In the 1850's, a Dr. Naegele determined the average length of human gestation was 266 days from conception, or 280 days (40 weeks) from last menstrual period. He assumed that the average woman had cycles that lasted 28 days and that she ovulated on Day 14 of her cycle. He used his data to come up with a mathematical calculation for due dates:
((1st day of your last menstrural period -LMP + 7 days) - 3 months) = Due Date. EX: ((January 1, 1996 + 7 days) - 3 months) = October 8, 1996
This is still the standard method used to calculate due date, despite the fact that it doesn't take into account: that many women are uncertain of the date of their last menstrual period and not all women ovulate on day 14. Other factors which affect term are: mother's age, ethnicity, prenatal care, prenatal nutrition, number of prior pregnancies.
Another Calculation
A more recent study (Mittendorf, 1990) indicates that term for uncomplicated pregnancies in first-time, Caucasian mothers, averages 274 days from ovulation. It averages 269 days for mothers who've given birth before. The abstract says non-Caucasian pregnancies are shorter than Caucasian, but doesn't give statistics.
*To calculate a due date: Take starting date of last menstrual period. Subtract 3 months. Add 15 days. (10 days if you're non-white, or this is not your first baby.)
What else can be done to determine fetal age?
If ultrasound is performed in the first half of pregnancy, it can indicate fetal age within a range of 7 - 10 days.
Fetal heart tones can be heard through Doppler starting at 9-12 weeks and by stethoscope at 18-20 weeks.
Some believe the baby will come five months after quickening, the first time the mother feels the baby move. This is hard to evaluate, as women can be more or less sensitive to these sensations, and may notice them at different times in their pregnancies. (First time mothers typically notice movement around 18-20 weeks. Mothers who have been pregnant before notice it as early as 16 weeks.)
Fundal height, the measurement of the uterus done throughout pregnancy, can indicate the size of the baby, which can give insight about fetal age.
When will my baby be born?
Only 4% of babies are actually born on their "due dates". 6-10% of babies are born early - prior to 37 weeks; 4-14% of pregnancies last more then 42 weeks.
If this leaves you very confused, just realize that predicting due dates is a very inexact science, and it's probably best to plan ahead of time for the baby to come anywhere between 2 weeks before and 2 weeks after your due date.
For more on due dates, read: Calculating Due Dates and the Impact of Mistaken Estimates of Gestational Age
Sources: "How long is too long?" by Penny Simkin, Childbirth Forum, Spring 1993. Abstract for "The Length of Uncomplicated Human Gestation" by Mittendorf et al
Obstetrics & Gynecology, V.75, N.6, June 1990. "When will my baby be born?" found at http://pregnancy.about.com/library/weekly/aa042197.htm "Pregnancy past your due date" by Terri Isidro-Cloudas on www.americanbaby.com
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