What Window Does My Due Date Have?

Cynthia Flynn's picture


Dear Midwife,
What does it mean when I am told that an ultrasound has a ten day window? And why do doctors say you can go 2-weeks before or after your due date? I know it's not an exact science. I am really confused on how the whole pregnancy thing works.


Ultrasounds compute expected date of delivery by measuring the size of the fetus. It is impossible to measure the fetus exactly, and the accuracy of the size/due date formula gets worse the farther along in the pregnancy you are. Early in the pregnancy, the estimate of the due date can be off as little as 5 days, meaning that if they say the 15th of the month, it may mean the 10th or the 20th is the real due date, or anywhere in between.

But we should really be talking about a "birth month" anyway, because some women make babies quickly and others are slow cookers. I tell my clients that first babies come (on an average) five days after the supposed "due date," but that it is ok for them to come up to 3 weeks before the due date (3 weeks and day is premature, so we don't want that) or up to 2 weeks after the due date (after that, the baby is truly late). There are 35 days during which it is perfectly normal and safe to have the baby -- any one of those days is as good as another from a medical point of view. I understand that they may not all be equal for you!

-- Cynthia, CNM