Pregnancy Due Date Calculator

Naegele's rule, named after German obstetrician Franz Karl Naegele, estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period. The rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. This method has been the clinical standard for over a century because it is simple and reasonably accurate for average cycles.

When your average cycle is longer or shorter than 28 days, we adjust the calculation by the difference. A 32-day cycle adds 4 days to the due date; a 24-day cycle subtracts 4 days. This correction accounts for later or earlier ovulation relative to the standard assumption.

If you know your conception date — from fertility tracking, IVF transfer date, or confirmed timing — the due date is calculated as 266 days (38 weeks) from conception. Pregnancy is counted in weeks from the first day of the last period in clinical practice, which is roughly two weeks before conception in a typical cycle. Both methods should produce similar results when cycle length and ovulation timing are average.

• LMP method: Due date = LMP + 280 days ± (cycle length − 28)
• Conception method: Due date = conception date + 266 days

Your estimated due date marks the midpoint of a normal delivery window, not a deadline. Full-term pregnancy is defined as 37 to 42 weeks. Most healthcare providers consider delivery between 39 and 40 weeks ideal for single, uncomplicated pregnancies. Babies born before 37 weeks are preterm; those born after 42 weeks are post-term.

The trimester breakdown helps organize prenatal care milestones. The first trimester (weeks 1 to 12) is when major organ development occurs and morning sickness is most common. The second trimester (weeks 13 to 26) often brings increased energy and fetal movement. The third trimester (weeks 27 to 40) involves rapid fetal growth and preparation for birth.

Your current week count tells you how far along you are today. Pregnancy weeks are counted from the first day of your last period, so you are considered "two weeks pregnant" at conception in medical terms. This can feel confusing but is the universal standard used by obstetricians and ultrasound technicians.

An early ultrasound, typically performed between 8 and 12 weeks, can refine your due date more accurately than LMP alone — especially if your cycle is irregular or you are unsure of your last period date. Always confirm your estimated due date with your prenatal care provider.

Naegele's rule assumes regular cycles and ovulation on day 14, which does not apply to everyone. Irregular cycles, hormonal conditions like PCOS, and uncertainty about the last period date can make LMP-based estimates less accurate. First-trimester ultrasound dating is the most reliable method when LMP is unknown or unreliable.

This calculator provides educational estimates only. It cannot replace prenatal care, ultrasound dating, or medical advice about your specific pregnancy. Always discuss your due date and pregnancy timeline with a qualified healthcare provider.