# Pregnancy Week Calculator > Work out how many weeks pregnant you are and your estimated due date from your last period. How the calculation is done and why ultrasound is more reliable. The tool below works out how many weeks pregnant you are and your estimated due date from **the date of your last period**. This is an **estimate** — precise dating is done by ultrasound. **In short:** A pregnancy week calculation takes the first day of the last menstrual period as the starting point and works out the week of pregnancy and the estimated due date. For the due date, 280 days are added to the date of the last period. Because it assumes a 28-day cycle and ovulation on day 14, the result is an estimate. Precise dating is done by ultrasound; if there is a difference, the ultrasound result is taken as the basis. ## Calculator **First day of your last menstrual period** Day select12345678910111213141516171819202122232425262728293031 Month selectJanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember Year select20262025 ## 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](https://www.muratalpay.com.tr/en/tedaviler/gebelikte-tarama-testleri/) 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 1. 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. 2. 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. - **Doctor**Op. Dr. Murat Alpay — Specialist in Obstetrics and Gynaecology - **Address**Op. Dr. Murat Alpay Practice, Tekelioğlu Cd. No:51 B Kat:2, Filo Apt., 07160 Muratpaşa / Antalya - **Telephone**0505 351 77 88 - **Hours**Monday – Saturday 09:00 – 18:00 · Closed on Sunday [← All treatments](https://www.muratalpay.com.tr/en/tedaviler/) --- Original page: https://www.muratalpay.com.tr/en/tedaviler/gebelik-haftasi-hesaplama/ Last updated: 2026-09-29 Op. Dr. Murat Alpay — Specialist in Obstetrics and Gynaecology Op. Dr. Murat Alpay Practice, Tekelioğlu Cd. No:51 B Kat:2, Filo Apt., 07160 Muratpaşa / Antalya 0505 351 77 88 · Monday – Saturday 09:00 – 18:00 · Closed on Sunday *This information is for general guidance only and does not replace examination, diagnosis or treatment.*