The Lie of the EDD: Why Your Due Date Isn’t when You Think
by Misha Safranski – http://voices.yahoo.com/the-lie-edd-why-due-date-1958162.html?cat=25
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”.
Percentage of babies that arrive at different stages of gestation
Before 37 weeks
37 – 42 weeks
After 42 weeks
After 43 weeks
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.
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.
LMP: August 4th, 2012
subtract 3 months: (May 4th)
add 10 days: (May 14th)
EDD: May 14th, 2013
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.
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.
What are Some Dangers to a Post Term (after 42 weeks) Pregnancy?
Your provider may express concerns about a condition called Postmaturity Syndrome. This condition,
when it occurs, does indicate a high level of risk for the baby. The symptoms include:
- Decreased soft tissue mass, particularly subcutaneous fat
- Peeling skin on the feet and hands
- Long finger and toenails
- Increased incidence of passage of meconium in utero. This can be serious, leading to meconium induced pneumonia in the baby after birth, but is not necessarily going to lead to a problem.
However, researchers in 1985 found that the symptoms of Postmaturity were not found exclusively in post term babies alone and do not necessarily mean that there is a problem with the baby or that the baby was postmature.
Will Carrying Past My Due Date Mean My Baby Will Be Too Big?
Depends on your definition of too big…many women have successfully birthed babies over 12 lbs, without any tears or assistance! Have faith in your body, and in your baby, to know when to go into labor. Studies have shown that larger babies have a lesser chance of getting “stuck” in the birth canal than smaller babies! And, just because you go past your due date, does not mean that your baby will be huge!
What About Stillbirth?
A study conducted in Canada in 1994 found that the stillbirth rate was approximately the same at 37 weeks (5.1 per thousand) as it was at 43 weeks (5.2 per thousand) of pregnancy! Also, the stillbirth rate was actually lower at 42 weeks (2.0 per thousand) of pregnancy than it was at 38 weeks (2.5 per thousand). Also, in this study, more than a quarter of the stillbirths were due to congenital anomalies, not due to being born post dates.
Your provider may monitor you and your baby more closely if you carry past your due date, to check for placental function, amniotic fluid levels, good fetal heart rate, and lots of fetal movement. All of these are indicators of how your baby is doing. This may be done with an ultrasound, Non stress test, or a biophysical profile.