Pregnancy due date calculator

Due date from conception or IVF transfer date

Add 266 days to the date of conception. ACOG dates an IVF pregnancy from the embryo's age at transfer, so a day-3 transfer on 18 July 2026 and a day-5 transfer on 20 July both mean conception on 15 July and a due date of 7 April 2027.

Counting 280 days from the first day of a last period on 1 July 2026, two weeks before that conception, gives the same date:

On 3 October 2026 that pregnancy is 13 weeks 3 days along, with 186 days to go.

Due date for a longer or shorter cycle

The 280-day count assumes ovulation on day 14 of a 28-day cycle. For regular cycles, move the due date one day later for each day over 28, or one day earlier for each day under. With a 32-day cycle, the 1 July 2026 period gives 11 April 2027.

Ovulation, and so conception, is then taken to come four days later, on 19 July, and the weeks count moves with it: on 3 October 2026 the pregnancy is 12 weeks 6 days, not 13 weeks 3 days.

To project ovulation from your cycle before pregnancy, use the ovulation calculator.

How due dates and pregnancy weeks are counted

Pregnancy is dated from the first day of the last menstrual period (LMP), about two weeks before conception. The due date is the LMP plus 280 days (40 weeks): a period that began on 3 March 2026 gives 8 December 2026. The count assumes a 28-day cycle.

Gestational age, the number of weeks pregnant, is the days since the LMP divided by 7. On 26 September 2026 that pregnancy is 207 days along, and 207 = 29 × 7 + 4, so it is 29 weeks 4 days: the 30th week is under way.

Because the count starts before conception, a pregnancy already measures 4 weeks 0 days on the day the next period would have been due in a 28-day cycle.

Naegele's rule versus the 280-day count

Naegele's rule, named after the German obstetrician Franz Naegele (1778–1851), puts birth nine months and seven days after the LMP, Lawson's 2021 review notes. By hand: add seven days, subtract three months and add one year.

ACOG and dating software count 280 days instead, and the two can disagree because months differ in length. For an LMP of 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.

The nine months from 10 March to 10 December run March to November: five 31-day months and no February, 275 days in all. With the extra week, Naegele lands 282 days after the LMP.

For an LMP of 1 January, the nine months from 8 January to 8 October include a 28-day February and total 273 days, so the methods agree. The gap for a period starting on the 1st of each month in 2026:

To check a date yourself, the add or subtract days calculator adds 280 days to any date.

Which dating method is most accurate

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

• Last period. Only about half of women recall their LMP accurately, the opinion notes. In one trial it cites, 40% of women randomized to a first-trimester scan had their due date changed because scan and LMP differed by more than 5 days.
• Conception date. The due date is 266 days after conception, the same day as 280 days from an LMP two weeks earlier. In this count, conception means the day of ovulation and fertilization.
• IVF. The embryo's age fixes the conception date, so the due date is 263 days after a day-3 transfer and 261 days after a day-5 transfer.
• Ultrasound. A first-trimester scan measuring crown–rump length, up to 13 weeks 6 days, is accurate to within 5 to 7 days.

Later scans are less precise, so ACOG moves the due date only when a scan and the LMP disagree by more than a margin that widens with gestational age:

Once the due date is set from the LMP or the first reliable scan, ACOG advises changing it only in rare circumstances.

Trimester and milestone dates

The first trimester runs to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days and the third from 28 weeks 0 days (ACOG). Each milestone date is the LMP plus its gestational age in days.

For an LMP of 3 March 2026 and a 28-day cycle:

The scan window follows ACOG's patient guidance that the standard ultrasound exam is usually done at 18 to 22 weeks. Early, full, late and post-term follow the definitions in ACOG Committee Opinion 579.

A conception date of 17 March 2026, or a day-5 IVF transfer on 22 March 2026, gives the same dates.

How often babies arrive on the due date

Few do. Bergsjø and colleagues analyzed 427,581 singleton births in Sweden from 1976 to 1980. Among women with reliable LMP dates, the median time from LMP to vaginal birth was 282 days, two days past the due date.

The spread was wide: a standard deviation of about 13 days, with 10% of those women giving birth after day 294, when post-term begins. On a normal curve with that spread, even the peak day would hold only about 3% of births.

Dating method changes the odds. In 34,249 pregnancies from the East Midlands, birth came within 7 days of the due date in 49.5% of cases dated by LMP and 55.2% dated by ultrasound; within 10 days, 64.1% and 70.3%.

First pregnancies run longer. Smith (2001) studied 1,514 women whose LMP and early scan agreed to within a day. Excluding planned inductions and cesareans, the median was 284 days for first-time mothers and 282 days for others.

Mistakes that shift the date

• Starting from the last day of bleeding. The count begins on the first day of the period. Counting from the last day of a five-day period puts the due date four days late.
• Treating the date of sex as conception. In Wilcox and colleagues' study of 221 women, pregnancies followed intercourse up to five days before ovulation, so conception can come several days after sex.
• Guessing an uncertain LMP. When you cannot pin down the date, or your cycles vary, ACOG advises dating the pregnancy by an early scan.

To work back from a due date to the likely conception window, use the conception calculator. The pregnancy weight gain calculator gives the IOM 2009 gain ranges by pre-pregnancy BMI.