Pregnancy week calculator
Find out how many weeks and days pregnant you are, which trimester you are in and how long until your due date.
Pregnancy calculator: what it is and how it works
Pregnancy weeks are counted from the first day of your last menstrual period, not from conception. So when we talk about a 40-week pregnancy, the baby has actually been developing for about 38 weeks. This system, called gestational age, is used by doctors and midwives because the date of the last period is usually easier to remember than the conception date.
Pregnancy is divided into three trimesters: the first runs until week 13+6, the second until 27+6 and the third until birth. This calculator shows your current week, your trimester, the days left until your due date and a table with the dates of the main milestones.
How to use it
- Enter the first day of your last period.
- Enter your usual cycle length.
- See your week of pregnancy, trimester and key dates.
Week of pregnancy = (today − adjusted last period) ÷ 7Worked example
If your last period started on 1 August 2026, on 2 November 2026 you are 13 weeks and 2 days pregnant, in the first trimester, with 187 days to go until your due date.
Frequently asked questions
How are pregnancy weeks counted?
From the first day of your last period. Conception usually happens about two weeks later.
How long does a pregnancy last?
About 40 weeks from the last period. Birth between weeks 37 and 42 is considered full term.
What does 13+2 mean?
Thirteen complete weeks and two days. It is the usual way of expressing gestational age.
When are the main scans?
Usually around weeks 12, 20 and 32–34, although it depends on each health system.