Overdue?
By Emily Jones
March 17, 2008
The average length of gestation for a human female is 266 days, or 38 weeks from the date of conception. However, since most women do not know the day they conceived, health care providers measure the length of pregnancy from the date of the last menstrual period (LMP), which works out to 40 weeks. The two extra weeks come from the fact that the average woman ovulates around 14 days after her last period, and this is when she is assumed to have conceived. This approach works for most women, but like all procedures and tests performed on pregnant women, does not work for everybody.
Because pregnancy is so long, and completely unpredictable, women have latched on to the concept of a “due date.” Women want someone to tell them when it will be all over. Women want something concrete to hold on to, to look forward to. Of course, we are all told from the beginning of our pregnancies that the estimated due date (EDD) is just that: an estimate, and not to pin our hopes too strongly to that particular day. Only about 5% of babies are actually born on their EDD. The EDD is really just a guess, a seemingly harmless target date for women to look forward to.
However, this “harmless” target date is becoming increasingly dangerous. Doctors seem to have forgotten the advice that they give women, that the EDD is not to be relied upon as the actual day of birth. Doctors are increasingly using the EDD as a date on which to start planning inductions, and even threatening c-sections, for going “overdue.” There are many reasons why this increasingly almost universal practice is flawed, and puts women at unnecessary risk.
First, let’s look at how the EDD is figured. As I mentioned, the standard practice is to base the length of gestation on the LMP. Because the average woman has a cycle length of 28 days, she is likely to have ovulated around 14 days after her last period. But this does not take into account any woman with a shorter or longer cycle. If a woman ovulated less than 14 days after her period, her baby will have an earlier due date, since the baby got started sooner. If a woman ovulated more than 14 days after her last period, then her baby will have a later due date.
But how many women know the length of their cycle? How many women know how many days after their period they ovulate? Not many. So they take the EDD as gospel, because they have nothing better to go on. Let me show you an example of how this can be a dangerous piece of information to have:
My cycles are 35 days long. I didn’t know this before I started having children, because I could not feel when I ovulated. I just noticed that my period sometimes “skipped” a month. After I had my first child, I felt pain at ovulation, so I began to notice when I ovulated. I realized that my periods come every 5 weeks, or 35 days, and that I ovulate about 21 days after my last period. How does this affect my EDD? When a care provider estimates my EDD using a 28-day standard cycle, they will place my EDD at one week earlier than it should be, since I actually ovulated one week later than the average woman. My first child was “overdue” by 6 days. My second child was “overdue” by 10 days. But these babies weren’t truly overdue! If my EDD had been figured according to MY cycle, my babies would have been born within three days of their due dates.
What does this mean for the woman whose menstrual cycle is not the standard 28-day cycle? This means that her EDD will be off by as much as 1-2 weeks in either direction. And if she has a doctor who likes to consider induction or c-section after the magical EDD, he may be wrong by as much as two weeks early!!
There is absolutely no justification for considering induction of a pregnant woman before 42 weeks of gestation, for just this reason. Unless the woman herself knows either the day she ovulated, or the day she conceived, her due date may be off by as much as two weeks in either direction. Two weeks is too much of a margin of error to consider taking a baby early.
There are also other factors to consider when determining EDD. Most women do not gestate for exactly 266 days. Some women have a history of early babies. Some women have a history of late babies. How much of this is due to ignorance of cycle length, I do not know. But I guess that since every woman is different, every pregnancy will also be different.
A woman who has a history of going 42 weeks or more in pregnancy will be greatly disadvantaged at an early induction. Her babies are meant to “bake” longer. If she were to be induced at 39 weeks, according to the doctor’s calculations, her typical 42-week babies will be born three weeks too early!
There is one other factor to consider: using ultrasound to determine due date. Studies have shown ultrasound measurement to be unreliable past the first trimester. Up to around 16 weeks, babies tend to grow at approximately the same rate. After that, babies begin to differentiate according to genetics, and then grow at various rates. Ultrasound estimation of due date in the second trimester can be off by a margin of error of up to two weeks. Ultrasound estimation of due date in the third trimester can be off by a margin of error of up to three weeks.
Determining due date by the LMP is a fairly accurate estimate of the due date, but because of the margin of error, it should NOT be used as a determining factor in suggesting induction or c-section. Ultrasound should NEVER be used to estimate due dates past the first trimester, because of the large margin for error.
EDDs should never be used to decide induction or c-section. They are exactly what they say they are: estimates. The baby is the best determinant of when it’s ready to be born. Sure, a woman gets uncomfortable and tired in the end of her pregnancy, and she’s more than ready for it to be over. But is she really willing to risk her baby’s health on a guesstimate? Could she live with her baby being born artificially up to three weeks to early? Could she accept the possibility of her baby ending up in NICU if it was born too early?
The best way to guess when your due date is is to get to know your own body. Pay attention to your cycles. Learn how to know when you ovulate. When you know when you ovulate, you can pretty accurately determine your own due date as exactly 38 weeks from the date of conception. Does it mean that you will have your baby on that day? Not likely. But you will certainly be closer to an idea of what’s appropriate for you, and this can help prevent unrealistic expectations, as well as give you an idea of how much your baby is really growing.
Look at your birthing history. If you haven’t yet had a baby, look at the birthing history of your mother and sisters. Do you/they have a history or going early or going late? Be prepared that your pregnancy may even be different from theirs, but use their experience as a general idea.
At the end of your pregnancy, if you are really tired and uncomfortable, and the idea of inducing seems good to you, and you are truly worried about going “overdue,” there are better ways to know what’s happening in your body then simply kicking the baby out. You can monitor the baby’s heart rate occasionally and pay attention to your body signals. There’s no need to cause an emergency situation when there isn’t one yet. Let your body and your baby tell you when it’s time.
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4 Responses to “Overdue?”
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My problem is that,the gestation period for my wife only took 35 weeks,is this normal? Or she may have got pregnant at ma absence?.I spent with her on 1187 and she delivered on 2048,with ma knowledge it seems it come earlier by 4 weeks,is it normal?
don’t worry about it- anywhere from 35-43 weeks is normal. what is 1187 and 2048?
I went to a doctor, which is not my gyn.. He asked my last period which i wasnt that sure I estimated.. he did a vaginal ultrasound and told me I was 6 weeks. when i should only be 4 weeks.. could the doctor be off by 2 weeks..
So I am past my due date by 9 days, would that change of the time of my first and last day of my period?
With my first son he came on his EDD, so wouldnt that be full term?
With both pregnancy I was both uncertian about the Last day of my period, how common is second pregnancy being over due by 9 - 10 days? I am being induce tomorrow - what are average women being induce with a nutral labout than a c - section?