How to use the pregnancy due date calculator
- Choose what the date you know is: the first day of the last period, or a conception date.
- Enter that date. For a last period, also enter your usual cycle length in days.
- Pick the date on which you want to see the stage. It starts as today.
- Read the estimated due date, the weeks and days on your chosen date and the key dates.
Formula
start of count = first day of last period + (cycle length - 28 days), or conception date - 14 days
due date = start of count + 280 days (40 weeks)
stage = days since the start of count, as whole weeks and days
A conception date is counted 266 days (38 weeks) ahead, which equals 280 days from a start 14 days earlier. A cycle longer or shorter than 28 days moves the start of the count by the difference.
A last period starting March 1, 2026
With a 28 day cycle the count starts on Sunday, March 1, 2026 and the estimated due date is 280 days later, Sunday, December 6, 2026. Week 37 begins on Sunday, November 15, 2026 and week 42 begins on Sunday, December 20, 2026. On October 10, 2026 the count stands at 31 weeks and 6 days.
A conception date of March 15, 2026 gives the same due date, December 6, 2026, since 14 days after March 1 plus 266 days is 280 days from March 1. A 35 day cycle would move the due date a week later, to December 13.
A date estimate and nothing more
This page does date arithmetic. It does not look at health, risk or what is normal, and no calculation can know when a particular baby will arrive. Few arrive on the estimated date. Many arrive in the weeks before or after it.
A clinician or midwife sets the due date that counts, often using an early ultrasound, which can differ from the date counted from a last period. Follow the dates you are given.
How the count works
The 40 weeks are counted from the first day of the last period, about two weeks before conception for a 28 day cycle. That is why a conception date is only 38 weeks, or 266 days, before the due date.
The weeks and days shown are simply the days since the start of the count. For other date questions, use the Date Calculator or find the week of the year with the Week Number Calculator.
Assumptions and limits
- The estimate counts 280 days from a 28 day cycle start. Other cycle lengths shift it by the difference.
- Dates are calendar dates without time zones.
- No medical conditions, multiple births, treatments or scan results are considered.
- Nothing you type is stored or sent anywhere.
- Dates from 1990 to 2090 are accepted.
Frequently asked questions
Is the due date the day the baby will be born?
No. It is a standard estimate, and most babies arrive on other days. It is mainly used to plan checkups and to talk about the stage of a pregnancy.
Why does cycle length matter?
The standard count assumes ovulation about 14 days after the period starts in a 28 day cycle. A longer or shorter cycle moves ovulation, so the count is shifted by the difference.
Which date should I use if my clinician gave me one?
Use the date from your clinician. This page is only a way to see where a date comes from, and it should not replace the date that care is planned around.
Can I use a date from IVF?
This page does not model assisted conception. Your clinic will give a due date based on the transfer, so follow their dates.