When Is Your Estimated Due Date? How Is It Calculated?

How Your Estimated Due Date (EDD) is Calculated

Discuss the best way of determining your expected due date with your caregiver. It may be wise and more realistic to determine a range of dates. I generally ask my clients to be prepared to be pregnant up to five days past their Woods' due date. Also, when dealing with those concerned, well-meaning questions of "when is your baby due?", try giving a due 'season' instead of a specific date. That way instead of calling and wondering if you're in labor on April 2, Aunt Susie is more likely to wait at least until after Easter. Most importantly, remember that due dates are often incorrect and we should never intend to announce (or decide!) the baby's birthday before it has even arrived. Your baby has the all-important job of initiating labor. Just sit back and wait for his (or her) cues!

There are several ways to calculate your expected due date, including using your date of conception, Naegle's rule, the Woods due date, lunar months, sonogram, amniocentesis, gestational wheel (based on Naegle's rule), and cues throughout pregnancy.

Naegle's rule:(this is the method used by the medical community) date of your last period - 3 months + 7 days = EDD.

This method assumes you have a 28-day menstrual cycle, ovulated on day 14, and is less accurate for first time moms who tend to go beyond the calculated due date. It also does not take leap year into account.
Wood's method:formulated by a Certified Nurse Midwife (who also is a Yale nurse midwifery professor) and takes into account individual menstrual cycle variations as well as if the pregnancy is the first or not. The formula goes as follows:

Add one year to the first day of the last menstrual period, then
For first time mothers, subtract 2 months and 2 weeks (14 days)
For second time (or more) mothers, subtract 2 months and 2 1/2 weeks (18 days)
Add or subtract the number of days your cycle varies from 28.

This method takes into account whether or not this is your first pregnancy, and your individual cycle length. It has been found to usually be accurate within 3-5 days.

Using LMP (last menstrual period):from the date of your last menstrual period add 280 days. Using the date of conception, add 266 days.

Using Pregnancy cues:After 20 weeks the fundal height approximately equals the number of weeks gestation. Baby's first movement can usually be first felt around 16-18 weeks. The baby's heartbeat can first be heard around 16-18 weeks with a fetoscope (earlier with a doptone) and should be heard by twenty weeks.

Due dates determined by sonogram are not very reliable, especially as the pregnancy progresses. This method is more accurate when determined very early in pregnancy and should always be used in conjunction with other methods.