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pregnancy-due-date-calculator

A pregnancy due date calculator estimates the expected delivery date from either the first day of the last menstrual period (LMP — using Naegele's rule of LMP + 280 days) or a known conception / IVF transfer date, providing a 40-week timeline plus weekly milestones. The ZTools Pregnancy Due Date Calculator is for general reference and education — actual delivery is highly variable, with only ~5% of babies born on the predicted date and most arriving within ±2 weeks of it. Always pair with prenatal care from a licensed clinician for medical decisions.

Use cases​

Early pregnancy planning​

Newly pregnant individuals get a starting due-date estimate plus week-by-week development milestones for context.

IVF / fertility-tracked conception​

Known transfer dates give a more precise due-date estimate than LMP.

Trimester planning​

Trimester boundaries (~13 wk, ~27 wk) anchor maternity-leave timing, scan appointments, baby-shower planning.

Provider hand-off communication​

A pregnant individual switching providers references the same Naegele-based date the clinician will use as a starting point.

How it works​

  1. Pick input type — LMP date (Naegele's rule, default), conception date, or IVF transfer date.
  2. Compute — LMP + 280 days = due date. Conception + 266 days. IVF day-3 transfer + 263 days; day-5 transfer + 261 days.
  3. Show milestones — Trimester boundaries, weekly fetal-development summaries, recommended scan windows.
  4. Inspect ±2-week window — 90% of babies are born within 2 weeks of the predicted date. Calculator shows the realistic range.
  5. Convert to weeks pregnant — Today's gestational age in weeks + days, the metric clinicians use most.

Examples​

Input: LMP 2026-04-01

Output: Due date 2027-01-06 · today 5w 0d (week of 2026-05-05)

Input: Conception 2026-04-15

Output: Due date 2027-01-06 (10 days earlier than LMP-based for the same conception)

Input: IVF day-5 transfer 2026-04-15

Output: Due date 2027-01-01

Frequently asked questions​

How accurate is Naegele's rule?

It assumes a 28-day cycle with ovulation on day 14 — true for ~30% of women. Longer or shorter cycles shift the real due date by days. The first-trimester ultrasound (CRL measurement) is the most accurate due-date method clinically.

Why 280 days?

Because the average pregnancy lasts 280 days from LMP (not from conception). Conception is ~14 days after LMP, so true gestation is ~266 days from conception.

Will my baby actually be born on this date?

Roughly 5% of babies are born on the predicted date; ~50% within 5 days; ~80% within 10 days. Treat the due date as the centre of a 4-week window, not a deadline.

Is this medical advice?

No. The calculator provides a general estimate. Real prenatal care, dating ultrasounds, and personalised medical decisions belong with a licensed obstetric provider.

What if I do not know my LMP?

A first-trimester ultrasound is the most reliable way to date the pregnancy. The calculator handles known-conception or IVF dates as alternatives.

Can the date change?

Yes — clinicians may revise based on first-trimester scan if it differs significantly from LMP-based dating. Mid- and late-pregnancy revisions are uncommon.

Tips​

  • Use first-trimester ultrasound dating as the gold standard — Naegele is the educational starting point.
  • Note your LMP precisely if you know it — even a few days off shifts the result noticeably.
  • Plan major time-sensitive events (e.g., maternity leave start) for around the due date but with two-week buffer either side.
  • Keep tracking weekly — gestational age in weeks is what providers reference.
  • For IVF, use the transfer date — far more precise than LMP-based estimation.

Try it now​

The full pregnancy-due-date-calculator runs in your browser at https://ztools.zaions.com/pregnancy-due-date-calculator — no signup, no upload, no data leaves your device.

Open the tool ↗

Last updated: 2026-05-05 · Author: Ahsan Mahmood · Edit this page on GitHub