Pregnancy Week Calculator
See how far along you are in weeks and trimester, from your last period or a known due date.
Pregnancy weeks are counted from the first day of your last period, so "week 10" means about eight weeks since conception. Weeks 1–13 are the first trimester, 14–27 the second, and 28–40 the third.
How it works
Pregnancy is measured in weeks from the first day of your last menstrual period, not from conception, and a full term runs about 40 weeks. Enter that date and the tool counts forward to today to show your current week and day.
The three trimesters break down as weeks 1 to 13, 14 to 27, and 28 to 40. Knowing which one you are in is a quick way to make sense of appointments, tests, and what is happening week to week.
If you only know your due date, the tool runs the math in reverse — a due date is simply the last period plus 280 days — and still lands on the right week.
Frequently asked questions
What is the difference between gestational age and fetal age?
Gestational age counts from your last period and is the number your doctor uses. Fetal age counts from conception and is about two weeks less.
When does each trimester start and end?
The first trimester is weeks 1 to 13, the second is 14 to 27, and the third is 28 until birth, usually around week 40.
Can this replace an ultrasound dating scan?
No. An early ultrasound measures the baby directly and is more accurate, especially if your cycle is irregular. This is a quick estimate between visits.