Why Pregnancy Due Dates Are Inaccurate

Pregnant women and their care providers, as well as society in general, put a lot of stock in due dates. But in reality, a due date is only an estimated date of delivery (EDD). Mothers-to-be, midwives, doctors, families, and others would do well to remember the following reasons why due dates are often inaccurate.

1. Length of the menstrual cycle. Every woman's due date is calculated with the assumption that she has a 28-day menstrual cycle. In addition, it is assumed she ovulated on day 14. Then, the due date is 40 weeks from her last menstrual period.

This method of calculating due dates is extremely out-dated, presumptuous, and flawed. First of all, not all women have regular menstrual cycles. Secondly, one woman may have a 28-day cycle, while a second woman's cycle is 35 days, and a third's is 40 days. Healthy women have menstrual cycles of various lengths, and the length can even change throughout a woman's life or from one pregnancy to the next.

As a result, due dates are often inaccurate.

2. Length of pregnancy. Believe it or not, not all pregnancies are 40 weeks, nor do they have to be. Technically, based on the 28-day menstrual cycle, a pregnancy whose due date is calculated using the above method would only be 38 weeks gestation. This is because, if ovulation happens on day 14, there were 2 weeks between the last period and ovulation. So, saying the pregnancy lasts 40 weeks is wrong, even with this faulty due date calculation.

However, not all pregnancies are 38 full weeks, either. This is simply because women are all different, and so are their babies. A healthy woman can have, say, 3 healthy babies, all born at different gestational ages. There is really no concrete way to determine how long a pregnancy should or will be. But one woman was pregnant for an entire year--52 weeks! The mother and baby were completely healthy and never in danger. A year-long pregnancy was just what that particular mother and baby needed. (I wouldn't want to experience that, though!)

3. Ultrasounds can be off. No matter how good ultrasound technology becomes, measurements of fetal weight and size can be off. So can the due dates ultrasounds provide. Dating a pregnancy by using ultrasound is only accurate within the first few weeks of pregnancy. But even then, the due date can be anywhere from 2 to 4 weeks earlier, or later.

4. Average gestation. There is no magic 40-week mark, as care providers would have mothers believe, and even as they themselves believe. Instead, the average length of pregnancy is actually 41 to 42 weeks. As a result, due dates cause needless worry in mothers and care providers. However, things like induction, if considered at all, shouldn't even be remotely thought of until at least 42.5 weeks gestation.

There is much to be improved, especially within the United States, in obstetrical care. And it may never be at the point where mothers and care providers have adult, enlightened, fact-based conversations about care during pregnancy. But both sides need to be truly informed. And one step in the right direction is realizing the flawed nature of due dates. If there is no logical, rational, medical reason to think mother or baby is in danger, the baby should be left alone as long as he or she wishes. That is, labor should be allowed to begin spontaneously.

This is an unpopular view, because in today's society, women want their babies out by elective Caesarians, because they're uncomfortable. Big deal! Would you rather have a baby born at "40 weeks" who was actually 4 weeks early, staying in the NICU, having trouble breathing, etc, just because you were uncomfortable? Or would you rather wait 4 weeks, be extremely uncomfortable and angry, but end up with a healthy baby to ease your mind and bring joy to your heart?