Are Sonograms Always Accurate with Due Dates?

Cynthia Flynn's picture
Experts: 

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.