Trimester week ranges checked against ACOG Committee Opinion No. 579.
The first trimester runs weeks 1 through 13, the second weeks 14 through 27, and the third from week 28 to delivery around week 40. The boundaries are a convention rather than a hard biological switch, but each stage really does bring a different set of developmental milestones and a different level of miscarriage risk.
The three trimesters at a glance
| Trimester | Weeks | Key milestones |
|---|---|---|
| First | 1 to 13 | Implantation, detectable heartbeat around week 6, major organ formation, highest miscarriage risk |
| Second | 14 to 27 | Fetal movement usually felt weeks 18 to 22, anatomy scan around week 20, viability threshold approaches near week 24 |
| Third | 28 to 40 | Rapid weight gain, lung maturation, head-down positioning, birth preparation |
First trimester: weeks 1 through 13
This is the highest-stakes stretch developmentally: the brain, spine, heart and limbs all begin forming in these weeks. Nausea, fatigue and breast tenderness are common, driven by rising hCG and progesterone. Most pregnancy losses that occur happen during the first trimester, which is the main reason many people wait until close to its end, around week 12 or 13, before sharing the news widely.
Second trimester: weeks 14 through 27
Many people find this stretch the most comfortable of the three. Nausea typically eases and energy returns. Fetal movement, often called quickening, is usually felt between weeks 18 and 22. A mid-pregnancy anatomy scan around week 20 checks structural development in detail and can often show the baby's sex. By week 24, near the boundary some sources draw between the second and third trimesters, survival outside the womb becomes possible with intensive neonatal care, though outcomes keep improving with every additional week from there.
Third trimester: week 28 to birth
Growth accelerates through this stretch: the baby gains most of its eventual birth weight, and the lungs continue maturing toward independent breathing. Sleep tends to become harder as the pregnancy advances, and Braxton Hicks practice contractions may start. Most babies settle into a head-down position ahead of birth. This trimester runs into the term categories described in the full-term guide: full term is 39 to 40 weeks under ACOG's definitions, though normal births range from 37 to 42 weeks.
Where the boundaries come from
The 1-13, 14-27, 28-40 split is the version used most often in U.S. obstetric care, though some sources set the first/second boundary at week 13 instead of 14. It is a convention for organizing prenatal visits and screening schedules around, not a line where development suddenly changes character. Use the Pregnancy Calculator to find your current trimester from your own dates.
Find your trimester
See your current week, trimester and days remaining from your own dates.