It is important to estimate your due date so you and your healthcare provider can schedule proper prenatal care and plan for your baby's birth. Your healthcare provider uses a method similar to this calculator to give you your due date.
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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 found 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 healthcare provider can use ultrasound and other techniques to help estimate your due date.
This calculator is not intended to replace the evaluation of a healthcare professional.
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Author: Lambert, J.G. M.D.
Online Medical Reviewer: Tara Novick BSN MSN
Online Medical Reviewer: Trevino, Heather, RN
Date Last Reviewed: 09/01/2025
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