How Pregnancy Due Date Is Calculated
The estimated due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period (LMP). This is because ovulation and conception typically occur about 14 days after the start of the period in a regular 28-day cycle. The 40-week pregnancy count begins from the LMP, not conception — meaning at conception you are already considered 2 weeks pregnant.
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Formula
Pregnancy Due Date
Estimate your pregnancy due date and current week from the last period.
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Worked Example
Given:
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FAQs
How accurate is Naegele's rule?
Naegele's rule assumes a regular 28-day cycle and ovulation on day 14. Only about 5% of babies are born on their exact due date — most are born within 2 weeks either side. Ultrasound dating in the first trimester is more accurate and is used to confirm or adjust the due date.
What are the three trimesters?
The first trimester covers weeks 1–12, the second trimester covers weeks 13–26, and the third trimester covers weeks 27–40. Each trimester involves distinct developmental milestones and health considerations for both mother and baby.
What is gestational age vs fetal age?
Gestational age is measured from the LMP and is the standard used by healthcare providers. Fetal age (conceptional age) is measured from conception and is typically 2 weeks less than gestational age. When a doctor says you are 10 weeks pregnant, they mean 10 weeks gestational age.