Pregnancy Calculator: How Many Weeks Pregnant
Find how many weeks and days pregnant you are, your trimester and your estimated due date from the first day of your last period, the conception date or an IVF transfer date.
Enter a date to see your estimated due date.
About the Pregnancy Calculator
The Pregnancy Calculator answers the question "how many weeks pregnant am I?" Enter the first day of your last period, a conception date or an IVF transfer date, and it tells you your gestational age today in weeks and days, which trimester you are in, how many days remain until your estimated due date, and when each major milestone falls. It is the same count that midwives and doctors use in appointments and on scan reports.
This is an estimate, not medical advice. Only about 4 percent of babies arrive on their due date, so please confirm your dates with a midwife or doctor, especially after your first ultrasound scan.
How pregnancy weeks are counted
Gestational age
Pregnancy is counted from the first day of your last menstrual period, not from conception. Conception usually happens about two weeks later, so at the moment of conception you are already described as two weeks pregnant. A pregnancy of 87 days is written as 12 weeks 3 days, often shortened to 12+3.
If you know the conception or transfer date
Choose Conception date or IVF transfer and the calculator works backwards to the equivalent last period. For a day-5 embryo, the transfer date is treated as 2 weeks and 5 days of pregnancy, and for a day-3 embryo as 2 weeks and 3 days.
The three trimesters
- First trimester, up to 13 weeks 6 days: the baby's organs form and many people have their first scan, often between 11 and 14 weeks.
- Second trimester, 14 weeks to 27 weeks 6 days: the anatomy scan usually takes place between 18 and 22 weeks, and 24 weeks is commonly used as the point of viability.
- Third trimester, from 28 weeks: the baby grows and gains weight. From 37 weeks a baby is considered full term, and the estimated due date falls at 40 weeks.
The formula
The due date follows Naegele's rule, and your gestational age is simply the number of days since the equivalent last period:
For example, with a last period on 1 January 2026, on 29 March 2026 you are 12 weeks 3 days pregnant, in the first trimester, with 193 days to go until 8 October 2026.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device. No dates are saved, and closing the page clears them.
Related tools
Count weeks and days between any two dates with the Date Calculator, see the full due date and milestone list on the Due Date Calculator, or work out an exact age with the Age Calculator.
Frequently asked questions
How do I know how many weeks pregnant I am?
Enter the first day of your last period. The calculator counts the full weeks and extra days from that date to today, so 87 days becomes 12 weeks 3 days. This is the same gestational age your midwife or doctor uses.
Why does pregnancy count from my last period and not from conception?
The first day of a period is a date most people know, while the exact day of conception is rarely known. Counting from the period adds about two weeks before conception, which is why a full-term pregnancy is described as 40 weeks.
When does each trimester start?
The first trimester runs to the end of week 13 (13 weeks 6 days). The second trimester runs from 14 weeks to the end of week 27, and the third trimester starts at 28 weeks and lasts until birth.
What does 'full term' mean?
A baby is considered full term from 37 weeks. Birth between 37 and 42 weeks is normal, and the due date at 40 weeks sits in the middle of that range.
Can a scan change how many weeks pregnant I am?
Yes. An early ultrasound measures the baby, and if the size points to a different age than your period date, your care team may move the due date. Use the scan date they give you from then on.
Is this calculator medical advice?
No. It gives an estimate from standard rules and does not replace care from a midwife or doctor. Contact them with any question about your pregnancy or your health.