Pregnancy Week Calculator

How to use the pregnancy week calculator

1. In Calculate from, choose Last menstrual period, Conception date, IVF transfer, day-3 embryo, IVF transfer, day-5 embryo or Known due date (for example the one from your dating scan).
2. Enter the matching date. For the last-period method, adjust the Average cycle length (days) if your cycles are not 28 days long.
3. On is set to today. Change it to see how far along you will be on another date, such as an appointment or a trip; Today resets it.
4. Read You are: weeks and days, with the obstetric notation (for example 8+3) and the week in progress. Below you see the Trimester, the time Since conception (embryo/fetal age), the Due date, the time Until the due date, and a bar showing Progress towards 40 weeks.
5. The steps show the count. Clear empties the date.

Everything is calculated in your browser; the dates stay on your device.

How pregnancy weeks are counted

Doctors and midwives count gestational age from the first day of the last menstrual period (LMP), not from conception. At the moment of conception, a pregnancy is therefore already “2 weeks” along. A full-length pregnancy is 280 days, or 40+0 weeks.

Worked example: with an LMP of 1 January 2026, on 1 March 2026 you are 59 days along, which is 8 weeks and 3 days (8+3). You are in the 9th week of pregnancy, because the first week runs from 0+0 to 0+6. The due date is 8 October 2026, 221 days later.

When you start from another date, the tool converts it to the same timeline:

• the conception date is counted as day 14 (2+0);
• a day-3 embryo transfer is counted as day 17, and a day-5 (blastocyst) transfer as day 19;
• a known due date is counted as day 280 (40+0);
• with the last-period method, a cycle longer than 28 days moves the count later by the difference, because ovulation probably happened later.

The Since conception (embryo/fetal age) figure is simply the gestational age minus two weeks. It is the age used in embryology texts, while scans and appointments use gestational age.

Trimesters

The tool uses the common convention of the American College of Obstetricians and Gynecologists:

• first trimester: up to 13+6;
• second trimester: 14+0 to 27+6;
• third trimester: from 28+0 until birth.

Other sources sometimes place the boundaries a week earlier or later, so your pregnancy guide may differ slightly.

Limits of the calculation

Every date here is an estimate built on averages. Ovulation does not always occur on the same cycle day, and the first day of the last period is often remembered approximately. An early ultrasound measuring the embryo gives a more accurate gestational age; once you have one, choose Known due date and enter the date from your scan report. Only a small share of babies, around 4 to 5%, are born on the due date itself. For any question about your pregnancy, rely on your midwife or doctor.