Pregnancy Due Date & Ovulation Calculator
Estimate your due date from your last period, conception date, or IVF transfer — or find your next ovulation and fertile window. A due date calculator and ovulation calculator due date estimator in one, no signup.
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How this is calculated
Due date (LMP): Naegele's rule — the first day of your last period plus 280 days (40 weeks), adjusted for cycles longer or shorter than the 28-day average. Due date (conception): conception date plus 266 days (38 weeks). Due date (IVF): transfer date plus 263 days for a day-3 embryo or 261 days for a day-5 blastocyst, so the math lines up with the same 266-day count from fertilization. Ovulation: last period date, plus your cycle length, minus your luteal phase (the fairly constant days between ovulation and your next period, ~14 on average). Fertile window: the 5 days before ovulation through ovulation day, since sperm can survive up to about 5 days and the egg is viable for roughly a day.
Worked example: three methods, one due date
A last period starting January 1, with a standard 28-day cycle, lands ovulation/conception around January 15 — and all three calculation methods agree on the same due date once you feed them equivalent dates:
| LMP Jan 1 + 280 days | Due Oct 8 |
| Conception Jan 15 + 266 days | Due Oct 8 |
| IVF day-5 transfer Jan 20 + 261 days | Due Oct 8 |
| IVF day-3 transfer Jan 20 + 263 days | Due Oct 10 |
The day-3 transfer lands 2 days later than the day-5 transfer on the same calendar date — because a day-3 embryo is 2 days younger, so it needs 2 extra days added to reach the same 266-day-from-fertilization point. A cycle longer than 28 days shifts the LMP due date later — a 32-day cycle on that same January 1 LMP pushes the due date to October 12, 4 days later than the 28-day default.
Worked example: ovulation & fertile window
Same January 1 last period, 28-day cycle, default 14-day luteal phase: ovulation lands January 15, the fertile window runs January 10–15 (5 days before ovulation through ovulation day), and the next period is expected January 29.
Not medical advice. These are statistical estimates based on average cycles, not a diagnosis or a guarantee — actual dates vary by individual, and an early ultrasound is the most reliable way to date a pregnancy. Talk to your doctor or midwife about your specific situation.
Frequently asked questions
How accurate is a due date calculated this way?
Only about 1 in 20 babies arrive exactly on their calculated due date — a full-term pregnancy spans roughly 39 to 41 weeks, so a due date is a statistical midpoint, not a deadline. An early ultrasound (typically 8-13 weeks) is considered the most reliable dating method because it measures the embryo directly instead of estimating from cycle averages.
Why does IVF use a different day count than LMP or conception dating?
Because the embryo's age at transfer is known precisely, IVF dating skips the uncertainty of ovulation timing entirely. A day-5 (blastocyst) transfer plus 261 days and a day-3 transfer plus 263 days both land on the same 266-days-since-fertilization point as conception dating — they're the same formula, just adjusted for how old the embryo already was on transfer day.
Why does cycle length change my due date if I'm using LMP?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. A longer cycle usually means later ovulation, so this calculator shifts the estimate forward by (your cycle length − 28) days; a shorter cycle shifts it earlier by the same logic.
Why does the luteal phase matter for ovulation timing?
The luteal phase (time between ovulation and your next period) is far more consistent from cycle to cycle than the time before ovulation — typically 12-14 days for most people. That's why ovulation is calculated by counting backward from the next expected period using the luteal phase, rather than forward from the last period using the follicular phase, which varies much more.