Due Dates

A due date does not mean there is only one safe day for your baby to be born. An Estimated Due Date or Estimated Date of Delivery is just that - estimated.

"Post date" is a huge reason that obstetricians give for needing to induce labor or schedule a C-section.
For example, these ridiculous posts on My OB Said WHAT?!

“Every day past 40 weeks, a baby dies more and more” – Family Practitioner on why they needed to schedule a mom’s induction at 39 weeks and 6 days.

“I let you go this long.” -OB to mothering at just 41 weeks, who did not want to be induced, had an uncomplicated pregnancy and had consistently been measuring one week behind.

“Now that you’ve reached your due date, there really just isn’t any good reason to be walking around putting yourself and your baby at risk. We need to do a cesarean tomorrow.” -OB to a mother one day past her due date, who was planning a VBAC. 

Very few woman actually deliver on their due date.  Pregnancy can last anywhere from 37 to 42 weeks. Some pregnancies are short and some are long.  Some post-date pregnancies are cases of miscalculated due dates! Some involve fetuses who are not yet ready to be born and need to stew a little longer in mom's tummy to grow and mature.

The Lie of the EDD: Why Your Due Date Isn't when You Think

by Misha Safranski

We have it ingrained in our heads throughout our entire adult lives-pregnancy is 40 weeks. The "due date" we are given at that first prenatal visit is based upon that 40 weeks, and we look forward to it with great anticipation. When we are still pregnant after that magical date, we call ourselves "overdue" and the days seem to drag on like years.

The problem with this belief about the 40 week EDD is that it is not based in fact. It is one of many pregnancy and childbirth myths which has wormed its way into the standard of practice over the years-something that is still believed because "that's the way it's always been done".

The folly of Naegele's Rule
The 40 week due date is based upon Naegele's Rule. This theory was originated by Harmanni Boerhaave, a botanist who in 1744 came up with a method of calculating the EDD based upon evidence in the Bible that human gestation lasts approximately 10 lunar months. The formula was publicized around 1812 by German obstetrician Franz Naegele and since has become the accepted norm for calculating the due date.

There is one glaring flaw in Naegele's rule. Strictly speaking, a lunar (or synodic - from new moon to new moon) month is actually 29.53 days, which makes 10 lunar months roughly 295 days, a full 15 days longer than the 280 days gestation we've been lead to believe is average. In fact, if left alone, 50-80% of mothers will gestate beyond 40 weeks.

Variants in cycle length
Aside from the gross miscalculation of the lunar due date, there is another common problem associated with formulating a woman's EDD: most methods of calculating gestational length are based upon a 28 day cycle. Not all women have a 28 day cycle; some are longer, some are shorter, and even those with a 28 day cycle do not always ovulate right on day 14. If a woman has a cycle which is significantly longer than 28 days and the baby is forced out too soon because her due date is calculated according to her LMP (last menstrual period), this can result in a premature baby with potential health problems at birth.

Recent research offers a more accurate method of approximating gestational length. In 1990 Mittendorf et Al. undertook a study to calculate the average length of uncomplicated human pregnancy. They found that for first time mothers (nulliparas) pregnancy lasted an average of 288 days (41 weeks 1 day). For multiparas, mothers who had previously given birth, the average gestational length was 283 days or 40 weeks 3 days. To easily calculate this EDD formula, a nullipara would take the LMP, subtract 3 months, then add 15 days. Multiparas start with LMP, subtract 3 months and add 10 days. The best way to determine an accurate due date, no matter which method you use, is to chart your cycles so that you know what day you ovulate. There are online programs available for this purpose (refer to links in resources section). Complete classes on tracking your cycle are also available through the Couple to Couple League.

ACOG and postdates
One of the most vital pieces of information to know when you are expecting is that ACOG itself (American College of Obstetricians and Gynecologists) does not recommend interfering with a normal pregnancy before 42 completed weeks. This is why knowing your true conception date and EDD is very important; if you come under pressure from a care provider to deliver at a certain point, you can be armed with ACOG's official recommendations as well as your own exact due date. This can help you and your baby avoid much unnecessary trauma throughout the labor and delivery. Remember, babies can't read calendars; they come on their own time and almost always without complication when left alone to be born when they are truly ready.

Sources:
Mittendorf, R. et al., "The length of uncomplicated human gestation," OB/GYN, Vol. 75, No., 6 June, 1990, pp. 907-932. ACOG Practice Bulletin #55: Clinical Management of Post-term Pregnancy