A pregnancy due date is a planning estimate. The usual count is 280 days from the first day of the last menstrual period, which is 40 weeks. If you know ovulation or conception, add 266 days instead, which is 38 weeks. Neither date diagnoses a pregnancy or schedules the birth.
The pregnancy due date calculator is the period version. The due date by ovulation calculator is the 266-day version. Both sit on the pregnancy calculators page.
Naegele’s rule
Naegele’s rule is the 280-day count written as a calendar shortcut:
Last period + 1 year − 3 months + 7 days.
Last period on 1 January 2026:
- Plus 1 year: 1 January 2027
- Minus 3 months: 1 October 2026
- Plus 7 days: 8 October 2026
That matches 1 January plus 280 days. The shortcut and the day count are the same estimate.
When the cycle is not 28 days
The rule assumes ovulation about 14 days before the next period, which is day 14 of a 28-day cycle. Shift the due date by the difference from 28.
| Cycle | Adjustment | Due date from 1 January 2026 |
|---|---|---|
| 24 days | −4 days | 4 October 2026 |
| 28 days | none | 8 October 2026 |
| 32 days | +4 days | 12 October 2026 |
If you already know ovulation, do not apply this shift on top of the 266-day count. The ovulation date already includes the later or earlier release. Due date from ovulation or conception works an example: ovulation on 14 March 2026 lands on 5 December 2026.
Why the period is the usual start
Most people can name the first day of bleeding. Fewer can name ovulation. Gestational age therefore starts at the period, and conception in this model is already about two weeks into the count. How many days a pregnancy is puts 280 and 266 side by side. Ovulation versus conception separates the egg release, fertilization, and implantation.
Weeks, months, and the fertile window
Days since the period, divided by 7, is the week count. The pregnancy week calculator leads with that number. How many weeks pregnant am I is the same division.
Pregnancy months are unequal groups of weeks. The weeks-to-months converter and the chart keep that separate from dividing by 4.
For trying to conceive, the ovulation calculator estimates ovulation 14 days before the next period and a fertile window of several days before it. That window is a cycle estimate. It does not confirm ovulation.
How to read the result
A figure often cited in prenatal references is that about 4% of births fall on the exact due date. Plan around a window. An early ultrasound replaces both the period estimate and the ovulation estimate when they disagree. Embryo-transfer dating is a different calculation, and these tools do not cover it. How accurate due dates are stays on that limit.