average error was 2.8 days for the AI vs 3.6 days for biometry. And when the sweeps came from untrained nurse midwives using the low-cost probe, the AI was as accurate as standard measurements: 4.9 days of error vs 5.4 days.
Key figures
- 3.9 daysaverage error for the AI, vs 4.7 days for a trained sonographer
- 86.0%of scans dated within 7 days by the AI, vs 77.0% by the sonographer
- 4,695pregnant volunteers in North Carolina and Zambia
Your pregnancy, one appointment at a time, with Mima
This AI was designed for places where ultrasound is hard to get. In the US, your gestational age is checked during your prenatal ultrasounds, and nothing replaces your scheduled visits with your OB-GYN or midwife.
Mima helps you keep track of those appointments and reports, and note how you feel from day to day. She also gets your questions ready, such as: "Has my due date been confirmed by ultrasound?"
What could this mean for pregnant women?
This technology is aimed first at countries where ultrasound is hard to get. A midwife with a small probe could give a reliable gestational age without a sonographer on site, helping to plan prenatal care and delivery.
In the US, a dating ultrasound is already a standard part of prenatal care. This study does not change those appointments. It mainly shows how AI could bring this exam to many more women around the world.
What are the limits of this study?
The authors note that there was no truly external validation group from other regions. Very few scans were done before 9 weeks of pregnancy, so the AI cannot estimate age that early. Data were also sparse after 37 weeks, and the AI tended to underestimate age at that stage in the untrained midwife group.
Finally, this study measures accuracy, not the effect on the health of mothers and babies. The authors say more research is needed to see whether the technology can be rolled out successfully in the health systems that need it.
Frequently asked questions
How is gestational age calculated?
It usually starts from the first day of the last menstrual period, then is confirmed by an early ultrasound that measures the baby. Ultrasound is the recommended check because cycles vary from woman to woman.
How accurate is a dating ultrasound?
Very accurate. In this study, a trained sonographer was off by 4.7 days on average. The AI did slightly better, at 3.9 days.
Will AI replace my pregnancy ultrasounds?
No. This AI was designed for places that lack ultrasound. It does not replace your scheduled ultrasounds or prenatal visits with your OB-GYN or midwife.
What is a blind ultrasound sweep?
It is a pass of the probe across the belly along a set path, without aiming for a specific image. In the study, nurse midwives with no ultrasound training did it with a low-cost probe.
The original study
Pokaprakarn T, Prieto JC, Price JT, et al. “AI Estimation of Gestational Age from Blind Ultrasound Sweeps in Low-Resource Settings” NEJM Evidence, March 28, 2022. DOI : 10.1056/EVIDoa2100058
Read the full study in NEJM EvidenceThis article summarizes a scientific study and is not medical advice.