Dr. Marjorie Greenfield
Specializing in pregnancy and birth.
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 Ask Dr. Greenfield
Different Due Dates Based on Baby's Size | | | QUESTION | | | Dear Dr. Greenfield,
My daughter-in-law found out the sex of her baby today. The nurse told her the due date according to the size of her head would be September 27, but that the baby's legs were small and the due date according to that would be October 4. Does this make any sense and what does it mean?— Grandmother in North Carolina | | | ANSWER |
August 17, 2001 | | | Dear Grandmother,
Due dates by ultrasound are not that exact. Remember that we are measuring a flat image of a three dimensional fetus, and then estimating how far along the pregnancy is by the millimeters of the leg bone or of a line drawn across the head. There is a lot of room for being a few millimeters high or low on all of these measurements, not to mention that babies of the same gestational age can be slightly different sizes.
By the time you can see the sex of the baby on ultrasound (around 18 weeks) the measurements are only accurate to about two weeks. In other words--if the baby measures 20 weeks size, he could really be anywhere from 18 to 22 weeks. In the third trimester, ultrasound dates are give or take three weeks. It is pretty common to get head and leg measurements that are a week off from each other, and it usually doesn't mean anything.
The most accurate estimate of the due date is a known date of conception or a certain last menstrual period in a woman with regular cycles. As long as the second trimester ultrasound is within two weeks of that due date, or the third trimester scan is within three weeks, we normally don't change when we think that the baby is due. And unless there is a three-week discrepancy between the head, abdomen, or leg bone, there are not usually any medical issues related to the baby’s size.
Have your daughter-in-law ask her doctor if there was any real concern about the size and shape of the baby, or whether the technician was just thinking out loud as she read you the measurements. Hopefully, this will clear up any confusion about the baby's well being and about the due date. Enjoy!— by Marjorie Greenfield, M.D. |
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