Pregnancy due date calculator
This due date calculator applies Naegele's rule: add one year, subtract three months, and add seven days to the first day of your last menstrual period (LMP), or equivalently add 280 days (40 weeks).
How to estimate your due date
1. Enter the first day of your last menstrual period (LMP) — day 1 of bleeding.
2. Read the estimated due date (EDD) based on Naegele's rule / 40 weeks.
3. Review trimester dates: first (0–13+6), second (14–27+6), third (28–40+).
4. Note milestone weeks; book prenatal care and ultrasound timing with your provider.
Due date examples
LMP 15 March 2026
EDD ≈ 20 December 2026 (280 days later). Second trimester begins ~16 June; third ~23 September. Many anatomy scans occur around weeks 18–22 (late June to late July).
LMP 1 January 2026
EDD ≈ 8 October 2026. Week 12 (~24 March) often marks end of first trimester screening discussions; week 37 (~8 September) starts early-term window.
Irregular cycles
If cycles are 35 days, LMP-based EDD may be less accurate. An early ultrasound (week 8–13) measuring crown–rump length often revises the date by several days.
When to use this tool
• When you know LMP and want a quick 40-week EDD before the first prenatal visit.
• When you map trimester boundaries for apps, journals, or workplace planning.
• When you explain Naegele's rule in childbirth education or study materials.
From LMP to EDD — the 40-week model
Pregnancy length is counted from LMP because most people know when their period started, not exact conception. Conception typically occurs ~2 weeks after LMP in a 28-day cycle, which is why gestational age at “4 weeks pregnant” is roughly 2 weeks after fertilization. Naegele encodes that convention. IVF pregnancies use embryo age plus 14 days or transfer-date rules instead — this calculator assumes spontaneous LMP dating.
Frequently asked questions
What is Naegele's rule?
From LMP day 1: add 7 days, subtract 3 months, add 1 year (or simply add 280 days). It assumes a 28-day cycle with ovulation on day 14.
When do trimesters start and end?
Commonly: 1st trimester weeks 1–13+6, 2nd weeks 14–27+6, 3rd weeks 28 until birth. Some sources round at week 13/27; your clinic handout may differ slightly.
How accurate is LMP dating?
About ±2 weeks for many pregnancies. Early ultrasound can narrow uncertainty, especially with irregular periods or hormonal contraception recently stopped.
What is full term vs early term?
Early term: 37+0 through 38+6 weeks. Full term: 39+0 through 40+6. Post-term often defined as 42+0 and may trigger monitoring.
Can my due date change?
Yes. First-trimester ultrasound often adjusts EDD if fetal size differs from LMP prediction by more than ~5–7 days.