How weeks of pregnancy are counted
Doctors and midwives count pregnancy from the first day of your last menstrual period (LMP), not from conception. Conception usually happens about two weeks later, so at "4 weeks pregnant" the embryo is about 2 weeks old. A full-term pregnancy is about 40 weeks, or 280 days, from the LMP.
Example: if your last period started on March 1, 2026, then on June 15, 2026 you are 106 days along: 15 weeks and 1 day, in the second trimester, with a due date of December 6, 2026.
Trimesters and milestones
- First trimester (weeks 1–13): organs form. A dating ultrasound in this window is the most accurate way to set the due date.
- Second trimester (weeks 14–27): the anatomy scan is usually around 18 to 22 weeks.
- Third trimester (week 28 to birth): 37 weeks is early term, 39 to 40 weeks full term, and 42 weeks post-term.
If an ultrasound gave you a different due date, enter that due date instead; your care provider's date is the one to go by. To find the due date from scratch, use the due date calculator.
Frequently asked questions
How many weeks pregnant am I if my last period was 8 weeks ago?
About 8 weeks pregnant. Pregnancy weeks are counted from the first day of your last period, even though conception happened around two weeks later.
How many months is 20 weeks pregnant?
About 4½ months. Twenty weeks is the halfway point of a 40-week pregnancy.
When does the second trimester start?
At 14 weeks. The third trimester starts at 28 weeks.
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