How is the due date calculated from the last menstrual period?

Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts the due date accordingly.

How accurate is an estimated due date?

Only about 5% of babies are born on their exact due date. Approximately 80% are born within 10 days either side. Early ultrasound (before 14 weeks) is the most accurate dating method.

What are the three trimesters?

The first trimester is weeks 1–13, the second trimester weeks 14–27, and the third trimester weeks 28–40. A pregnancy is considered full term from 37 weeks.

How is the IVF due date calculated?

For a day-3 embryo transfer, the due date is calculated as 263 days after the transfer date (equivalent to 38 weeks from conception, minus 3 days for the embryo age at transfer).

Pregnancy Due Date Calculator

How the due date is calculated

The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of the last menstrual period. This assumes a standard 28-day cycle and ovulation on day 14. If your cycle length differs, the calculator adds or subtracts the difference.

Pregnancy milestones

Week 6–10: First prenatal visit and viability ultrasound.
Week 12–13: End of first trimester; nuchal translucency scan.
Week 18–20: Anatomy scan (anomaly scan).
Week 24: Fetal viability threshold. Survival outside the womb is possible with intensive care.
Week 37: Full term. Delivery before this point is considered preterm.
Week 40: Estimated due date.
Week 42: Post-term; most clinicians recommend induction by this point.

Gestational age vs. fetal age

Gestational age is counted from the last menstrual period, which means the fetus is typically about 2 weeks younger than its gestational age. A 12-week gestational age corresponds to approximately 10 weeks of actual fetal development.

Trimester summary

Trimester	Weeks	Key milestones
First	1–13	Implantation; heart begins beating (~week 6); all major organs start forming; nuchal translucency scan (week 12–13)
Second	14–26	Fetal movement felt (~week 18–20); anatomy scan; sex can often be determined; viability threshold reached (~week 24)
Third	27–40	Rapid weight gain; lung maturation; full term at week 37; estimated due date at week 40

Ultrasound dating

After the first trimester, ultrasound biometric measurements — crown-rump length (CRL), femur length, and head circumference — can revise the estimated due date by several days. When ultrasound dating and Naegele’s rule differ by more than 5–7 days, clinicians typically use the ultrasound date as it is based on direct fetal measurement rather than assumed cycle timing.

How common is delivery on the due date?

Only about 5% of babies are born on their exact due date. Approximately 80% of births occur within two weeks either side of the EDD (between 38 and 42 weeks). The due date is best understood as the center of a normal distribution, not a deadline.

This calculator is for general information and is not medical advice. Consult a healthcare professional for personalized guidance.