When Are You Due?

The Lie of the Due Date - Why Your EDD Isn't When You Think

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 40weeks, 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.

The inaccuracy of ultrasound

First trimester: 7 days

14 - 20 weeks: 10 days

21 - 30 weeks: 14 days

31 - 42 weeks: 21 days

Calculating an accurate EDD

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

Don't Let Them Induce You

Gloria Lemay: "Midwives are always yammering on about “evidence-based practice”. This little catch-phrase seems to over-ride common sense and clinical experience. If the evidence sounds fishy and doesn’t support patience and kindness towards pregnant women and unborn babies, be very sceptical about whose evidence it really is. This is an expose of the poor “science” that started the induction epidemic back in the 1990s. Women continue to be induced to this day despite the publication of this article. "

From the article:

"Approximately one-quarter of pregnant women have not laboured by 41 weeks. Their stillbirth rate in the subsequent week without fetal surveillance is approximately 1 in 1000. Routine induction at 41 weeks is ritual induction at term, unsupported by rational evidence of benefit. It is unacceptable, illogical and unsupportable interference with a normal physiologic situation.

"Almost a quarter of a century ago, the prescient authors of an article entitled Intervention and Causal Inferences in Obstetric Practice cautioned that “as … interventions are applied to an increasingly large proportion of the obstetric and fetal population, a threshold will inevitably be reached beyond which the marginal risks of the procedure will outweigh the marginal benefits”[ 45 ].

The ‘evidence’ on which current practice and popularity of routine or as we prefer to think of it, ritual induction at 41 weeks, is based is seriously flawed and an abuse of biological norms. Such interference has the potential to do more harm than good, and its resource implications are staggering. It is time for this nonsensus consensus to be withdrawn."
 
Read the full article at the link.
Routine induction of labour at 41 weeks gestation: nonsensus consensus

BJOG: An International Journal of Obstetrics and Gynaecology Vol: 109 Issue: 5pp: 485-491 PII: S1470032802010042. Copyright © 2002 RCOG

The Wood's Method of calculating your EDD - essential information for every pregnant woman to know.