Calculating a baby's due date generally involving moving 40 weeks ahead of the first day of the mother's most recent period, according to BabyCenter. Determining the conception date more precisely requires tracking ovulation patterns and identifying the day on which the egg had to have been fertilized.
Between conception and delivery, the average human gestation period is approximately 38 weeks, but knowing the precise time of conception is extremely difficult, notes BabyCenter. Even for people who only have sex one time during their window of fertility, the sperm can loiter in the fallopian tube for a couple of days until the egg comes along, and then it fertilizes the egg, causing conception.
For a woman with an average menstrual cycle, conception generally happens about two weeks after the beginning of the most recent period, reports BabyCenter. If the physician notices that the baby's progression is at odds with the due date, then he may adjust it after viewing the baby on sonograms.
Only women who have had such procedures as in vitro fertilization know exactly when they conceived, notes the American Pregnancy Association. In that case, the due date is calculated 38 weeks ahead of that point in time. Even knowing the conception date is no lock on the day of delivery, as only about 1 baby in 20 is born on his exact due date.