How is a pregnancy due date calculated?

A due date is calculated using Naegele's Rule — add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days the due date is adjusted accordingly. Your doctor may also use ultrasound measurements to confirm or adjust the estimated due date.

How accurate is a due date calculator?

Due date calculators give an estimate based on average pregnancy length. Only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. Ultrasound dating in the first trimester is more accurate than date-based calculations.

What is full term pregnancy?

Full term pregnancy is 39 to 40 weeks of gestation. Early term is 37 to 38 weeks. Late term is 41 weeks and post-term is 42+ weeks. Babies born before 37 weeks are considered premature. Most doctors recommend waiting until at least 39 weeks for planned deliveries.

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Pregnancy Due Date Calculator — When Is Your Baby Due?

Calculate from your last period or conception date — with trimester timeline and milestones

Quick Answer

A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period — known as Naegele's Rule. If your last period started January 1 your estimated due date is October 8. Only 5% of babies are born on their exact due date. Most arrive within two weeks before or after. Full term is 39 to 40 weeks.

An estimated due date (EDD) marks the end of the 40th week of pregnancy counted from the first day of your last menstrual period. Pregnancy is divided into three trimesters: first (weeks 1–12) second (weeks 13–26) and third (weeks 27–40). Your healthcare provider may adjust your due date based on ultrasound measurements. This calculator is for informational purposes only — always consult your doctor.

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First day of last menstrual periodAverage cycle length — 28 days

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Last updated: June 2026

Methodology: Due date calculated using Naegele's Rule: first day of last menstrual period plus 280 days (40 weeks), adjusted for cycle length deviation from 28-day baseline. Conception date method adds 266 days (38 weeks) from estimated conception date. Trimester boundaries: first trimester weeks 1-12, second trimester weeks 13-26, third trimester weeks 27-40. Full term defined as 39-40 weeks per American College of Obstetricians and Gynecologists (ACOG) guidelines. Weekly milestones sourced from ACOG and Mayo Clinic pregnancy week-by-week data. This calculator is for informational and planning purposes only — not a substitute for medical advice. Always consult your healthcare provider for clinical due date determination.