Calculator
First day of your last menstrual period
How is the calculation done?
The common method takes the first day of the last menstrual period as the starting point:
- Week of pregnancy: the number of days from the last period to today is divided by seven. The result is written as "which week and how many days".
- Estimated due date: 280 days (40 weeks) are added to the date of the last period. If you want to work it out by hand: go back 3 months from the date of your last period and add 7 days.
An example: if the last period was on 10 January, three months back is 10 October, and adding seven days gives an estimated due date of 17 October.
Why is this only an estimate?
This calculation assumes that the menstrual cycle is 28 days and that ovulation takes place on day 14. In fact:
- The length of the cycle varies from person to person and from month to month.
- Ovulation does not always happen on day 14.
- The date of the last period is not always remembered clearly.
Precise dating is done by ultrasound. According to the guidelines, the crown–rump length measured in the first trimester of pregnancy is the most reliable measurement for determining the week of pregnancy. If the week found by ultrasound differs from the week calculated from the last period, the ultrasound result is taken as the basis — and all subsequent care is planned according to it. For details you can see the Screening Tests in Pregnancy page.
The periods of pregnancy
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: from week 28 until the birth
The birth is not expected to take place on the estimated date; the estimated due date is not a target but the midpoint of a range.
Frequently Asked Questions
From when are the weeks of pregnancy counted?
In the common method the starting point is not the day of conception but the first day of the last menstrual period. For this reason there is in fact no pregnancy yet in the first two weeks of "pregnancy" — that is how the calculation is set up.
How do I work out the estimated due date by hand?
Go back 3 months from the date of your last period and add 7 days. An example: if the last period was on 10 January, three months back is 10 October, and adding seven days gives an estimated due date of 17 October. This gives the same result as adding 280 days to the date of the last period.
The calculator and the ultrasound gave different answers — which is right?
The ultrasound is taken as the basis. According to the guidelines, the crown–rump length measured in the first trimester is the most reliable measurement for determining the week of pregnancy. If there is a difference, the week found by ultrasound is used and all subsequent care is planned according to it.
Why is it only an estimate?
Because the calculation assumes that the menstrual cycle is 28 days and that ovulation takes place on day 14. In fact the length of the cycle varies from person to person and from month to month, ovulation does not always happen on day 14, and the date of the last period is not always remembered clearly.
Does the birth happen on the estimated date?
Generally no. The estimated due date is not a target but the midpoint of a range. The birth is not expected to take place on exactly that day.
My periods are irregular — does the calculation still apply?
With an irregular cycle, a calculation based on the date of the last period is less reliable. In that case dating by an ultrasound done early becomes particularly important; use the calculator only to get a rough idea.
Sources
- ISUOG Practice Guidelines — Updated performance of the 11–14-week ultrasound scan. The reliability of the crown–rump length in determining the week of pregnancy, and taking ultrasound dating as the basis.
- Naegele's rule — calculation of the estimated due date from the date of the last period (date of the last period + 280 days).
The information on this page is based on the national and international guidelines listed above. The page is reviewed whenever those guidelines are updated.
Practice and Contact Details
The information on this page is general. Please speak to your doctor about your own situation.
- DoctorOp. Dr. Murat Alpay — Specialist in Obstetrics and Gynaecology
- AddressOp. Dr. Murat Alpay Practice, Tekelioğlu Cd. No:51 B Kat:2, Filo Apt., 07160 Muratpaşa / Antalya
- Telephone0505 351 77 88
- HoursMonday – Saturday 09:00 – 18:00 · Closed on Sunday