Disagreement over due date | Frequently Asked Questions About Pregnancy

A: During the first trimester, ultrasound of the embryo is very reliable. It becomes less reliable during the third trimester when the baby gains weight but less organ development occurs.

Acceleration, or the mother’s perception of fetal movement, is more subjective. Activity in the gut can sometimes be misinterpreted as a fetal movement. Mothers who have already felt movement in a previous pregnancy can usually notice the acceleration earlier (at about 16 weeks of pregnancy), while mothers who have started for the first time feel it a little later (at 18 weeks).

Such a discrepancy in dates would be unusual if you were very regular (cycles from 28 to 32 days) and you had no bleeding or were not using a hormonal method of contraception.

If a mom is very confident about her dates and she has regular monthly cycles, most providers try not to change due dates based on one ultrasound. However, with a big discrepancy like yours, most carriers would probably change an expiration date.

Ultrasound is only as good as the equipment, the technician and the interpreter. It is possible that an error has been made. Or maybe a previous pregnancy was lost and a new one started. You may want to request another ultrasound in a month or six weeks, especially if your uterus isn’t growing at the new estimate.