The first trimester of pregnancy lasts from week 1 through the end of week 13, so it’s about 3 months long.

Quick breakdown

  • It usually runs from the first day of your last menstrual period (week 1) up to 13 weeks and 6 days.
  • Many medical sources describe it simply as “0–13 weeks.”

If you’re pregnant or think you might be, a healthcare professional or midwife can confirm your exact gestational age and tell you which week you’re in right now.