A labor nurse explains how your OB will determine your due date. And a word of advice? When your dates vary – be flexible.

due date on calendar
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When exactly is this baby due? Is it based on your last period, the ultrasound or the date the doctor gives you? If all three are different days, how do you know when to plan for delivery? Good questions, right? I'll bet there are hundreds of pregnant women out there right now who thought they had a due date all nailed down. They went in for their first doctor's appointment and he/she gave them some other date.

Then they went for an ultrasound and were told yet another. What's a mother to do?

Here's how we figure out when to expect that baby. We start out with the first day of your last menstrual period. Let's take, say: January 5th, 2009. Let's assume this was a normal period for you. A couple weeks later, oh, right about inauguration day, you started "celebrating." You got caught up in all that national love and hope and, well, what do you know? A couple weeks later, no period. Uh-huh – you're pregnant.

Now, you know when you got pregnant, more or less. It was after the inaugural ball, right? Or maybe it was a few days before that or that other time. Oh well, you kind of know. You go to the doctor sometime in February, thinking you're right around four weeks "with child." Then your doctor tacks on a couple weeks and says you're six weeks pregnant. What the heck? That's because he's counting the two weeks before you got "inspired" as part of your cycle. Your due date is based on the first day of your last period; even though you weren't pregnant then.

We say a normal pregnancy lasts 40 weeks but figure that anything after 38 is "due." So, if we're basing this on a January 5th period that makes your baby due: October 12th. You can expect a lot of women to share the same, approximate due date. Those were some really fun inauguration/commiseration parties.

What if you have no idea when your last period was or your doctor wants to double-check. It's ultrasound time. The ultrasonographer waves his/her magic wand over your belly and takes measurements of the little squiggle in your uterus. He/she is looking for the length from the crown of the head to the "rump." They call that the CRL or crown to rump length. They base an approximation of when the baby is due on that CRL.

The earlier the ultrasound, the more accurate they are in guestimating a due date. The further along the pregnancy is, the less accurate because, babies vary in size. The margin of error (that's fancy-talk for "we get to make mistakes") is one to two weeks. They're making what a friend of mine calls a SWAG (Scientific Wild A—Guess). It's close, but even the sonographer will tell you, "This is a more or less due date."

What happens if you go to another doctor/midwife and they give you yet a third due date? That could happen. Did he/she use a date wheel? We line the day you say your period starts with a little line on the dial. Then we look for another line that marks 40 weeks and pick that date. The problem is, those wheels are kind of small and those lines are tiny. Those of us of a certain age may need our glasses to see the exact date. If our finger slips on the wheel, we're off a day or two. Or maybe your doctor is using a computer program and accidently plugs in a day that's a bit off from your dates. Eh - mistakes happen. Computer programs vary. Babies grow the way they're going to grow anyway. If it's not wildly off from your original date and not too far off from your ultrasound; so be it. Your due date is an estimate – that's all.

Babies don't keep calendars in there. They mature at somewhat different rates and we still don't know exactly what triggers labor. Approximately 5% of babies are born spontaneously on their due date (not counting the ones induced on their due date).

It's reassuring for women to know there's a light at the end of the tunnel – that light would be a delivery room light and that tunnel, well, I'll let you figure that one out. We need to know the "end is in sight" and we won't just be pregnant indefinitely. Get your doctor/midwife to commit to one date and then, be flexible. Maybe you'll deliver a prompt, stick-to-the-schedule, kind of baby. You have a 5% chance of that. 95% come when they're ready or when we say they're ready. This is your first lesson in flexibility – the key to good parenting. Have fun with it.