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Due Date Calculator

Calculate pregnancy due date

Result

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Formula

Due Date = LMP + 280 days
(40 weeks from last period)
Conception ≈ LMP + 14 days

Pregnancy Timeline

First Trimester	Weeks 1-12
Second Trimester	Weeks 13-26
Third Trimester	Weeks 27-40
Full Term	39-40 weeks

Complete Guide to Due Date Calculator

Your estimated due date (EDD) is calculated as 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is the standard method used by healthcare providers worldwide, based on Naegele's Rule developed in the 1800s.

Understanding Pregnancy Dating

Pregnancy weeks count from LMP, not conception. This means you're already "2 weeks pregnant" at conception and "4 weeks pregnant" when you miss your period. Actual conception typically occurs around day 14 of a 28-day cycle.

How Accurate Is the Due Date?

Only about 5% of babies arrive on their exact due date. Most arrive within 2 weeks before or after. First-time mothers tend to deliver later than the EDD. The due date is an estimate, not a deadline.

When Dating May Be Adjusted

First trimester ultrasounds (8-12 weeks) can refine the due date if it differs from LMP calculation by more than 7 days. Ultrasound dating is most accurate early in pregnancy when embryos develop at consistent rates.

Term Pregnancy Definitions

Early Term: 37-38 weeks
Full Term: 39-40 weeks
Late Term: 41 weeks
Post Term: 42+ weeks

Frequently Asked Questions

Add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. Your provider may adjust based on ultrasound measurements.

Early ultrasounds (8-12 weeks) measure fetal development which can differ from LMP dating by up to 7 days. If the difference is significant, your provider may adjust the EDD based on ultrasound.

Only about 5%. Most babies are born within 2 weeks of the due date. Consider the due date the middle of a 4-week window rather than an exact delivery day.

Medical convention counts from LMP because it's a known date, while exact conception date is usually unknown. This means you're 'pregnant' for about 2 weeks before actual conception occurs.

With irregular cycles, LMP dating may be less accurate. A first trimester ultrasound provides better dating. Tell your provider about irregular cycles for more accurate estimates.

Gestational age counts from LMP (the standard). Fetal age counts from conception and is about 2 weeks less. Medical records use gestational age.

Full term is 39-40 weeks. 37-38 weeks is 'early term,' 41 weeks is 'late term,' and 42+ is 'post term.' The definition was updated in 2013 - babies benefit from staying in utero until 39 weeks.