How many ultrasounds are safe in pregnancy?

What does the evidence say about ultrasound frequency during pregnancy?

1. A normal, low-risk pregnancy typically only calls for two standard ultrasounds: one between 11 and 14 weeks to date the pregnancy and confirm viability, and one between 18 and 22 weeks to check fetal anatomy. 1
2. Diagnostic ultrasound has typically been associated with little to no risk to the fetus, though the research base is more limited than for some other areas of prenatal care, which is part of why non-medical keepsake ultrasounds aren't recommended. 1
3. Ultrasound energy can raise tissue temperature, which is why safety guidance calls for keeping the machine's displayed thermal index at or below 1.0 and following the ALARA principle so sonographers use the least energy and time needed for a clear picture. 2
4. A systematic review of obstetric ultrasound safety studies, including one covering more than 2,300 pregnancies, found no evidence linking routine diagnostic exposure, even scans with elevated thermal or mechanical index readings, to differences in a baby's birth size. 3
5. Extra scans get added for a specific reason, like tracking preeclampsia, gestational diabetes, twins, or suspected growth restriction with repeat ultrasound and Doppler checks, not because ultrasound itself becomes riskier with repetition. 4

Is ultrasound frequency safe in each trimester?

• First trimester. The first routine ultrasound usually happens between 11 and 14 weeks, mainly to confirm dating, check viability, and look for twins.
• Second trimester. The anatomy scan is typically done between 18 and 22 weeks to look closely at fetal organs and structures. For a low-risk pregnancy, this is often the last routine scan.
• Third trimester. A third-trimester scan isn't automatic for every low-risk pregnancy, but it gets added when there's a specific concern like slower growth. Twin and other high-risk pregnancies follow a set schedule of repeat scans through this trimester instead.