Due Date Calculator

What it is

This calculator turns the one date you know into an estimated due date and a picture of where you are right now. Start from the first day of your last period, an embryo transfer date, or the day you ovulated or had insemination. You get the estimated due date, the gestational age on the day you check, the trimester, the number of days to go, when the next trimester begins, and the calendar date each pregnancy week starts, from week 4 to week 40. The page opens with an example, a period that began eight weeks before today, so you see a result straight away. Change the dates and everything updates. Nothing you type leaves your browser.

How to use

1. Choose what you are starting from. For most people that is the last period. Embryo transfer and ovulation or insemination are for fertility treatment.
2. Enter the date. For the last period, check the average cycle length too. It starts at 28 days.
3. For an embryo transfer, pick a day 3 or day 5 embryo.
4. The date to check starts as today. Change it to see the gestational age on any other day.
5. Read the result, the math lines and the week-by-week start dates.

How it works

• Last period: due date = first day of last period + 280 days + (cycle length - 28) days. The 280-day count and the trimester split follow the Korea Disease Control and Prevention Agency guidance on normal pregnancy care.
• Embryo transfer: due date = transfer date + 266 days - embryo age, so 263 days for a day 3 embryo and 261 days for a day 5 embryo, as in ACOG Committee Opinion 700 (2017). Ovulation or insemination: date + 266 days, the 280-day rule minus its 14-day ovulation assumption. The clinic’s date takes priority.
• Gestational age: the whole weeks and leftover days between the calculated start (due date minus 280 days) and the date you check.
• Trimesters are blocks of 14 weeks. The first runs from 0w0d to 13w6d, the second from 14w0d to 27w6d and the third from 28w0d until birth.
• Limits: cycle length 20 to 45 days, and the date checked can be at most 364 days after the calculated start. From 42 weeks the page tells you that you are past the due date.
• Dates are plain calendar days, with no time zone or daylight saving effects.

Examples

A last period starting on 10 January 2026 with a 28-day cycle.

Item	Math	Result
Estimated due date	10 Jan + 280 days	2026-10-17
Week 8 starts	10 Jan + 56 days	2026-03-07
Second trimester (week 14)	10 Jan + 98 days	2026-04-18
Third trimester (week 28)	10 Jan + 196 days	2026-07-25

With a 32-day cycle the due date moves four days later, to 2026-10-21. These dates are an example only.

Good to know

Every result here assumes an average pregnancy. If you have an ultrasound, follow the due date your clinic sets. A normal pregnancy lasts about 280 days (40 weeks) and the actual birth date often differs. This is a rough estimate for reference and does not diagnose or treat anything, so talk to a doctor if you are worried.