How the due date is worked out
Pregnancy is counted from the first day of the last menstrual period, about two weeks before conception. By that count a pregnancy lasts 280 days, or 40 weeks. ACOG’s Committee Opinion 700, Methods for Estimating the Due Date, sets out the methods this calculator uses.
From your last period: due date = first day of last period + 280 days + (cycle length − 28)
From conception: due date = conception date + 266 days
After IVF: due date = transfer date + 266 − embryo age (day 5: + 261, day 3: + 263)
From an ultrasound: due date = scan date + 280 days − age at the scan
Weeks pregnant today = 280 days − days left until the due date, written as weeks and days. The cycle-length adjustment is a common clinical rule of thumb, not part of ACOG’s guidance; leave the cycle at 28 to get the plain 280-day date.
Worked example
Your last period started on January 1, 2026, and your cycle is 28 days. January 1 is day 1 of the year, and 280 days later is day 281, which is October 8, 2026. On May 1, 2026 there are 160 days to go, so you are 280 − 160 = 120 days along: 17 weeks and 1 day, in the second trimester. With a 32-day cycle the date moves 4 days later, to October 12. A conception date of January 15 gives the same October 8 date (January 15 + 266 days), and so does a day 5 embryo transferred on January 20 (+ 261 days).
When an ultrasound changes the date
ACOG says ultrasound measurement in the first trimester, up to and including 13 weeks 6 days, is the most accurate way to set or confirm the due date. If the scan and the period-based date differ by more than the amounts below, the due date is usually changed to the scan date. The age range is the age by your last period on the day of the scan.
| Age by last period at the scan | Measured by | Difference that supports changing the date |
|---|---|---|
| Up to 8 weeks 6 days | Crown-rump length | More than 5 days |
| 9 weeks to 13 weeks 6 days | Crown-rump length | More than 7 days |
| 14 weeks to 15 weeks 6 days | Head, abdomen and thigh bone | More than 7 days |
| 16 weeks to 21 weeks 6 days | Head, abdomen and thigh bone | More than 10 days |
| 22 weeks to 27 weeks 6 days | Head, abdomen and thigh bone | More than 14 days |
| 28 weeks and later | Head, abdomen and thigh bone | More than 21 days |
Source: ACOG Committee Opinion 700, Table 1. ACOG adds that third-trimester redating needs special care, because a small baby may be growing slowly rather than younger than thought.
Trimesters and term
- First trimester: up to 13 weeks 6 days. Second: 14 weeks to 27 weeks 6 days. Third: 28 weeks onward.
- Early term is 37 weeks to 38 weeks 6 days, full term 39 weeks to 40 weeks 6 days, late term 41 weeks to 41 weeks 6 days and post-term 42 weeks onward. These are the ACOG and SMFM definitions from Committee Opinion 579, Definition of Term Pregnancy (2013), summarized by the National Institute of Child Health and Human Development.
Limits
- ACOG notes that only about half of women recall their last period accurately, and ovulation does not always fall on day 14. That is why an early scan usually wins.
- A due date is a midpoint, not a deadline. Birth any time in the term window is normal.
- This page does not give medical advice. If you have bleeding, pain or any worry, call your doctor or midwife.
Questions people ask
How is a due date calculated?
By convention the estimated due date is 280 days (40 weeks) after the first day of your last menstrual period. That is how the American College of Obstetricians and Gynecologists (ACOG) describes it in Committee Opinion 700. If you know the conception date, the due date is 266 days after it, because pregnancy is counted from a period that started about two weeks before conception.
What if my cycle is longer or shorter than 28 days?
The 280-day rule assumes ovulation on about day 14 of a 28-day cycle. A common clinical adjustment moves the due date by the difference: a 32-day cycle adds 4 days, a 25-day cycle takes off 3. This is a widely used rule of thumb rather than part of ACOG’s guidance, and an early ultrasound settles it either way.
How do I work out my due date after IVF?
ACOG says the due date after IVF should be set from the age of the embryo and the date of transfer. For a day 5 (blastocyst) transfer the due date is 261 days after the transfer; for a day 3 transfer it is 263 days after. Both are 266 days from conception, less the days the embryo had already grown.
Why did my due date change after an ultrasound?
A first-trimester ultrasound (up to 13 weeks 6 days) is the most accurate way to date a pregnancy, according to ACOG. If the scan and your period disagree by more than a set number of days, your provider will usually change the date. Before 9 weeks the cut-off is more than 5 days; from 9 weeks to 15 weeks 6 days it is more than 7 days. Enter both dates on the ultrasound tab to see the comparison.
How many weeks pregnant am I?
Pregnancy weeks are counted from the first day of the last period, not from conception. So on the day you conceive you are already counted as about 2 weeks pregnant. The calculator shows weeks and days as of today from whichever date you enter.
What are the trimesters?
The first trimester runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth.
How likely am I to give birth on my due date?
Not very. The due date marks 40 weeks, the middle of the full-term window. ACOG and SMFM call 39 weeks 0 days to 40 weeks 6 days full term, and most babies arrive somewhere in the weeks around the date rather than on it.