A pregnancy due date calendar predicts a baby's birth date by using the starting date of the mother's last menstrual period and adding 280 days, or 40 weeks, according to Perinatology.com. If a woman is sure of the date of conception, the calendar calculates the due date by adding 266 days or 38 weeks to the date of conception. Calculators on the websites for The Bump, What to Expect and BabyCenter may be used to estimate due dates.
Another way to calculate estimated delivery date is to subtract three months and add one week to the date of the last menstrual period, as reported by What to Expect. Most health care providers date pregnancy from the first day of the last menstrual period. Under this system, the pregnancy begins two weeks prior to conception, assuming a 28-day menstrual cycle with ovulation occurring on day 14, according to Perinatology.com.
Due dates are only estimates, and only about 5 percent of babies are born on their due dates, states What to Expect. Women whose menstrual cycles are not 28 days long or who ovulate before or after cycle day 14 may be given less accurate due dates, with errors of up to two weeks using the last menstrual period dating method, notes Perinatology.com. The duration of pregnancy varies from 38 to 42 weeks, according to What to Expect.