Due Dates

March 28, 2011 at 10:44 am (Education, Informed Consent)

There are two common questions that everyone asks a pregnant woman:  Are you having a boy or girl, and when is your due date?  There’s a lot of emotion surrounding this date- moms look forward to it eagerly as they anticipate meeting their baby, and friends keep track of it so that they know when to expect the big day.  When a mother has her baby before or after the Big Day, people are often concerned by this deviation from their expectation.  And for mamas with a set date in their mind, it can be a big emotional challenge to go past that date…..possibly weeks past it.  Why is the Due Date such a big deal, and what does it really mean?

The Due Date is just an estimate

This all-important date is based on the assumption that pregnancies go on for exactly 280 days.  These 280 days, or 10 lunar months, are calculated with the assumption that the pregnancy started exactly two weeks after the start of the mother’s last menstrual period.  There are a couple of problems with this method of calculation.  First of all, several studies have been done to see how long the average pregnancy is when labor is not induced.  The average pregnancies described by these studies range from 279-288 days.   So we’re already shortchanging our natural gestational cycles by estimating on the low end of this range of averages.  And if a mom has a cycle that’s longer than 28 days, then her ovulation will happen later in that cycle, pushing her true due date even later.  Since the standard calculation method doesn’t take into account differences in cycle length, some women may have a due date that’s as much as a week earlier than it should be, if all physiological factors are taken into account.  Knowing this, it really helps to keep in mind that the due date is just an estimate, and potentially an inaccurate one- it’s not a definite date!

The Due Date is the midpoint of a range

Normal physiological pregnancy can last anywhere from 37-42 weeks, with the due date falling on week 40, right in the middle.  In all studies on length of pregnancy, the majority of women had their babies somewhere in that 5-week range, with a few percent having their babies before 37 or after 42 weeks.  A pregnant woman is not truly post-dates until she progresses beyond 42 weeks.  However, with our cultural fascination with the due date, we’ve led many women to think that there’s something wrong if they go past that midpoint date, instead of helping them to wait until 42 weeks before they consider themselves late.  Our obsession with The Date makes it really hard for the 50% of women who go past their due date (and that percentage is higher for first time moms).  Keeping all this in mind, it’s a good idea to just pretend that your due date is a week after the date your doctor has given you. That way, if you have your baby earlier, you’ll be happily surprised, but if you go to the date, you’ll still feel like you’re normal.  An alternative is to tell everyone that you have a “due month”- “I’ll probably have my baby sometime between June 8th and July 4th.  We’ll see when she decides to come!”

In thinking about your due date, just remember that it’s a number that’s estimated based on a formula and that doesn’t take into account your body and your baby’s individuality.  As long as you don’t have a medical reason to have your baby by the date on your chart, give yourself the freedom to gently ignore your due date.  Remember that it’s normal to be pregnant up to 42 weeks, and that you have upwards of a 95% chance of having your baby by then.  Relax, give yourself time, and schedule a pampering session for your due date….and the next few days after, as well.  And let your friends know that the baby will come when she’s ready, on her own schedule, and that you’ll be sure to let them know when the Birth Date (instead of the Due Date) arrives!


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