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Pregnancy Due Date Calculator: From Last Period, Conception or IVF, with Trimester Dates

Pick the first day of your last period and the tool gives your estimated due date by Naegele's rule, how many weeks and days pregnant you are on any date you choose, and the dates the second and third trimesters begin.

Maintained by Hirantha Sandaruwan Lakshan. Formulas checked 13 October 2026 against ACOG Committee Opinion 700, Methods for Estimating the Due Date, May 2017 and 4 more sources. General information, not medical advice.

About this tool

You can adjust for your usual cycle length, or work from a conception date or an IVF embryo transfer date instead. Only about 5 per cent of babies arrive on the due date, and a clinician confirms dating, usually by ultrasound. This is general information, not medical advice. Nothing you type leaves your device.

How it works

By convention the estimated due date is 280 days (40 weeks) after the first day of the last menstrual period (ACOG Committee Opinion 700). Naegele's rule reaches nearly the same date by counting back three calendar months from that day and adding one year and seven days. The rule assumes a regular 28-day cycle with ovulation on day 14. This tool counts exactly 280 days, as ACOG does. The calendar version can land a day away because months differ in length: for a period that began on 9 September 2023, University Hospitals' worked example of the calendar rule gives 16 June 2024, while 280 days gives 15 June 2024.

Gestational age is counted from the first day of the last period, so by this count you are already about two weeks pregnant at conception. The tool turns every method into an estimated due date and then counts the gestational age on your chosen date as 280 days minus the days left to the due date, so all four methods are counted the same way. It shows completed weeks and days, and also the pregnancy week people usually quote, which is the completed weeks plus one.

A cycle longer or shorter than 28 days moves ovulation. A 1994 study of 750 women at Cairo University found cycle length was the most significant factor in the length of pregnancy and proposed a modified Naegele's rule that adds or subtracts the days a cycle runs over or under 28. This tool applies that adjustment when you enter a cycle length: 35 days adds 7 days, 24 days takes off 4. It is a common convention, not part of ACOG's guidance, which uses 280 days whatever the cycle length. Leave the cycle at 28 for the standard date.

From conception the due date is 266 days (38 weeks) later, which is the 280 days minus the 14 days from the period to ovulation. For IVF, ACOG says the due date should be assigned from the age of the embryo and the date of transfer: 261 days after the transfer of a day-5 embryo and 263 days after a day-3 embryo. If your clinic has given you a date, use theirs.

The trimester ranges follow ACOG Committee Opinion 700: the first trimester runs up to and including 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days and the third from 28 weeks 0 days. The term labels follow ACOG Committee Opinion 579: early term 37 0/7 to 38 6/7 weeks, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7 and post-term from 42 0/7. The tool gives the date each one begins.

A due date is a middle point, not a deadline. The Victorian Government's Better Health Channel says only about 5 per cent of women give birth on the exact date they are due and that most babies arrive between 37 and 41 weeks. ACOG says about half of women accurately recall their last period, and that an ultrasound measurement of the embryo or fetus in the first trimester (up to 13 6/7 weeks) is the most accurate way to establish gestational age. A clinician changes the due date to match the scan only when the two differ by more than a set amount, shown in the table below.

Worked examples

Last period began 1 August 2026, 28-day cycle

1 August 2026 plus 280 days is Saturday 8 May 2027. Naegele's calendar rule gives the same date: 1 August minus 3 months is 1 May, plus one year is 1 May 2027, plus 7 days is 8 May 2027. The second trimester begins on 7 November 2026 (14 weeks) and the third on 13 February 2027 (28 weeks). On 13 October 2026 you would be 10 weeks and 3 days pregnant, in week 11 and the first trimester.

The same period with a 35-day or a 24-day cycle

With a 35-day cycle the due date moves 7 days later, to 287 days after 1 August 2026, which is 15 May 2027. With a 24-day cycle it moves 4 days earlier, to 276 days after, which is 4 May 2027. These adjustments are a convention, so a clinician's ultrasound date takes priority.

IVF and conception dates

A day-5 embryo transferred on 1 September 2026 gives 1 September + 261 days = 20 May 2027. A day-3 embryo transferred the same day gives + 263 days = 22 May 2027. A conception date of 15 August 2026 gives + 266 days = 8 May 2027, the same date as a 1 August last period with a 28-day cycle.

When ultrasound dating replaces last-period dating (ACOG Committee Opinion 700, Table 1)

Gestational age by last periodMeasurementDifference that supports changing the due date
Up to 8 6/7 weeksCrown-rump lengthMore than 5 days
9 0/7 to 13 6/7 weeksCrown-rump lengthMore than 7 days
14 0/7 to 15 6/7 weeksFetal measurements (BPD, HC, AC, FL)More than 7 days
16 0/7 to 21 6/7 weeksFetal measurements (BPD, HC, AC, FL)More than 10 days
22 0/7 to 27 6/7 weeksFetal measurements (BPD, HC, AC, FL)More than 14 days
28 0/7 weeks and beyondFetal measurements (BPD, HC, AC, FL)More than 21 days

Frequently asked questions

How is the due date calculated?

By convention it is 280 days (40 weeks) after the first day of your last period. Naegele's rule is the calendar way of doing the same sum: add one year, subtract three months and add seven days. Both assume a regular 28-day cycle. If you know the date of conception it is 266 days later, and for IVF it is 261 days after a day-5 embryo transfer or 263 days after a day-3 transfer.

How accurate is a due date?

It is an estimate. Only about 5 per cent of women give birth on the exact date they are due, and most babies arrive between 37 and 41 weeks. ACOG notes that about half of women accurately recall their last period, and that a first-trimester ultrasound is the most accurate way to set the date. Ask your doctor or midwife which date they are using.

How many weeks pregnant am I?

Pick the first day of your last period and the tool shows your gestational age in completed weeks and days on the date you choose, counted from that day. People often quote the pregnancy week, which is the completed weeks plus one, so 10 weeks and 3 days is in week 11. Both are shown.

Does my cycle length change the due date?

Under Naegele's rule as ACOG states it, no: it is 280 days from the last period. A longer or shorter cycle moves ovulation, and a 1994 Cairo University study proposed adding or subtracting the days a cycle runs over or under 28. This tool applies that convention when you enter a cycle length and says so. Leave it at 28 for the standard date.

What if I do not know the date of my last period?

ACOG says that if you are unsure of your last period, dating should be based on ultrasound, ideally at or before 13 weeks 6 days. About half of women recall the date accurately, so this is common. Ask your doctor or midwife about a dating scan, and use the date they give you instead of this calculator's.

How is an IVF due date worked out?

ACOG says the due date for an IVF pregnancy should be assigned using the age of the embryo and the transfer date. Its examples are 261 days after the transfer for a day-5 embryo and 263 days for a day-3 embryo. Pick the matching option and the date of transfer. Your clinic's date takes priority over any calculator.

When does the second trimester start?

On this calculator, which follows the ACOG week ranges, the first trimester runs up to and including 13 weeks 6 days, the second trimester starts at 14 weeks 0 days and the third at 28 weeks 0 days. The tool gives the calendar date for each start from your due date.

When is a baby considered full term?

ACOG's 2013 definitions are early term 37 0/7 to 38 6/7 weeks, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7 and post-term 42 0/7 and beyond. The tool shows the date each begins. Your doctor or midwife will plan your care around your own situation.

Updated Oct 13, 2026