Pregnancy due date calculator
Enter the first day of your last period to see your due date, the week of pregnancy you are in today and how many days are left.
How the pregnancy is dated
The 40 weeks are counted from the first day of the last period.
From the first day of one period to the first day of the next. Each day over 28 moves the due date one day later, and each day under 28 one day earlier.
Estimated due date
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Pick a date to see the due date and the week of pregnancy.
Where you are today
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- Trimester-
- Time to the due date-
- Weeks counted from-
- First trimester ends13+6-
- Term begins37+0-
- Due date40+0-
- Post-term begins42+0-
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Only an ultrasound dates a pregnancy exactly, and a doctor or midwife sets the due date.
This table shows the day each week of pregnancy begins.
Pick a date in the calculator above and this table shows the day each week of pregnancy begins.
A week begins on the day the pregnancy reaches that many full weeks, so week 12 begins the day you are 12 weeks and 0 days along.
Add 280 days to the first day of the last period and you have the due date.
- Last period. Add 280 days (40 weeks) to the first day of the period, then add the days your cycle is over 28 or take off the days it is under (NHS, ACOG).
- Conception. Add 266 days (38 weeks) to the day of conception, because the 40 weeks are counted from a last period that began about two weeks before it (ACOG).
- IVF transfer. Add 261 days to the day a 5-day embryo was transferred and 263 days to the day a 3-day embryo was transferred, which is 266 days less the age of the embryo (ACOG).
For example, a period that began on 12 January 2027 with a cycle of 30 days: 12 January 2027 plus 280 days is 19 October 2027. The cycle is 2 days longer than 28, so the due date is 21 October 2027.
2027-01-12 + 280 = 2027-10-19, + 2 = 2027-10-21A pregnancy is counted in weeks and days, and these are the points that matter.
| Point | Week and day | Days from the start |
|---|---|---|
| The start, the first day of the last period | 0+0 | 0 |
| The first trimester ends | 13+6 | 97 |
| The second trimester ends | 27+6 | 195 |
| Term begins | 37+0 | 259 |
| The due date | 40+0 | 280 |
| Post-term begins | 42+0 | 294 |
Doctors write 12 weeks and 3 days as 12+3. The trimesters follow ACOG: the first ends at 13 weeks 6 days and the second at 27 weeks 6 days. The NHS counts a pregnancy as normally lasting from 37 to 42 weeks.
These three pages are the ones parents-to-be open next.
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People ask these questions about a due date.
How accurate is a due date?+
Only about 1 in 20 babies is born on the due date, and the NHS says a pregnancy normally lasts from 37 to 42 weeks. A due date from the last period assumes a 28-day cycle with ovulation on day 14, so an ultrasound in the first trimester measures the pregnancy more exactly.
Why are the weeks counted from the last period and not from conception?+
The day of conception is rarely known, and the first day of the last period usually is. Conception comes about two weeks after that day, so 40 weeks from the last period is 38 weeks, or 266 days, from conception.
What does full term mean?+
A birth from 37 weeks up to 42 weeks is a birth at term, and this page marks it from 37 weeks 0 days. ACOG divides it further: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and post-term is 42 weeks 0 days and over.
Which weeks are the first, second and third trimester?+
The first trimester runs from the first day of the last period to 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until the birth.
How does the length of the cycle change the due date?+
The rule assumes a 28-day cycle, with ovulation on day 14. A cycle one day longer puts ovulation one day later, so the due date moves one day later, and a cycle one day shorter moves it one day earlier. A 35-day cycle adds 7 days, and a 24-day cycle takes off 4.
How is the due date worked out after IVF?+
The age of the embryo and the day it was transferred set it. The due date is 261 days after the transfer of a 5-day embryo and 263 days after the transfer of a 3-day embryo, which is 266 days less the age of the embryo.