How to use it
- Choose what you know: the first day of the last period, a conception date, or an IVF embryo transfer (day 5 or day 3).
- Enter the date. The estimated due date appears, with the gestational age today in weeks and days and the days remaining; change the second date to see the gestational age on any other day.
How it calculates
The American College of Obstetricians and Gynecologists’ Committee Opinion 700, Methods for Estimating the Due Date, sets the convention: the due date is 280 days after the first day of the last menstrual period. For pregnancies from assisted reproduction it is counted from the embryo’s age at transfer, 261 days after a day-5 transfer and 263 days after a day-3 transfer. From a known conception date the calculator adds 266 days, the figure StatPearls (NCBI) gives for the average length after conception. Gestational age is the number of days since the date 280 days before the due date, shown as weeks and days. Dates are handled as whole calendar days, so time zones cannot move the result.
The calculation from the last period assumes a regular cycle with ovulation about two weeks in, which is why ACOG recommends confirming or revising the date with an early ultrasound when the period date is uncertain. Treat this as an estimate to discuss with your obstetric care provider, who sets the due date used for your care.
What we tested
On October 14, 2026 we compared the page’s own functions with Python’s calendar on 4,003 random dates from 1990 to 2050, with all four methods and a gestational age on a random second date, running the page code in three time zones (UTC, Los Angeles and Kiritimati, 14 hours ahead): every due date and gestational age matched, and invalid dates such as February 29, 2026 were rejected. A version using 261 days for day-3 transfers failed the test. In Chromium: with the browser set to UTC+14, a last period on January 1, 2026 showed Thursday, October 8, 2026, and on May 15 a gestational age of 19 weeks 1 day with 146 days to go; a day-5 transfer on March 10 showed November 26 and a day-3 transfer November 28; conception on January 15 showed October 8; the page fitted a 390 px phone with no errors.
| Starting point | Estimated due date |
|---|---|
| Last period began January 1, 2026 | October 8, 2026 |
| Day-5 embryo transfer March 10, 2026 | November 26, 2026 (2 weeks 5 days on the transfer day) |
| Last period began February 29, 2024 | December 5, 2024 |
Sources
Checked on .
Frequently Asked Questions
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