This article is from the General Pregnancy FAQ, by swnymph@FensEnde.com (Sabrina Cuddy) with numerous contributions by others.
From: email@example.com (Chicar)
This is how to calculate your due day.
The due month is: your LMP month + 9.
The due date is: your LMP date plus 7.
For Example, if your LMP was 3/10/95:
3 + 9 = 12 (Dec), 10 + 7 = 17 (17th day).
If the added month exceeds 12, then you subtract 12.
For example, if you LMP was 6/20/95, the month will be (6 + 9) - 12 = 3
(March), and the date will be 20 + 7 = 27,
so your due date will be 3/27/96.
Traditionally, this is calculated by Naegele's rule, which says
you take the first day of your last menstrual period, count back
3 months, then add 7 days. The following reference questions
that approach (which is several hundred years old... Nutrition
has improved significantly since then!)
The Length of Uncomplicated Human Gestation
Mittendorf, R. et al
Obstetrics & Gynecology, V.75, N.6, June 1990 pp. 929-932.
(From the Dept. of Epidemiology, Harvard School of Public Health;
Harvard School of Dental Medicine; St. Margaret's Hospital for Women;
and Tufts University School of Medicine, Boston MA)
By retrospective exclusion of gestations with known obstetric
complications, maternal diseases, or unreliable menstrual histories, we
found that uncomplicated, spontaneous labor pregnancy in private-care
white mothers is longer than Naegele's rule predicts. For primiparas,
the median duration of gestation from assumed ovulation to delivery was
274 days (P=.0003). For multiparas, the median duration of pregnancy
was 269 days, also significantly longer than the prediction (P=.019).
Moreover, the median length of pregnancy in primiparas proved to be
significantly longer than that for multiparas (P=.0032). Thus, this
study suggests that when estimating a due date for private-care white
patients, one should count back 3 months from the first day of the last
menses, then add 15 days for primiparas or 10 days for multiparas,
instead of using the common algorithm for Naegele's rule.