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Due Dates

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One of the first questions asked both by a woman expecting a new baby and by almost everyone she comes into contact with throughout her pregnancy is "when is the baby due?" The length of human gestation has traditionally been considered to be between 38 and 42 weeks from the date of the woman's last menstrual period (LMP). The "estimated due date" (EDD) or "estimated date of confinement" (EDC) is usually assigned 280 days, 40 completed weeks, from the LMP date, or 266 days from the date of conception. This assumes that all women have cycles that are of the average 28 days, and that conception occurred exactly 14 days after the beginning of the woman's last period. This calculation is based upon a formula developed in the 1700s by Harmanni Boerhaave and later popularized in the 1800s by a German obstetrician, Franz Carl Naegele. This formula is now referred to as "Naegele's Rule" and is the most widely used method of calculating the EDD today. Recent research has cast doubts upon the accuracy of this method of calculating the due date. At a time when scheduled inductions are at an all-time high, the risks associated with inducing labor using due dates that are not based on scientific evidence are unacceptable. Labor induction for non-medical indications (medical indications include things like pre-eclampsia, intra-uterine growth retardation, or diabetic mother) results in many unnecessary cesareans and often infants requiring respiratory support, suffering from nursing difficulties, and remaining hospitalized for severe jaundice which may have been avoided if labor were allowed to start spontaneously. The following due date calculator utilizes the formula developed by Drs. Robert Mittendorf and Michelle Williams1 rather than Naegele's Rule. While it is a much more realistic estimate, it is important to remember that babies have their own timetables (only 5% of babies are born on their "due dates") and in most cases labor and birth will go most smoothly when clocks and calendars aren't part of the decision to intervene in a healthy pregnancy.
Calculate using:

Month:    Day:    Year: 

Cycle Length:            First Baby? 

Is Mother of African Descent? 

[Ed: Unfortunately, the calculator feature is not working.]

Why is an accurate due date important?
Many different prenatal tests are only accurate within a specific time frame during pregnancy, for example, the Triple Screen test must be performed between 15-18 weeks and the time at which the blood sample was drawn needs to be carefully recorded to help minimize false positive or negative results. Your 'due date' is also used to determine how far your pregnancy has progressed for other markers such as fetal growth, which is determined at each visit starting around 20 weeks by measuring from the top of your pubic bone to the top of your uterus, the fundus. If the measurement is considered too large or small for 'dates' you may be referred for further testing including multiple ultrasound scans to assess the growth of your baby.

Another important issue surrounding your EDD is the likelihood of being subjected to further tests and procedures if you have not gone into labor by your due date. Many care providers start routine Non-Stress Testing or Biophysical Profiles after 40 weeks. Increasingly, inductions are being scheduled at 41 weeks with no medical indication and without explaining the risks involved. Women are told that their placentas are unable to nourish their babies after a certain point or that their baby won't have enough amniotic fluid if the pregnancy continues, regardless of reassuring results on these tests of fetal well-being. Some birth centers and midwives will "risk-out" women who have gone beyond 42 weeks.

The American College of Obstetricians and Gynecologists (ACOG) guidelines for the management of so-called post-term pregnancies2 do not support this routine testing until 42 completed weeks of pregnancy have passed. This is because research has shown no difference in outcome for babies, and increased risk of operative delivery for women, when antenatal surveillance or routine induction is recommended based solely on dates, especially in light of the most recent research indicating that normal pregnancy lasts closer to 41 weeks.

For more information on postdates pregnancy, visit the Pregnancy Concerns section.

1. Mittendorf R, Williams MA, Berkey CS, Cotter PF. The length of uncomplicated human gestation. Obstet Gynecol. 1990 Jun;75(6):929-32.

2. American College of Obstetricians and Gynecologists. ACOG practice patterns. Management of postterm pregnancy. Int J Gynaecol Obstet. 1998 Jan;60(1):86-91.


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