How to Calculate Your Pregnancy Due Date (2026)
By Rui Barreira · Last updated: 18 June 2026
A pregnancy due date — formally called the estimated due date (EDD) — is a 40-week projection from the first day of your last menstrual period (LMP). Only about 5% of babies arrive on their exact due date, but the EDD is clinically important: it anchors prenatal appointments, screening windows, and delivery planning. Use the Pregnancy Due Date Calculator to get your EDD instantly.
The Naegele's Rule Formula
The standard method used by midwives and obstetricians worldwide is Naegele's Rule, developed in 1812 and still the clinical default:
EDD = LMP + 280 days (40 weeks)
The shorthand version: take the first day of your last period, subtract 3 months, then add 7 days and 1 year. For example, if your LMP was 10 September 2025 — subtract 3 months to get 10 June 2025, then add 7 days — your EDD is 17 June 2026.
This formula assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, your midwife may adjust the EDD after an early ultrasound.
Key Milestones in a 40-Week Pregnancy
| Milestone | Week | What happens |
|---|---|---|
| End of first trimester | Week 12–13 | Miscarriage risk drops sharply; dating scan usually performed |
| Anatomy scan | Week 18–21 | Structural review of organs; sex may be visible |
| Viability threshold | Week 24 | Survival outside the womb becomes medically possible |
| Third trimester begins | Week 28 | Rapid weight gain; GDM screening typically done |
| Term range | Week 37–42 | Birth at any point in this window is considered full term |
| Due date (EDD) | Week 40 | Statistical midpoint of the normal delivery window |
Why the Due Date May Be Adjusted
An early ultrasound (ideally before week 14) measures crown-rump length and gives a more accurate gestational age than LMP alone, particularly when periods are irregular. If the scan-based estimate differs from the LMP date by more than 5–7 days, clinicians typically adopt the ultrasound date as the official EDD. IVF pregnancies use the egg retrieval or transfer date instead of LMP, making the calculation more precise from the start.
Calculate your estimated due date, trimester boundaries, and key milestones with the Pregnancy Due Date Calculator — no account required, everything runs in your browser.
Frequently Asked Questions
- Is this tool free?
- Yes — completely free, no signup required. All processing happens in your browser.
- Does the tool work offline?
- Once loaded, most features work without an internet connection since everything runs client-side.