Due Date Calculator

Worked example

A last period beginning 1 April 2026 with a 28-day cycle gives a due date of 6 January 2027 — 280 days later. On 17 August that pregnancy is 19 weeks and 5 days along, in the second trimester, with 142 days to go.

How to work it out yourself

1. 1.Enter the first day of your last period, not the day it ended. If you know the date of conception instead, switch the selector and the count runs 266 days from there.
2. 2.Set your usual cycle length. The standard rule assumes 28 days and puts ovulation on day 14; a 32-day cycle pushes the due date four days later.
3. 3.Read the due date and the gestational age. Weeks are counted from the last period, which is why a pregnancy is "four weeks" at about the time the first period is missed.
4. 4.Take the date to a first-trimester ultrasound. A scan before 14 weeks dates a pregnancy more accurately than any calculation from a remembered date.

The formula

1. Cycle-length adjustment1 * (28 - 28)
2. Estimated due date2026-05-17 + 1 * (280 + 28 - 28) + 0 * 266
3. Days of pregnancy so far2026-10-02 - (2027-02-21 - 280)
4. Completed weeksfloor(138 / 7)
5. Days into the current week138 - 19 * 7
6. Days remaining2027-02-21 - 2026-10-02
7. Trimester((2))
8. Confirm dating by this date(2027-02-21 - 280) + 154
9. End of the first trimester (14 weeks)(2027-02-21 - 280) + 98
10. Anatomy scan window opens (18 weeks)(2027-02-21 - 280) + 126
11. Anatomy scan window closes (22 weeks)(2027-02-21 - 280) + 154
12. Glucose screening usually offered (24 weeks)(2027-02-21 - 280) + 168
13. Third trimester begins (28 weeks)(2027-02-21 - 280) + 196
14. Full term begins (39 weeks)(2027-02-21 - 280) + 273
15. Post-term begins (42 weeks)(2027-02-21 - 280) + 294

Source: ACOG Committee Opinion 700 — Methods for Estimating the Due Date, NICHD — pregnancy dating and gestational age

Questions people actually ask

How accurate is a due date?

It is an estimate of the middle of a range, not an appointment. About 4% of babies arrive on the estimated date, and most births fall within two weeks either side. The figure exists to schedule care and to judge whether growth is on track, not to predict a day.

Why 280 days from the last period?

Naegele's rule counts 280 days — forty weeks — from the first day of the last menstrual period, which assumes a 28-day cycle with ovulation on day 14. Pregnancy itself lasts about 266 days from conception; the extra fortnight is the two weeks before you conceived, which is why the count starts before the pregnancy did.

Does a long or short cycle change the date?

Yes, and this is where a plain 280-day count goes wrong. Ovulation moves with cycle length while the luteal phase stays near 14 days, so a 35-day cycle ovulates about a week later than a 28-day one and the due date moves with it. This calculator applies that shift; most consumer tools do not offer the field at all.

When does the due date need to be confirmed by ultrasound?

ACOG treats a pregnancy that reaches 22 weeks and 0 days without an ultrasound confirming the dating as suboptimally dated, which affects how later decisions are made. That date is shown in the derivation so it can go in a calendar rather than being discovered late.

Related

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• BMI CalculatorBody mass index from height and weight in either metric or US units, with the CDC category cut-offs.
• TDEE CalculatorHow many calories you burn in a day, from the Mifflin-St Jeor equation and an activity multiplier, with cut and bulk targets beside it.
• Ovulation CalculatorOvulation day and the six-day fertile window from your last period, counted back from the next period rather than forward from day 14.
• Period CalculatorYour next six periods from the last one, with the prediction window widening the further out it goes — because cycle length varies month to month.
• Pregnancy Weight Gain CalculatorWhere your weight should be at this point in pregnancy, from the 2009 Institute of Medicine ranges for your pre-pregnancy BMI.

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