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Since your last menstrual period was on , your baby's estimated birth date is .

How is a baby's estimated birth date calculated?

Naegele's Rule (Standard Pregnancy Dating Method)

Franz Naegele, a German obstetrician, came up with the first due date calculations around 1850. He determined that the average length of pregnancy was about 38 weeks (266 days). This is about 40 weeks (280 days) after the first day of a pregnant woman’s last menstrual period. Naegele based his figures on an average menstrual cycle of 28 days, with ovulation on day 14 of the cycle. That meant the egg could not be fertilized until 2 weeks after the first day of the menstrual cycle. He used his data to come up with a calculation for due dates.

Today, most authorities agree that many factors affect a due date. Your health care provider can use ultrasound and other techniques to help estimate your due date.