Pregnancy Due Date Calculator — Naegele's Rule + Conception Date
Calculate pregnancy due date by last menstrual period (Naegele's rule) or conception date. Free, in-browser.
About 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
- Pick input type. LMP date (Naegele's rule, default), conception date, or IVF transfer date.
- Compute. LMP + 280 days = due date. Conception + 266 days. IVF day-3 transfer + 263 days; day-5 transfer + 261 days.
- Show milestones. Trimester boundaries, weekly fetal-development summaries, recommended scan windows.
- Inspect ±2-week window. 90% of babies are born within 2 weeks of the predicted date. Calculator shows the realistic range.
- 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.
Pro 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.
Reviewed by Ahsan Mahmood · Last updated 2026-05-05 · Part of ZTools.
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