A full-term pregnancy lasts about 40 weeks, divided into three trimesters of roughly 13 to 14 weeks each. The exact boundaries shift slightly depending on the source, but the standard medical breakdown is straightforward: the first trimester covers weeks 1 through 13, the second trimester runs from week 14 through week 27, and the third trimester spans week 28 through week 40.
First Trimester: Weeks 1 Through 13
The first trimester begins on the first day of your last menstrual period, not at conception. That means you’re technically “pregnant” for about two weeks before fertilization even happens. The American College of Obstetricians and Gynecologists defines this trimester as lasting through 13 weeks and 6 days.
This is when the most dramatic development takes place. After fertilization in the fallopian tube, the fertilized egg divides into a cluster of cells, travels to the uterus, and implants in the lining. For the first eight weeks after fertilization, the developing baby is called an embryo. During this stretch, the placenta forms, major organs begin to take shape, and the nervous system starts developing. By the end of the first trimester, all major organ systems are in place, though they’ll continue maturing for months.
Most pregnancy symptoms, like nausea, fatigue, and breast tenderness, are at their peak during these weeks. The risk of miscarriage also drops significantly once you pass the first trimester.
Second Trimester: Weeks 14 Through 27
The second trimester lasts 14 weeks. For many people, this is the most comfortable stretch of pregnancy. First-trimester nausea typically fades, energy levels improve, and the belly becomes visibly rounder.
The major milestone of this trimester is the anatomy scan, an ultrasound performed between weeks 18 and 22 that checks on fetal development in detail. This is the appointment where many parents learn the sex of the baby, and it’s also when the care team evaluates the baby’s organs, limbs, and growth. Most people begin feeling fetal movement (sometimes called “quickening”) somewhere between weeks 16 and 25, though first-time parents often notice it later in that range.
Third Trimester: Weeks 28 Through 40
The third trimester is the longest stretch, covering 13 weeks. It’s defined by rapid weight gain for the baby and increasing physical demands on you. At week 28, the baby weighs about 2.25 pounds. By week 32, that doubles to roughly 3.75 pounds. At week 38, the average baby weighs around 6.5 pounds, and by week 40, about 7.5 pounds, though individual variation is wide.
Week 31 marks a turning point: most major development is finished, and the remaining weeks are primarily about gaining weight and maturing. Lung development is one of the last systems to complete. By week 28, the baby’s nervous system can trigger breathing movements visible on ultrasound, but the lungs continue refining their ability to function independently well into the final weeks.
Not all deliveries at “term” are the same. The medical community breaks it down further: 37 to 38 weeks is considered early term, 39 to 40 weeks is full term, 41 weeks is late term, and 42 weeks or beyond is post-term. Babies born at full term (39 weeks or later) generally have better outcomes than those born even a week or two earlier, which is why most providers avoid elective induction before 39 weeks.
Why Sources Sometimes Disagree
You may notice that some apps or websites say the second trimester starts at week 12, while others say week 14. The confusion comes from how the math shakes out. A 40-week pregnancy doesn’t divide evenly into three parts. ACOG defines the first trimester as ending at 13 weeks and 6 days, which effectively means the second trimester begins at week 14. But some providers and pregnancy apps round differently, placing the boundary at the start of week 13. The difference is clinically insignificant. If your provider tells you you’ve entered a new trimester and your app disagrees by a week, neither is wrong.
The “Fourth Trimester”
You may also hear about a fourth trimester. This isn’t a stage of pregnancy but rather the first 12 weeks after delivery. The American Academy of Family Physicians uses this term to describe the postpartum period, recognizing it as a critical adjustment window for both the parent and the newborn. During these 12 weeks, the body recovers from birth, hormones shift dramatically, and the baby adapts to life outside the womb. Thinking of it as a trimester helps frame this period as one that deserves the same level of attention and care as pregnancy itself.