Pregnancy Due Date Calculator
Estimate your baby's due date from your last menstrual period (LMP) using Naegele's rule — the standard OB/GYN method in the U.S.
What is Pregnancy Due Date Calculator?
The pregnancy due date calculator uses Naegele's rule — the method every U.S. OB/GYN relies on — to estimate when your baby will arrive. It counts 280 days (40 weeks) from the first day of your last menstrual period (LMP).
Only about 5% of babies are born exactly on the due date, but roughly 80% arrive within two weeks of it. If your cycle is longer or shorter than 28 days, the calculator adjusts so the estimate stays accurate.
Formula
Naegele's rule: EDD = LMP + 280 days
For cycles other than 28 days, add (cycle length − 28) days. A 32-day cycle pushes the due date 4 days later; a 26-day cycle pulls it 2 days earlier.
Trimesters (ACOG convention):
- 1st trimester: weeks 1–12
- 2nd trimester: weeks 13–26
- 3rd trimester: weeks 27–40
Worked example
If your LMP was January 1, 2026 with a 28-day cycle:
- EDD = January 1 + 280 days = October 8, 2026
- 2nd trimester starts ~April 2, 2026 (week 13)
- 3rd trimester starts ~July 9, 2026 (week 27)
How to use this calculator
- Pick the first day of your last menstrual period — not the last day, not when you think you conceived.
- Adjust cycle length if yours is not 28 days (most women are 26–32 days).
- Read your due date, current gestational week, and key trimester dates.
For the most accurate dating, ask your provider about a first-trimester ultrasound — it can tighten the estimate to within ±5 days if done between weeks 8–13.
Frequently asked questions
How accurate is a due-date calculator?
The LMP-based estimate is within ±1 week for most women with a regular 28-day cycle. A first-trimester ultrasound (crown-rump length, weeks 8–13) is more accurate — within ±5 days — and will be used to confirm or revise your EDD at your first OB appointment.
What if I don't know my LMP?
Use an ultrasound due date instead. If you know your conception date (e.g., via IVF or ovulation tracking), your EDD is that date + 266 days. Without either, wait for your first prenatal ultrasound to establish dating.
What counts as full-term?
The American College of Obstetricians and Gynecologists (ACOG) defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37–38+6, late term is 41+, and post-term is 42+ weeks.
Is it safe if my baby is born a week or two early?
Babies born at 37 weeks or later are considered term and usually do well, though 39–40 weeks is ideal. Preterm (before 37 weeks) carries increased health risks and typically requires extra monitoring. Always discuss timing with your OB/GYN or midwife.
How do I count pregnancy weeks?
Pregnancy is counted from the first day of your last period, not conception. That means by the time your period is a week late, you are already considered "5 weeks pregnant" — even though conception happened only about 3 weeks earlier.