One of the first things a newly pregnant woman does is calculate her due date. This estimation is the closest thing we have to our baby's future birthday and gives us something for which to prepare. This date plays a huge emotional role for us and it's easy to sometimes forget what it really means. The concept of expected due dates is credited to a German Doctor by the name of Naigele, who, through observation of his patients, estimated that a pregnancy lasts for 10 lunar months. Today, estimated due dates are calculated by adding 280 days to the date of the last menstrual period. This calculation does assume that mothers ovulate on day 14 of a 28 day cycle, which may not be a correct assumption for all women . . . so it's not an exact science. Many providers feel that ultrasound measurements are the most accurate way to predict a due date, but that has a margin of error as well.
Based on the premise that 280 days (from the LMP) is the average length of a pregnancy, 80% of babies are born sometime between 37-42 weeks with 10% born premature (before 37) and 10% post-term (after 42 weeks). As with any AVERAGE, there is a range of normal both before and after the due date. Some studies indicate that women of different races have slightly different pregnancy lengths, for example, asian and african american women have shorter gestation periods.
As much as a hate to be the bearer of bad news, it stands to reason that if some women have slightly shorter pregnancies, some women will have slightly longer pregnancies. Sometimes we think of the pregnancy due date like a due date on a term paper: must be completed by ______. This is simply not the case. Many women see their due dates come and go and feel exasperated because their little one has not appeared. They often experience stress and loose confidence in their ability to birth all because of a fundamental misunderstanding of what a due date actually is . . . an estimation. Sometimes this leads to unnecessary inductions and birth complications.
Some providers express concern about mothers going "overdue". There are some risks that women experience with post-term babies, but post-term babies are defined as 42 weeks+ NOT 40 weeks+. One Canadian study compared the stillbirth rate with gestation periods and found an increase of stillbirths at 43 weeks (0.52%), but at 42 weeks (0.20%), it was even less than those who birthed at 38 weeks (0.25%).
So, if your due date comes and goes, and you find yourself wondering "what to do", you can remind yourself (and your provider, if necessary) that you are still normal. As long as you and your baby are doing fine, there is no reason to panic. Breathe deeply and rest assured that no pregnancy lasts forever. You'll be there soon.
Take it from a mother of three, all 8-10 days past their EDDs.
Roshni R Patel, Philip Steer, Pat Doyle, Mark P Little and Paul Elliott. "Does gestation vary by ethnic group? A London-based study of over 122 000 pregnancies with spontaneous onset of labour". International Journal of Epidemiology, Volume 33, Number 1, pp. 107-113
Shi, W. W., Joseph, K.S., Kramer, M.S., Demissie, K., Oppenheimer, L., Liston, R., & Allen, A. 2001, "Recent Trends in Fetal and Infant Outcomes Following Post-Term Pregnancies", Fetal and Infant Mortality Study Group, Canadian Perinatal Surveillance System, Chronic Diseases in Canada, vol. 22 no. 1.
Cindy Crosby is a Certified Doula and Childbirth Educator, as well as blogger at www.birth-smart.com. She currently lives in Derby, Kansas with her husband and three children.