Due Date Calculator
Find your estimated due date and how many weeks pregnant you are. Based on the method of the NHS due date calculator.
Days from the start of one period to the start of the next. If your cycle is usually shorter or longer than 28 days, change it here.
Estimated due date
Enter the first day of your last period to see your estimated due date.
How your due date is calculated
Pregnancy is dated from the first day of your last period [4]. The NHS due date calculator asks for that date and gives an estimated due date (EDD). It also lets you adjust the cycle length, because your menstrual cycle may not be exactly 28 days [1].
This calculator adds 280 days (40 weeks) to the first day of your last period. If your cycle is not 28 days, it moves the date by the difference: later for a longer cycle, earlier for a shorter one.
Worked example
If your last period started on September 15, 2026 and your cycle is 28 days, your estimated due date is June 22, 2027. With a 35-day cycle it is June 29, 2027, and with a 21-day cycle it is June 15, 2027.
Weeks of pregnancy are counted from the same date. With a 28-day cycle, on October 25, 2026 that count is 5 weeks and 5 days.
Why your due date is a range
The NHS says pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period [1]. So the calculator also shows the dates you reach 37 weeks and 42 weeks. In the worked example above, 37 weeks falls on June 1, 2027 and 42 weeks on July 6, 2027.
If you choose a cycle length other than 28 days, the calculator moves the 37-week and 42-week dates by the same number of days as the due date.
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Your dating scan gives the due date to use
In England, the NHS offers an ultrasound scan at around 10 to 14 weeks of pregnancy, called the dating scan [2]. It is used to see how many weeks pregnant you are and to work out your due date [2].
The NHS due date calculator page says this scan "can more accurately estimate how many weeks pregnant you are" [1]. A calculator like this one gives an estimate from your period dates. The due date your midwife or doctor gives you is the one to use.
The 20-week screening scan is usually done between 18 and 21 weeks of pregnancy [5]. Your own care team may schedule scans differently.
Trimesters by week
The NHS week-by-week guide groups pregnancy weeks into 3 trimesters [3]:
| Trimester | Weeks in the NHS guide |
|---|---|
| 1st trimester | Weeks 4 to 12 [3] |
| 2nd trimester | Weeks 13 to 27 [3] |
| 3rd trimester | Weeks 28 to 41 [3] |
The NHS guide's weekly pages start at week 4, and the NHS says the 1st trimester is "up until week 12" [4]. This calculator counts from the first day of your last period, so it shows the 1st trimester up to 12 weeks and 6 days, the 2nd from 13 weeks to 27 weeks and 6 days, and the 3rd from 28 weeks.
Not sure of the date of your last period?
You need the first day of your last period to use this calculator. The NHS says: "If you do not know it or are unsure, speak to a midwife or GP" [1]. The dating scan is also used to work out how many weeks pregnant you are [2].
What this calculator can't tell you
It can't tell you whether you are pregnant. The NHS says that if you think you might be pregnant, you can find out by doing a pregnancy test [6].
It doesn't date a pregnancy from a conception date, an IVF transfer date or a scan. It uses only the first day of your last period and your cycle length.
Frequently asked questions
How is my due date calculated?
This calculator adds 280 days (40 weeks) to the first day of your last period, then moves the date by the difference between your cycle length and 28 days. Pregnancy is dated from the first day of your last period [4].
How many weeks pregnant am I?
Enter the first day of your last period above. The calculator counts the days from that date to today, adjusted for your cycle length, and shows them as weeks and days.
Can I use my conception date instead?
No. This calculator uses the first day of your last period, which is how pregnancy is dated [4]. It has no conception-date option. The NHS says you're more likely to get pregnant around the time you ovulate (release an egg), which is "usually about 14 days before the start of your next period but depends on how long your menstrual cycle is" [7].
What if my cycle isn't 28 days?
Choose your usual cycle length, from 20 to 44 days. A longer cycle moves your due date later, and a shorter cycle moves it earlier. The NHS calculator also lets you adjust the cycle length [1].
Will my baby be born on my due date?
The due date is an estimate. The NHS says pregnancy normally lasts from 37 weeks to 42 weeks from the first day of your last period [1], so the calculator shows that range as well.
Does this work for IVF pregnancies?
This calculator has no IVF option. It uses only the first day of your last period and your cycle length.
Sources and how we check this tool
Last checked against NHS: October 27, 2026
This calculator is based on the method of the NHS due date calculator [1]: the first day of your last period plus 280 days, moved by the difference between your cycle length and 28 days. Trimesters follow the NHS week-by-week guide [3][4]. Scan timings are from NHS pages about care in England [2][5].
We re-check our sources every 6 months and whenever a source updates.
- NHS. Pregnancy due date calculator. Page last reviewed: August 21, 2024. https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/
- NHS. 12-week scan. Page last reviewed: November 8, 2023. https://www.nhs.uk/pregnancy/your-pregnancy-care/12-week-scan/
- NHS Best Start in Life. Week-by-week guide to pregnancy. https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/
- NHS Best Start in Life. 4 weeks pregnant guide. https://www.nhs.uk/best-start-in-life/pregnancy/week-by-week-guide-to-pregnancy/1st-trimester/week-4/
- NHS. 20-week scan. Page last reviewed: October 1, 2024. https://www.nhs.uk/pregnancy/your-pregnancy-care/20-week-scan/
- NHS. Signs and symptoms that might mean you're pregnant. Page last reviewed: July 31, 2026. https://www.nhs.uk/pregnancy/trying-for-a-baby/signs-and-symptoms-that-might-mean-youre-pregnant/
- NHS. Trying to get pregnant. Page last reviewed: June 2, 2026. https://www.nhs.uk/pregnancy/trying-for-a-baby/trying-to-get-pregnant/
This calculator is for general information and is not medical advice. It does not diagnose any condition or confirm a pregnancy. The due date your midwife or doctor gives you is the one to use.
Source: based on the NHS due date calculator method | med.com/due-date-calculator