How is the due date calculated?

The estimated due date (EDD) is calculated using Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). For cycles other than 28 days, the calculator adjusts the ovulation offset accordingly.

When does each trimester start?

First trimester: weeks 1–13. Second trimester: weeks 14–26. Third trimester: weeks 27 through birth (around week 40).

Home Health & Fitness Pregnancy Calculator

Pregnancy Due Date & Ovulation Calculator

Find your estimated due date, fertile window, and full trimester timeline.

First Day of Last Menstrual Period (LMP)

Average Cycle Length (days)

Typical range: 21–45 days. Default is 28.

Your due date appears here

Enter your LMP and click Calculate.

Due Date Formula — Naegele's Rule

The standard method for calculating EDD (Estimated Due Date) is Naegele's Rule:
EDD = LMP + 280 days (40 weeks)

For cycles ≠ 28 days, the ovulation day is adjusted: Ovulation = LMP + (cycle length − 14). The EDD shifts accordingly so the 40-week count remains accurate relative to actual ovulation.

Step-by-Step Example

LMP: January 1, 2025 | Cycle: 30 days

Ovulation offset = 30 − 14 = Day 16

Ovulation Day = Jan 1 + 16 = January 17, 2025

Fertile Window = January 12 – January 18

Due Date = Jan 1 + 282 days = October 9, 2025

Frequently Asked Questions

How accurate is this calculator?

This calculator uses Naegele's Rule adjusted for custom cycle lengths. Ultrasound measurements provide more accurate gestational age, especially in the first trimester.

What is the fertile window?

The fertile window is the 6-day period ending on ovulation day. Sperm can survive up to 5 days in the fallopian tubes, so intercourse in this window can lead to conception.

Can I use this if my cycle is irregular?

Use your average cycle length over the last 3–6 months. For highly irregular cycles, consult your healthcare provider for a more accurate assessment.

Last reviewed: October 2024 Sources: ACOG — Methods for Estimating the Due Date· NHS — Pregnancy due date calculator Due-date estimates assume a 28-day cycle and 40-week gestation. Actual dates are confirmed by ultrasound.

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