Pregnancy week calculator

Subtract the date of the first day of your last menstrual period (LMP) from today's date. Divide the number of days by 7 — the integer part is the completed weeks of pregnancy and the remainder is the days in the current week. For example, 77 days = 11 weeks + 0 days.

A normal pregnancy lasts about 40 weeks (280 days) counted from the first day of the last menstrual period. A birth is considered full-term between 37 and 42 weeks. A birth before 37 weeks is premature.

The 2nd trimester starts at week 13 and lasts through the end of week 26. It is generally considered the most comfortable stage — morning sickness usually subsides and the bump is not yet very large.

The 3rd trimester begins at week 27 and lasts until delivery (around week 40). During this time the foetus gains weight rapidly and prepares for life outside the womb.

The calculator uses Naegele's rule: it adds 280 days (40 weeks) to the first day of the last menstrual period. This is the estimated due date (EDD). A doctor may adjust it after an ultrasound scan.

Officially, pregnancy weeks are counted from the first day of the last menstrual period (LMP), not from the date of conception. That is why pregnancy "lasts 40 weeks" even though actual fertilisation occurred about 2 weeks later, around the time of ovulation.

Gestational age is counted from LMP and is the standard in obstetrics. Fetal age (conceptional age) is counted from fertilisation and is about 2 weeks shorter. Doctors and pregnancy calculators report gestational age by default.

No. The calculator is informational and is based solely on the entered LMP date. Accurate determination of the due date requires an ultrasound, and the course of pregnancy should be supervised by a gynaecologist or midwife.

The first dating ultrasound is recommended between weeks 11 and 14. It provides an accurate gestational age, detects multiple pregnancies, and allows preliminary assessment of chromosomal risk (nuchal translucency — NT measurement).

Yes. The calculator assumes a standard 28-day cycle with ovulation on day 14. With irregular cycles (e.g. 21- or 35-day cycles) the result may be off by a few days. In such cases, gestational age should be confirmed by ultrasound.