Pregnancy Due Date Calculator

If your last period began on January 1, 2026, your due date is Thursday, October 8, 2026

The convention is simple: the estimated due date (EDD) falls 280 days, or 40 weeks, after the first day of the last menstrual period (LMP). Because the count starts at the LMP, which is counted as 0 weeks 0 days, conception itself usually happens around week 2. This calculator also works from an ultrasound, a conception date or an IVF embryo transfer, and it shows how far along you are today, your term window and the dates of the main milestones.

Below you will find how the calculation works, how the four starting points differ, why Naegele's rule can differ from 280 days, when an ultrasound changes the date, how term is divided, common mistakes and the limits of any due date.

How the calculation works

1. Choose what to calculate from: last period, ultrasound, conception date or IVF transfer. Only the fields that apply to that choice are shown, and dates already typed under another choice are kept when you switch.
2. The calculator fixes 0 weeks 0 days. With the last period, that is the first day of the period, moved by the difference if your cycle is not 28 days. With an ultrasound, it is the scan date minus the gestational age given at the scan. With a conception date, it is 14 days before conception. With IVF, it is the transfer date minus 14 days and minus the age of the embryo in days.
3. The due date (40 weeks 0 days) is that 0 weeks 0 days plus 280 days.
4. Today's gestational age, the term window and the milestone dates all count from the same 0 weeks 0 days, so the due date always lands on exactly 40 weeks 0 days.

The American College of Obstetricians and Gynecologists (ACOG) states that, by convention, the EDD is 280 days after the first day of the LMP, and that this convention assumes a 28-day cycle with ovulation on day 14. For regular cycles that are not 28 days, the Merck Manual (professional version) gives the adjustment LMP + 280 days + (cycle length − 28 days). A 32-day cycle therefore moves the date 4 days later, and a 25-day cycle moves it 3 days earlier. Public calculators such as the one from the NHS also ask for cycle length. The dates you type here are not stored or sent to a server, and a copied link keeps its values only after the # sign, which the page reads and then removes.

How the four starting points differ

The examples describe one and the same pregnancy entered four ways, so every row ends on October 8, 2026. Conception is counted at week 2, so the Merck Manual's conception + 266 days is the 280 days minus 14. In assisted reproduction the embryo is already 3 or 5 days old at transfer, so ACOG gives 261 days for a day-5 embryo and 263 days for a day-3 embryo.

Why Naegele's rule and 280 days sometimes differ

Naegele's rule, the usual way to do it by hand, subtracts 3 months from the first day of the last period and adds 7 days (and a year). Months have different lengths, so the result can be up to 3 days later than a straight count of 280 days.

Counting back three months passes through months of different lengths. For last periods that start in 2026, March and April starts come out 2 days later, most May starts 3 days later, July and December starts 1 day later, and the other months match. In a year that contains February 29, each difference can shift by a day. This calculator adds the 280 days directly, as in the ACOG convention. If a pregnancy booklet or another tool shows a date a day or two off, this may be the reason.

When an ultrasound changes the due date

ACOG considers a first-trimester ultrasound, up to and including 13 6/7 weeks, the most accurate way to date a pregnancy, with the crown-rump length (CRL) accurate to about 5 to 7 days. If the dating from the LMP and from the scan differ by more than the amounts below, the due date is changed to the one from the scan.

If your clinic has already set a due date from an ultrasound, choose Ultrasound in the calculator and enter the scan date and the weeks and days you were given. You should get the same due date.

How term is divided

Since 2013, ACOG has recommended using these four categories instead of calling every birth from 37 weeks onward simply term. The term window in the results runs from 37 weeks 0 days to 41 weeks 6 days, and once you pass 37 weeks the calculator shows which category you are in. Trimesters follow the boundaries ACOG uses for dating: up to 13 weeks 6 days is the first, 14 weeks 0 days to 27 weeks 6 days the second, and 28 weeks 0 days onward the third.

Common mistakes

• Counting from conception as if it were week 0: gestational age counts from the first day of the last period, so conception already falls around week 2. Adding 280 days to a conception date gives a due date two weeks too late.
• Adjusting the due date for cycle length but not the weeks: with a 35-day cycle the due date moves 7 days later, and the weeks must move the same 7 days. Otherwise the due date itself would show as 41 weeks 0 days. This calculator uses one 0 weeks 0 days for both.
• Treating Naegele's rule and 280 days as the same thing: as the table shows, they can differ by up to 3 days depending on the month the period started.
• Holding on to the period date after an early ultrasound has set the date: when a first-trimester scan has dated the pregnancy, that date is the working one. Later scans that measure the baby larger or smaller do not normally change it.
• Expecting birth on the due date: in a study of 18,708 singleton births with spontaneous labour in Australia, only 5% of births occurred on the estimated date, and about 66% were within 7 days of it.

Limits

• A due date is a calculated reference date. Actual birth dates spread over several weeks.
• If your cycles are irregular or you are unsure of your last period, the date from the LMP can be well off. In that case the ultrasound gestational age is the better guide.
• Twin pregnancies and planned deliveries are managed by your care team. Follow their plan rather than this result.
• Today's date is read from your device. If the device clock is wrong, the weeks and days remaining will be wrong too.
• This calculator gives general information and does not replace medical care. For your dating, follow the due date set by your own clinician.

Checked on The rules and figures in this article were checked on 2026-10-08 against ACOG Committee Opinions 700 and 579 (reaffirmed 2025), the Merck Manual Professional Version, and Khambalia et al. (2013).

Sources

• 01ACOG, Committee Opinion No. 700: Methods for Estimating the Due Date (2017, reaffirmed 2025)LMP + 280 days, first-trimester ultrasound, CRL accuracy, redating thresholds, 261 and 263 days for IVF
• 02ACOG, Committee Opinion No. 579: Definition of Term Pregnancy (2013, reaffirmed 2025)Early term, full term, late term and postterm
• 03Merck Manual Professional Version, Evaluation of the Obstetric Patient (2024)Cycle adjustment: LMP + 280 days + (cycle length − 28 days); conception + 266 days
• 04Khambalia AZ et al., Predicting date of birth and examining the best time to date a pregnancy, Int J Gynaecol Obstet 2013;123:105-1095% of births on the estimated date, about 66% within 7 days
• 05NHS, Due date calculatorA public calculator that also asks for cycle length

Frequently asked questions

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