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Are Sonograms Always Accurate with Due Dates?

QUESTION

Dear Midwife,
Are sonogram due dates always accurate?

Every time I get a sonogram, they change my due date. And I don't know if I'm 7 and a half or 8 and a half months. Is there any way I can know how far along I am without a sonogram? Is there anything I can do to know?

ANSWER

The earliest sonogram is the most accurate, and the one we stick with for your due date if we can't tell from your period. If later sonograms say that your "due date" is earlier, that just means that your baby will get to be 7 1/2 pounds sooner than your due date, and you will probably have a baby that is somewhat larger. If they say that your "due date" is later, by the same reasoning you will probably have a baby that is smaller than 7 1/2 pounds, since all the ultrasound does is take measurements, and obviously some normal children (of tiny people) are 6 pounds and other normal children (of tall people) are 9 pounds at birth.

--Cynthia, CNM. PhD.

Cynthia Flynn

Cynthia Flynn, CNM. PhD, is the General Director of the Family Health and Birth Center which provides prenatal, birth, postnatal, gynecological and primary health care to underserved women and their families in Washington, D.C. Recently Cynthia served as Associate Professor of Nursing at Seattle University. There she not only taught, but remained in full scope clinical midwifery practice at Valley Medical Center where she cared for pregnant and birthing women, and practices well-woman gynecology, family planning, and treatment of sexually transmitted diseases.

Cynthia founded Columbia Women's Clinic and Birth Center, where she took care of pregnant women and infants up to two weeks of age and attended both birth center and hospital births. Before Cynthia earned her CNM, she worked as a registered nurse in labor and delivery and postpartum and is a certified Doula and Doula trainer.