Due Date Calculator: Naegele's Rule & Trimester Guide

Weeks

Checkup

Wk 6-8

Ultrasound: confirm intrauterine & heartbeat

Wk 11-13+6

NT scan

Wk 15-20

Down syndrome screening / NIPT

Wk 20-24

Anatomy scan

Wk 24-28

OGTT glucose test

Wk 30-32

Growth scan & position

Wk 36+

Weekly visits & NST

Key Takeaways

Naegele's rule: LMP + 1 year − 3 months + 7 days = due date (assumes 28-day cycle)

Cycle adjustment: +1 day for each cycle day beyond 28 (35-day cycle → +7 days)

The window: full-term 37-42 weeks; only ~4-5% arrive on the exact date

Ultrasound wins: first-trimester dating (CRL) is accurate to ±5-7 days and overrides LMP

Core answer: Due date (EDD) = first day of last menstrual period (LMP) + 280 days (40 weeks), assuming a 28-day cycle — Naegele's rule. Shortcut: LMP − 3 months + 7 days (+ 1 year). Example: LMP 2025-03-10 → EDD 2025-12-15. Only ~4–5% of babies arrive on the exact date; 37–42 weeks is the normal window. Long cycles add days: cycle 35 days → EDD = Naegele + 7.

Naegele's rule, worked

LMP March 10, 2025 → minus 3 months = Dec 10, 2024 → plus 7 days = Dec 17, 2024 → adjust year: Dec 17, 2024? No — count carefully: Mar 10 + 280 days = Dec 15, 2025. The −3 months + 7 days + 1 year shortcut gives the same: Dec 17 is the arithmetic for some edge alignments — always verify with the day count. Our calculator does the 280-day addition exactly.

What shifts the estimate

Factor	Adjustment
28-day cycle	baseline
35-day cycle	+ 7 days
21-day cycle	− 7 days
Early ultrasound (8–13 wks)	most accurate — overrides LMP (±5 days)
IVF	EDD = transfer date + (266 − embryo age) days
Irregular cycles	LMP unreliable; ultrasound rules

The timeline that matters

Weeks 1–12 (first trimester): organ formation; NT scan at 11–13+6 weeks; morning sickness peaks ~week 9.
Week 12: miscarriage risk drops sharply — the traditional announcement point.
Weeks 13–27 (second): anatomy scan at 20–24 weeks; first kicks felt 18–22 (first baby) or 16–18 (subsequent).
Weeks 28–40 (third): glucose test 24–28; growth scans; weekly checkups from 36.
37 weeks: full term begins. 41 weeks: most hospitals discuss induction.

Worked examples

Example 1 — Standard cycle. LMP 2025-06-01, 28-day cycle → EDD 2026-03-08. Conception ≈ 2025-06-15 (ovulation day 14).

Example 2 — Long cycle. LMP 2025-06-01, 35-day cycle → ovulation ~day 21 → EDD = 280 + 7 = 287 days → 2026-03-15.

Example 3 — IVF. Day-5 blastocyst transferred 2025-07-10 → EDD = transfer + 261 days = 2026-03-28.

Common mistakes and myths

Pregnancy = 9 months — it's 40 weeks ≈ 9.2 calendar months; doctors count from LMP (2 weeks BEFORE conception), which confuses everyone.
The date is a deadline — it's the center of a distribution: 50% of first babies arrive after 40+3; "overdue" starts at 42, not 40.
Ultrasound always overrides — early scans (8–13 wk) are gold; third-trimester scans have ±3 weeks error and should NOT rewrite a solid early EDD.
Conception date = LMP date — conception ≈ LMP + 14 (28-day cycle); the baby is ~2 weeks younger than "weeks pregnant" suggests.
Planning events around 40w exactly — schedule the baby shower for week 32–34, not 38.

Frequently Asked Questions

How is my due date calculated?

Naegele's rule: first day of your last menstrual period + 280 days. Shortcut: LMP + 1 year − 3 months + 7 days. LMP May 10, 2025 → May 10 + 7 days = May 17, − 3 months = Feb 17, + 1 year = Feb 17, 2026. It assumes ovulation on day 14 of a 28-day cycle — adjust for longer cycles.

How does cycle length change my due date?

Ovulation (and conception) happens ~14 days before the NEXT period, not on day 14 universally. 35-day cycle → ovulation ~day 21 → due date 7 days later than Naegele. 24-day cycle → 4 days earlier. Formula: due date = LMP + 280 + (cycle length − 28). Irregular cycles make LMP dating unreliable — use early ultrasound.

How is the due date calculated for IVF?

From the embryo transfer: for a day-5 blastocyst, due date = transfer date + 261 days; for a day-3 embryo, + 263 days. Egg retrieval day counts as conception day: due date = retrieval + 266 days. IVF dating is the most precise of all methods since the fertilization moment is known exactly.

How accurate is the due date really?

Only ~4-5% of babies arrive on the exact date; ~70% within ±10 days. The normal window spans 37-42 weeks (before 37 = preterm, after 42 = post-term, usually triggering induction). First-trimester ultrasound (crown-rump length) dates to ±5-7 days and overrides LMP when they disagree by >7 days. Think of it as a "due month", not a deadline.

What are the trimester milestones?

First trimester (weeks 1-13): organ formation, highest miscarriage sensitivity, dating scan. Second (14-27): anatomy scan at 20-24 weeks, first kicks around 18-22 (earlier in second pregnancies), viability border ~24. Third (28-40+): rapid growth, glucose screening, kick counting from 28, weekly checkups from 36. Full-term at 37; due date at 40.

Why do doctors count from the last period, not conception?

Historical practicality: most women know their LMP date; few know their ovulation/conception date. The convention adds ~2 weeks before fertilization even happened — so "week 6 of pregnancy" means ~4 weeks post-conception. All medical milestones (scans, screenings, viability) use gestational age from LMP. With IVF or tracked ovulation, conception-based dating is exact.