How are pregnancy weeks counted?
Pregnancy is counted from the first day of your last menstrual period, not from the day you conceived. That means the first two weeks of a 40-week pregnancy happen before conception. If you know your due date, the calculator counts back 280 days to find the same starting point.
Doctors write pregnancy age as weeks plus days, for example 13 weeks and 2 days (often written 13+2). When you are 13 weeks and 2 days pregnant, you are in your 14th week.
Pregnancy weeks by trimester
- First trimester: week 1 to 13 weeks and 6 days
- Second trimester: 14 weeks to 27 weeks and 6 days
- Third trimester: 28 weeks until birth
- Full term: 39 weeks to 40 weeks and 6 days
Which date should I use?
If you have had a dating ultrasound, use the due date your doctor gave you. It is usually more accurate than a date based on your last period, especially if your cycles are irregular.
Frequently asked questions
Why am I counted as pregnant before I conceived?
Because the count starts on the first day of your last period. Ovulation and conception usually happen about two weeks later, so at conception you are already counted as about 2 weeks pregnant.
What is the difference between 12 weeks pregnant and the 12th week?
At 12 weeks and 0 days you have finished 12 full weeks and are starting your 13th week.
How many months pregnant am I?
Doctors count pregnancy in weeks because months have different lengths. A full-term pregnancy of 40 weeks is a little over 9 calendar months.
Is my information saved?
No. The calculation runs in your browser and nothing is stored.