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Expecting A Baby — But When? Your Due Date

by Jane Wangersky | September 10th, 2013 | Pregnancy

mixed up dates“When are you due?” While a pregnant woman usually has an official due date to answer this question, the reality is that, with all our progress in understanding pregnancy, no one really knows when the baby is coming until it already is.

There are exceptions, like elective C-sections and induced labor. (And there are unlikely events like me going into labor the morning I was scheduled for an induction.) There are pregnancies that go on too long, and those that end too soon. But we do have some guidelines for the majority of pregnancies.

A new home pregnancy test can tell you how far along you are, if you want to pay a few extra dollars and use two test strips instead of one (the second is to measure the level of your pregnancy hormones, which rise predictably during the first three weeks). Instead of a precise number, the test tells whether you’re one to two, two to three, or more than three weeks pregnant. Since full-term pregnancies last about 40 weeks, this gives you a pretty good idea, however.

This test can be really helpful if you haven’t been keeping track of your menstrual cycles, or if they’re irregular. If you’re fairly certain about your cycles, there are easier ways.

A low tech way to calculate your due date without even doing much math is to go by Naegaele’s Rule — count three months, less a week, backwards from the first day of your last period. Your due date is that same date the following year.

Womenshealth.gov has an online calculator for due dates (also one for predicting ovulation if you haven’t managed to get pregnant yet). It explains: “Your due date is estimated by adding 40 weeks (280 days) to the approximate first day of your last menstrual period. Only five percent of women deliver their babies on their due date, so this date should be used as an estimate.”

Your doctor will have his/her own favorite way of calculating due dates. During my first pregnancy, my doctor used a chart printed on some cardboard disks — probably obsolete now. When she herself got too pregnant to deliver babies, and I had to switch to her partner, he recalculated and came up with another date. As far as I remember, the baby wasn’t born on either.

Your first trimester ultrasound will give the doctor a lot of information on how far your baby’s developed. One more reason to put up with the discomfort.

Though all these methods will give you a rough idea of when your baby’s joining you in the outside world, ultimately the best attitude to take is “We’ll expect you when we see you.”

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