Pregnancy due date calculator

Naegele's rule and the 280-day count

The German obstetrician Franz Naegele (1778–1851) gave his name to the rule that a pregnancy lasts nine months and seven days from the first day of the last period, Lawson's 2021 review notes. In its usual form: take the first day of the last period, add seven days, subtract three months and add one year. ACOG's convention is a plain count of 280 days, and this calculator follows it.

The two methods can disagree by a few days because calendar months are not all the same length. For a last period starting on 3 March 2026:

1. Naegele: 3 March + 7 days = 10 March; minus three months = 10 December 2025; plus one year = 10 December 2026.
2. 280-day count: 3 March 2026 + 280 days = 8 December 2026.

Naegele's rule adds nine calendar months and a week, and the length of those nine months depends on which months they are. From 10 March to 10 December they cover March to November, five months of 31 days and no February, 275 days in all, so the rule lands 282 days after the period. From 8 January to 8 October the nine months include a 28-day February and come to 273 days, so the two methods agree. The table shows the gap for a period starting on the 1st of each month in 2026:

To check a date by hand, the add or subtract days calculator will add 280 days to any date.

Dating by period, conception, IVF or ultrasound

ACOG Committee Opinion 700, written with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine and reaffirmed in 2025, ranks the methods.

• Last period. The 280-day count assumes a 28-day cycle with ovulation on day 14. The opinion notes that only about half of women recall the date of their last period accurately. In one trial it cites, 40% of women randomised to a first-trimester scan had their due date moved because the scan and the period date differed by more than 5 days.
• Conception date. The due date is 266 days after conception, the same point as 280 days from a period two weeks earlier. A conception date entered in the calculator is treated as the day of ovulation and fertilisation.
• IVF. ACOG says a pregnancy from assisted reproduction should be dated from the embryo's age and the transfer date, not from a period.
• Ultrasound. A first-trimester scan measuring crown–rump length, up to 13 weeks 6 days, is the most accurate method, to within 5 to 7 days. Later scans are less precise, so ACOG advises changing the due date only when a scan disagrees with the period date by more than a threshold that widens with gestational age:

Once the due date is set from the period date or the first reliable scan, ACOG advises changing it only in rare circumstances. The calculator has no scan input: if your clinician has set a date from a scan, use that date.

Trimester and milestone dates: a worked example

For a last period starting on 3 March 2026 with a 28-day cycle, the milestones fall on these dates. Each is the period date plus the number of days in the gestational age.

The scan window follows ACOG's patient guidance that the standard ultrasound exam is usually done at 18 to 22 weeks. A conception date of 17 March 2026, or a day-5 IVF transfer on 22 March 2026, gives the same 8 December due date. With a 35-day cycle instead of 28, every date moves 7 days later and the due date becomes 15 December.

Gestational age is counted in completed weeks and days. On 26 September 2026 the calculator shows this pregnancy at 29 weeks 4 days: 207 days since 3 March, in the third trimester, 73.9% of the way to 280 days, with 73 days to go. A reading of 29 weeks 4 days means the 30th week is under way. With the 35-day cycle the calculator counts from 10 March, 7 days after the period started, so the same day reads 28 weeks 4 days with 80 days to go.

How often babies arrive on the due date

Few do. Bergsjø and colleagues analysed 427,581 singleton births in Sweden from 1976 to 1980. Among women with reliable period dates, the median time from the last period to a vaginal birth was 282 days, two days past the due date, with a standard deviation of about 13 days, and 10% of those women gave birth after day 294, when post-term begins. If births followed a normal curve with that spread, about 3% would fall on any single day at its peak.

Dating method changes the odds of arriving close to the date. In 34,249 pregnancies from the East Midlands, birth came within 7 days of the due date in 49.5% of cases when the period date was used and 55.2% when ultrasound was used; within 10 days, 64.1% and 70.3%.

First pregnancies run longer. Smith (2001) studied 1,514 women whose period date and early scan agreed to within a day, looking at births that were not planned inductions or caesareans: the median was 284 days for first-time mothers and 282 days for others.

Mistakes that shift the date

• Using the day of a positive test. The count starts on the first day of the last period, about two weeks before conception in a 28-day cycle, not on the day a test turned positive.
• Adjusting for an irregular cycle. The cycle-length field assumes the extra days all fall before ovulation. When cycle length varies from month to month there is no single right number to enter, and ACOG advises dating by an early scan when the period date is uncertain.
• Entering the date of sex as the conception date. In Wilcox and colleagues' study of 221 women, pregnancies came from intercourse up to five days before ovulation, so conception can follow sex by several days.

The calculator refuses a start date after today, and any date that would put the pregnancy past 44 weeks, with a prompt to check the year. For cycle tracking before pregnancy, the ovulation calculator estimates fertile days from the same cycle-length input.