Due date calculator
Estimated due date, gestational age and the term window from the last period.
Add a year to the first day of the last period, subtract three months, add seven days — which is 280 days, or 40 weeks. Naegele set this out in the early 1800s and it still anchors how gestational age is counted. It assumes ovulation on day 14, so a different cycle shifts it, and babies rarely arrive on the date itself. The term window is shown alongside for that reason.
Last period
The day it began, not the day it ended.
Anything other than 28 shifts the date by the difference.
Date and gestational age
Estimated due date
8 October 2026
Term window
17 September 2026 – 22 October 2026
Estimated ovulation
15 January 2026
Full term runs from 37 weeks 0 days to 42 weeks 0 days. About one in twenty babies arrives on the date itself.
Based on the last period alone. A date adjusted by ultrasound is more accurate. Not medical advice.
A year forward, three months back, seven days on: 280 days
Enter 1 January 2026: a year on is 1 January 2027, three months back is 1 October 2026, seven days on is 8 October, the due date. Adding 280 days to 1 January lands on the same 8 October. The 280 is 14 days from the last period to ovulation plus the 266 days (38 weeks) taken as conception to birth. That is why gestational age starts counting before conception — at two weeks pregnant there has been no conception yet — and why the estimated ovulation shows as 15 January, from that same 14.
A different cycle shifts every date together
The cycle length minus 28 is added to the due date, the term window and the estimated ovulation alike, and subtracted from the days elapsed when the current week is counted. Change the cycle to 35 and the due date becomes 15 October, ovulation 22 January, and the age seen on 4 September drops from 35 weeks 1 day to 34 weeks 1 day. The cycle is rounded to whole days and accepts up to 60. With an irregular cycle this correction swings with whichever length you enter, so it is not much to rely on.
Weeks, trimester and days remaining are relative to today
Today is this browser's date. Days since the last period are divided by 7: the quotient is weeks, the remainder days — 246 days from 1 January to 4 September is 35 weeks 1 day. Under 14 weeks is trimester 1, under 28 is trimester 2, beyond is 3; sources draw those lines a little differently, some at 12 or 13 weeks. Days remaining is the due date minus today, and once the date has passed the screen says so. The term window runs from 37 weeks 0 days to 42 weeks 0 days, which for 1 January is 17 September to 22 October.
An estimate only — the clinic's ultrasound date takes precedence
This screen works from a single date, the first day of the last period. When ovulation actually happened is unknown; it is assumed to be day 14. If a due date has been set from fetal measurements on ultrasound, that one takes precedence, and a disagreement between the two does not mean this screen is right. It does not determine whether someone is pregnant, says nothing about the fetus, and has no method for dating from an IVF transfer. It is not medical advice; questions belong at an appointment.
Common questions
QWhat if my cycle is not 28 days?
Entering the cycle length shifts the date by (cycle − 28) days. A 35-day cycle means ovulation a week later, so the date moves a week out. With an irregular cycle even this correction is rough, and a scan is needed.
QThe clinic gave me a different date.
A date adjusted from fetal measurements on ultrasound is more accurate, and a first-trimester scan is good to within a few days. This works from the last period alone, so follow the clinic.
QWill the baby arrive on the date?
About one in twenty do. Most arrive somewhere between 37 weeks 0 days and 42 weeks 0 days, which is why the window is shown rather than a single date.