Due Date Calculator: Naegele's Rule and Trimester Boundaries

Summary

A due date calculator estimates when a baby is likely to be born based on the first day of the mother’s last menstrual period (LMP) or a known conception date. The most widely used method is Naegele’s rule, which adds 280 days (40 weeks) to the LMP. While only about 5% of babies arrive on their exact due date, the estimate provides a critical reference point for prenatal care scheduling and monitoring fetal development.

Medical disclaimer: This calculator provides estimates for informational purposes only. Every pregnancy is different, and actual delivery dates vary. Always consult a qualified healthcare provider for medical advice, prenatal care, and any concerns about your pregnancy.

How it works

The calculator supports two input methods:

1. From LMP (last menstrual period) — adds 280 days to the first day of the last period. This is the standard clinical approach because most people can recall their LMP more reliably than the exact date of conception.
2. From conception date — adds 266 days (38 weeks) to the known conception date. This method is useful when conception timing is precisely known (e.g., through IVF).

Both methods yield the same estimated due date when the cycle is a typical 28 days, since ovulation and conception are assumed to occur around day 14 of the cycle (280 - 14 = 266).

The formulas

Worked examples

Trimester boundaries

Pregnancy is divided into three trimesters, each marking a distinct phase of fetal development:

• First trimester (weeks 1-12): Major organs begin forming. This is the period of highest risk for miscarriage.
• Second trimester (weeks 13-27): Rapid growth, movement felt by the mother, and the anatomy scan typically occurs around week 20.
• Third trimester (weeks 28-40): Final growth and preparation for delivery. The lungs mature, and the baby gains most of its birth weight.

Assumptions & limitations

• Naegele’s rule assumes a 28-day cycle with ovulation on day 14. For longer or shorter cycles, the due date may shift accordingly.
• Only ~5% of deliveries occur on the exact due date. A normal full-term delivery can happen between 37 and 42 weeks.
• Ultrasound dating in the first trimester is often more accurate than LMP-based estimates and may be used to adjust the due date by a healthcare provider.
• Multiple pregnancies (twins, triplets) typically have shorter gestational periods and require specialized medical guidance.