Due Date Calculator Guide: How Pregnancy Dating Actually Works
The three ways to calculate a pregnancy due date — last period, conception date, and IVF transfer — with the exact formulas and worked examples for each.
Every method of estimating a due date works backward from the same target: 40 weeks (280 days) of gestation, counted from the first day of the last menstrual period (LMP) — even when you're not actually dating from the LMP directly.
Method 1: Last menstrual period (Naegele's Rule)
Add 280 days to the first day of your last period. A last period starting January 1, 2026 gives a due date of October 8, 2026. If your average cycle isn't exactly 28 days, adjust by the difference — a 32-day cycle shifts the estimate 4 days later, to October 12, 2026, since a longer cycle means ovulation likely happened later that month.
Method 2: Conception or ovulation date
Add 266 days (38 weeks) directly to the conception date. This skips the LMP entirely and is more precise if you tracked ovulation — a conception date of January 15, 2026 also lands on October 8, 2026, since conception typically falls about two weeks after the LMP in a standard cycle.
Method 3: IVF embryo transfer date
Because an IVF embryo has a known age at transfer, subtract that age from the standard count: add 261 days for a day-5 (blastocyst) transfer, or 263 days for a day-3 transfer. A transfer on January 20, 2026 gives October 8, 2026 for a day-5 transfer, or October 10, 2026 (2 days later) for a day-3 transfer — the 2-day gap exactly matches the embryo's younger age at transfer.
Why these three methods agree with each other
All three formulas are really the same 280-day count, just anchored to different, equally valid starting points — LMP itself, conception (~14 days after LMP), or an IVF transfer (~14 + embryo age days after LMP). That's why a matched LMP, conception, and IVF date in the examples above can land on the identical due date.
The Due Date Calculator runs whichever formula matches what you know, and shows your current gestational age and trimester milestones alongside the estimated due date.