When Will My Baby Be Born?
Want to know what day your baby is supposed to be born? I'm afraid there is no clear answer for that question. There are many problems with the modern notion of a "due date" which make its ability to predict your baby's birth much less than desired.
Pregnancy is believed to last 280 days, which is 10 lunar months or 40 weeks, from the first day of your last period. The formula used by your health care provider will be to take the date you're your last period began, add 7 days and then subtract three months. So, if your last period began on March 6, you will add 7 days to get March 13 and subtract three months to have an estimated due date of December 11.
Where did this date come from? According to Henci Goyer "It was established by a German obstetrician in the early 1800s. He simply declared that a pregnancy should last ten moon months, that is, ten moths of four weeks each. However, when researchers in a 1990 study followed a group of healthy, white women, they discovered that pregnancy in first-time mothers averaged eight days longer than this, and the average was three days longer in women with prior births."
You may be thinking you should consider your baby's real due date 8 days after the one given by the formula? Not only is the length of gestation wrong, by counting from your last menstrual period you have a very high chance of getting the actual start date of your pregnancy wrong (the date of conception). The last menstrual period gives a general guess as to the date of conception, but assumes you have a regular cycle of 28 days. If your cycle is different, or if your body was under stress or medication that changed ovulation your last menstrual period may not be a good indicator of when conception happened.
Even if you know the date of conception, you still have the problem of the length of pregnancy being average. This means half the women give birth before this day, and half give birth after. It is the center of a range of possible dates for your baby to be born. At best, you can be pretty certain your baby has a pretty good chance of being born a week on either side of the 288 day mark.
Perhaps you think your due date is more accurate because it was confirmed by ultrasound. Unfortunately, this method is not as accurate as you may like. Although it may be considered the gold standard in birth prediction, calculations from early ultrasound still allow for plus or minus 5 days, giving you a 10 day window of possibility. Ultrasound done later in pregnancy is even less accurate.
If due dates are so inaccurate, why are they so important? Due dates are only important to modern obstetrical management. After 42 weeks pregnancy, some women experience a problem with their placenta called post-maturity. When this happens, placental function decreases putting the baby at risk. The standard used to be that a baby was considered healthy, even after 42 weeks, unless there was an indication of a problem. With more sophisticated obstetrical techniques, intervention to begin labor at 42 weeks became the standard to prevent post-maturity.
Even that is changing as many caregivers will now routinely induce at 41 weeks. But as Henci Goyer points out, "The earlier and earlier time limit has come about mainly through tests intended to evaluate the baby's well-being. All these tests have poor positive predictive values, meaning that when the test says the baby of a healthy mother is in jeopardy, it is probably wrong." She goes on to say, "Most obstetricians have an unfounded faith in the accuracy of tests. A healthy baby only reinforces the erroneous idea that timely intervention (induction) saved him."
What is most difficult about this forced end to pregnancy is the intervention deemed necessary to save the baby, induction, increases the risk of fetal distress and the incidence of cesarean section. But these problems are then attributed to the baby's "poor condition" noted from the faulty test instead of the induction that caused the problem in the first place. The American College of Obstetricians and Gynecologists recognizes there is no evidence of improved outcomes by doing these routine tests of fetal well-being, but recommends doing them anyway because they feel there is no evidence of adverse effects of testing. Their recommendation ignores the high false positive rate of these tests, and the increased risks associated with inducing a normal, healthy pregnancy.
All of this can put you in a very difficult situation if your due date has come and gone. If the tests are poor predictors of a baby's health, and the interventions cause problems, what do you do? First and foremost understand your due date is no more than an estimate of a time range when your baby will be ready to be born. Don't feel pressured to deliver according to anyone's schedule. You might want to refuse routine tests of fetal-well being. If you notice a change or suspect a problem you can always use them to reassure you, but have them done just because you are past your due date.