How is the due date calculated?

Add 280 days (40 weeks) to the first day of your last period — Naegele's rule. The shortcut version is: add one year, subtract three months, add seven days. A last period starting 1 January gives 8 October.

How many weeks pregnant am I?

Count the days since the first day of your last period and divide by seven. On 15 May, with a last period on 1 January, that is 134 days — 19 weeks and 1 day. Set the 'as of' date and the page does it for any day.

Why is pregnancy 40 weeks if conception is two weeks later?

Because the count starts at the last period, about two weeks before ovulation and conception. The baby develops for about 38 weeks; the extra two are a convention that lets the count start on a date people know.

Can I calculate my due date from the conception date?

Yes: add 266 days (38 weeks). Conception on 15 January gives 8 October, the same as a last period on 1 January in a 28-day cycle. Choose 'Conception date' above.

How do I calculate an IVF due date?

From the transfer date and the embryo's age, as ACOG recommends: 261 days after a day-5 (blastocyst) transfer, 263 days after a day-3 transfer. Both work out to the date a natural pregnancy would have with the same embryo age.

How accurate is a due date?

It is the centre of a range. Full term covers 39 weeks to 40 weeks 6 days, and a first-trimester ultrasound measuring the embryo is the most accurate way to set the date; dates from the last period alone carry more uncertainty, especially with irregular cycles.

Pregnancy Due Date Calculator (LMP, Conception or IVF)

Due date from your last period, conception date or IVF transfer, with how far along you are, the trimesters and the term dates week by week.

Inputs

Calculate from

Date

Usual cycle length(days)

21 days45 days

Show progress as of

Include CalculateEveryday mark

Estimated due date

Thursday, October 8, 2026

Gestational age is counted from the first day of the last period, about two weeks before conception

Milestone	Gestational age	Date
Second trimester begins	14w 0d	Thursday, April 9, 2026
Mid-pregnancy anatomy scan, usually	18–22 weeks	Thursday, May 7, 2026 – Thursday, June 4, 2026
Third trimester begins	28w 0d	Thursday, July 16, 2026
Early term begins	37w 0d	Thursday, September 17, 2026
Full term begins	39w 0d	Thursday, October 1, 2026

Rule: LMP + 280 days (Naegele's rule)
Gestational age on Friday, May 15, 2026: 19 weeks 1 day — second trimester
Days to the due date: 146 days
Estimated conception: Thursday, January 15, 2026
Full term (39w0d – 40w6d): Thursday, October 1, 2026 – Wednesday, October 14, 2026

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What each Pregnancy Due Date input means

Calculate from: Last period is the usual route. Use conception only if you know the date, for example from ovulation tracking; for IVF, choose the transfer type your clinic used.
Date: For the last-period method, the first day of bleeding, not the last. For the other methods, the conception or transfer date.
Usual cycle length: Days from the first day of one period to the first day of the next. 28 is the assumption in Naegele's rule; the date shifts one day for each day your cycle differs.
Show progress as of: The day to show progress for — today by default. Change it to see how far along you will be on a particular date.

Pregnancy Due Date formula

Due date = first day of last period + 280 days (+ cycle length − 28) · conception + 266 days · day-5 embryo transfer + 261 days · day-3 transfer + 263 days

Pregnancy is dated from the first day of the last menstrual period, not from conception, because that day is usually known and conception is not. A pregnancy is counted as 40 weeks — 280 days — from that date, which is Naegele's rule, and the estimate from it is the starting point in ACOG's Committee Opinion 700 on estimating the due date. The rule assumes a 28-day cycle with ovulation around day 14, so a longer cycle moves the date later by the extra days and a shorter one earlier. If you know the conception date, the due date is 266 days (38 weeks) after it; after IVF, ACOG dates the pregnancy from the embryo's age at transfer, which gives 261 days after a day-5 transfer and 263 after a day-3 one. Gestational age is counted the same way, from the dating start, so at conception a pregnancy is already 'two weeks'. ACOG says a first-trimester ultrasound is the most accurate way to confirm or revise the date, and that the ultrasound date wins when the two disagree by more than set limits. The table gives the dates for each trimester and the term categories.

Pregnancy Due Date worked example

Last period started on 1 January 2026, with a 28-day cycle. 280 days later is Thursday, October 8, 2026. On 15 May 2026 the pregnancy is 19 weeks 1 day, in the second trimester, with 146 days to go. Conception most likely happened around 15 January, and 'full term' — 39 weeks to 40 weeks 6 days — runs from 1 to 14 October.

More Pregnancy Due Date examples

With a 32-day cycle, ovulation comes about four days later than in a 28-day one, so the due date moves four days too: 10 March + 280 + 4 days is Saturday, December 19, 2026. On 1 July that pregnancy is 15 weeks 4 days.

A day-5 embryo transferred on 20 April 2026: the due date is 261 days later, Wednesday, January 6, 2027. On 1 June the pregnancy is 8 weeks 5 days by the usual count.

Where the Pregnancy Due Date calculator can give the wrong answer

A due date is an estimate. Few babies arrive on the day itself; most are born within the two weeks either side, which is why ACOG defines term as a range.
Irregular cycles, recent hormonal contraception or breastfeeding make the last-period date a weaker guide. ACOG considers a pregnancy without an ultrasound confirming the date before 22 weeks to be suboptimally dated.
If an early ultrasound gives a different date, the clinician's revised date replaces this one; ACOG sets how big a difference triggers the change at each stage.
The cycle-length adjustment assumes the luteal phase (ovulation to period) is the usual 14 days. If ovulation was tracked, the conception method is more direct.
Gestational age on the as-of date changes daily. Weeks and days are counted from the dating start, so '19 weeks 1 day' means 134 days after it.

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Pregnancy Due Date reference table

Term categories and trimesters
Stage	Gestational age	Source
First trimester	Up to 13 weeks 6 days	ACOG
Second trimester	14w 0d to 27w 6d	ACOG
Third trimester	28w 0d to birth	ACOG
Early term	37w 0d to 38w 6d	ACOG Committee Opinion 579
Full term	39w 0d to 40w 6d	ACOG Committee Opinion 579
Late term	41w 0d to 41w 6d	ACOG Committee Opinion 579
Post-term	42w 0d and beyond	ACOG Committee Opinion 579

Source: American College of Obstetricians and Gynecologists, Committee Opinions 579 and 700

Pregnancy Due Date FAQ

: Add 280 days (40 weeks) to the first day of your last period — Naegele's rule. The shortcut version is: add one year, subtract three months, add seven days. A last period starting 1 January gives 8 October.
: Count the days since the first day of your last period and divide by seven. On 15 May, with a last period on 1 January, that is 134 days — 19 weeks and 1 day. Set the 'as of' date and the page does it for any day.
: Because the count starts at the last period, about two weeks before ovulation and conception. The baby develops for about 38 weeks; the extra two are a convention that lets the count start on a date people know.
: Yes: add 266 days (38 weeks). Conception on 15 January gives 8 October, the same as a last period on 1 January in a 28-day cycle. Choose 'Conception date' above.
: From the transfer date and the embryo's age, as ACOG recommends: 261 days after a day-5 (blastocyst) transfer, 263 days after a day-3 transfer. Both work out to the date a natural pregnancy would have with the same embryo age.
: It is the centre of a range. Full term covers 39 weeks to 40 weeks 6 days, and a first-trimester ultrasound measuring the embryo is the most accurate way to set the date; dates from the last period alone carry more uncertainty, especially with irregular cycles.