Trimester Calculator

Enter your last menstrual period to find out which trimester you are in, the dates of each trimester, and which trimester key milestones fall in.

ACOG Trimester Boundaries

Source: ACOG Committee Opinion, Definition of Term Pregnancy.

What Happens in Each Trimester

First Trimester (0 – 13 6/7 weeks)

• Embryo develops into a fetus
• All major organs form by week 13
• First-trimester screening (NT scan) at 11-13 weeks
• Common symptoms: nausea, fatigue, breast tenderness

Second Trimester (14 0/7 – 27 6/7 weeks)

• Quickening (first movement) typically 16-22 weeks
• Anatomy scan at 18-22 weeks
• Energy often returns
• Common symptoms: back pain, round ligament pain, heartburn

Third Trimester (28 0/7 – birth)

• Rapid fetal growth
• Baby begins to drop (lightening) in the weeks before birth
• Common symptoms: shortness of breath, frequent urination, Braxton Hicks contractions
• Term begins at 37 0/7 weeks

Trimester Boundary Variations

Some sources use slightly different boundaries for the second trimester. For example, some define it as 13-26 weeks or 13-27 weeks. We follow ACOG, which defines the second trimester as 14 0/7 – 27 6/7 weeks.

Where sources disagree, we name the authority we follow and note the variance. This is more useful than a single number that may not match your care provider's framework.

Related Tools

• Gestational Age Calculator — full results with gestational age
• How Many Weeks Pregnant — conversational framing
• Pregnancy Milestone Dates — dated list of key milestones
• Due Date Calculator — four modes: LMP, conception, IVF, ultrasound