You may have calculated your due date using the exact date of your last period, or your practitioner may have made a prediction based on an ultrasound and measurements of your uterus. Either way it's unlikely that you'll have your baby on exactly that day. Believing that due dates are real is just one more comforting fantasy we humans hold dear.
In reality most women deliver healthy babies anywhere between 37 and 42 weeks after getting pregnant--a pretty broad window--with no more than 5 percent of women actually having their babies on their due dates.
The calculations. You and your provider formulated this magical date by taking the first day of your last menstrual period and adding 280 days, or 40 weeks. However, this method assumes that your periods are a tidy, predictable 28 days long, when in fact they might be 23 days or 32 days or so varied that you don't even bother keeping track.
Typically, if you have longer menstrual cycles, you're more likely to deliver your baby after your due date, but you can never know for certain ahead of time. In fact, even a 1st-trimester sonogram--the most trustworthy way to calculate a woman's due date--has to be taken with the proverbial grain of salt because nobody really knows what triggers labor.
Educated guesses. So how do you know when your baby will arrive? You really don't until the Big Day. Until then you can make an educated guess. For instance, if this is your first pregnancy, you're likely to deliver early if your mother did. Caucasian mothers tend to have the longest pregnancies, and so do women under 30. Girl babies are more likely to be born early than boys. The most popular birthday in the United States is Tuesday, if that's any help, while weekends have noticeably lower birthrates--probably because cesareans and induced births are typically scheduled during the workweek for the sake of the doctors performing them.
If your pregnancy extends to 41 weeks, your practitioner will probably perform weekly nonstress tests, but many won't intervene until 42 weeks as long as you and your baby seem to be doing fine. So relax and enjoy a few more romantic dinners when you don't have to tote along an infant seat, a bulging diaper bag, and--oh, yes--a baby!
Originally published in You & Your Baby: Pregnancy.
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