One trimester of pregnancy lasts roughly 13 to 14 weeks, depending on which trimester you’re counting. A full pregnancy spans about 40 weeks, and those weeks are divided into three trimesters of slightly unequal length. The first trimester covers weeks 1 through 13, the second runs from week 14 through week 27, and the third stretches from week 28 through week 40.
How Each Trimester Breaks Down
The American College of Obstetricians and Gynecologists (ACOG) defines the three trimesters with precise cutoffs:
- First trimester: First day of your last menstrual period through 13 weeks and 6 days (about 14 weeks total)
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days (exactly 14 weeks)
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days (about 13 weeks)
So while people often say each trimester is “about three months,” the math doesn’t divide evenly into calendar months. Pregnancy lasts 280 days, or 40 weeks, which works out to closer to 10 lunar months (28 days each) than the 9 calendar months most people picture. Calendar months vary between 28 and 31 days, so mapping trimesters onto months always creates some awkwardness. Weeks are the more reliable unit.
Why Pregnancy Weeks Start Before Conception
One detail that trips people up: “week 1” of pregnancy doesn’t begin at conception. It begins on the first day of your last menstrual period, often called the LMP. Doctors use this date because most people know when their period started but can’t pinpoint the exact day of fertilization. The standard assumption is that conception happens around day 14 of the cycle, but ovulation timing varies widely from person to person and cycle to cycle. LMP dating can overestimate gestational age by as much as several weeks in some cases.
This is why a first-trimester ultrasound is considered the most accurate way to confirm how far along you are. If your ultrasound date and your LMP date disagree, your provider will typically go with the ultrasound measurement.
What Happens in Each Trimester
The trimester divisions aren’t arbitrary. They reflect real shifts in fetal development and in what you experience physically.
First Trimester
The first trimester is when fertilization occurs and all major organs begin to form. By the end of week 13, the basic structures of the brain, heart, lungs, and limbs are in place. This is also the period most associated with nausea, fatigue, and breast tenderness. The risk of miscarriage is highest during this stretch and drops significantly once you enter the second trimester.
Second Trimester
Weeks 14 through 27 are a period of rapid growth. The fetus gains length and weight quickly, and movements become strong enough for you to feel them, usually somewhere between weeks 16 and 22. Many people find the second trimester physically easier than the first: nausea typically fades, and the belly hasn’t grown large enough to cause the discomforts of late pregnancy. The anatomy scan ultrasound, which checks the baby’s organs and structure in detail, is usually done around weeks 18 to 22.
Third Trimester
From week 28 through delivery, the focus shifts to weight gain and organ maturation. The lungs, brain, and liver are still developing and won’t be fully ready for life outside the womb until the final weeks. This is when you’re most likely to feel shortness of breath, back pain, and difficulty sleeping as the baby takes up more space. Prenatal visits also become more frequent, shifting from monthly to every two weeks and then weekly as your due date approaches.
When “Full Term” Actually Starts
Not all deliveries at the end of the third trimester are equal. ACOG breaks the final weeks into distinct categories:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because babies born even a week or two before 39 weeks have higher rates of breathing problems and feeding difficulties compared to those who reach full term. The last few weeks of pregnancy allow the brain and lungs to complete critical final development.
The Fourth Trimester
You may also hear the phrase “fourth trimester,” which refers to the first 12 weeks after birth. It isn’t a stage of pregnancy, but it’s increasingly recognized as a distinct period of recovery and adjustment. During these weeks, parents are adapting to newborn care while the birthing parent’s body is going through major physiological changes: hormone shifts, uterine healing, and, for those who breastfeed, milk production. ACOG now recommends that postpartum care focus on maternal health throughout this period rather than relying on a single checkup at six weeks.