A trimester is one of three roughly equal stages that divide the 40 weeks of pregnancy into periods of about three months each. The first trimester runs from conception through week 12, the second from week 13 through week 27, and the third from week 28 through week 40. Each trimester brings distinct changes for both the developing baby and the pregnant person, from organ formation in the earliest weeks to rapid weight gain near delivery.
First Trimester: Weeks 1 Through 12
The first trimester is when the most foundational development happens. By the end of week 12, all major organ systems have begun forming. Cartilage for the limbs, hands, and feet starts to take shape, though it won’t harden into bone until later. The heart begins beating, the neural tube (which becomes the brain and spinal cord) closes, and facial features start to emerge. Despite how much is happening inside, the baby is still only about two to three inches long by the end of this stage.
For the pregnant person, the first trimester is often the most physically uncomfortable relative to what’s visible on the outside. Rising progesterone levels cause fatigue that can feel overwhelming, especially in the early weeks. Nausea (sometimes called morning sickness, though it can strike at any hour), food aversions, bloating, constipation, headaches, and mood swings are all common. Hormonal shifts slow the digestive system and can make certain smells intolerable. Many people also experience breast tenderness and frequent urination as the uterus begins to expand.
The first trimester is also when most early prenatal screening takes place. Blood tests and an early ultrasound help confirm the pregnancy’s viability, estimate a due date, and screen for chromosomal conditions.
Second Trimester: Weeks 13 Through 27
The second trimester is often described as the most comfortable stretch of pregnancy. First-trimester nausea and fatigue typically ease, energy returns, and the pregnancy becomes visibly apparent. This is also when many people first feel fetal movement, an experience called quickening. It usually happens between weeks 16 and 20. If you’ve been pregnant before, you may notice those flutters closer to 16 weeks; first-time pregnancies more commonly feel movement around week 20.
The baby grows considerably during this stage. Soft, downy hair called lanugo covers the body, and the sucking reflex begins developing. By weeks 21 through 24, kicks and turns grow stronger and more noticeable. Bones continue to harden, fingerprints form, and the baby starts responding to sound. By the end of the second trimester, the baby is roughly 14 inches long and weighs about two pounds.
Several important screenings happen during this trimester. The anatomy ultrasound, typically performed around weeks 18 to 22, gives a detailed look at the baby’s organs, limbs, and growth. Glucose screening for gestational diabetes is standard between weeks 24 and 28.
Third Trimester: Weeks 28 Through 40
The third trimester is defined by rapid weight gain for the baby and increasing physical demands on the pregnant person. At week 28, the baby weighs about 2¼ pounds. By week 31, most major development is complete and the focus shifts to putting on weight quickly. The baby nearly doubles in size over the next several weeks: roughly 3¾ pounds at week 32, more than 4½ pounds at week 34, and about 6½ pounds by week 38. At week 40, the average baby weighs around 7½ pounds, though some reach 9 pounds or more.
Lung and brain maturation are the critical milestones of this period. By week 28, the central nervous system can regulate body temperature and trigger rhythmic breathing movements visible on ultrasound. Fat accumulates under the baby’s skin to help with temperature regulation after birth, and by week 36 the skin becomes noticeably smoother as that fat layer fills in.
For the pregnant person, the third trimester often brings back some first-trimester discomforts along with new ones. Fatigue returns as the body supports a much larger baby. Shortness of breath, back pain, heartburn, frequent urination, and difficulty sleeping are all common as the uterus presses against surrounding organs. Braxton Hicks contractions (irregular, painless tightening of the uterus) may become noticeable as the body prepares for labor.
When Is a Pregnancy Considered Full Term?
Not all deliveries at the end of the third trimester carry the same level of readiness. The American College of Obstetricians and Gynecologists breaks it down into specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a couple of weeks early may face more breathing difficulties and feeding challenges than those born at 39 weeks or later. The final weeks of pregnancy allow for critical lung maturation and brain growth that improve outcomes after birth.
Weight Gain Across All Three Trimesters
How much weight you gain over the full course of pregnancy depends on your pre-pregnancy body mass index. The CDC recommends the following total ranges for a single pregnancy:
- Underweight (BMI below 18.5): 28 to 40 pounds
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds
- Overweight (BMI 25 to 29.9): 15 to 25 pounds
- Obese (BMI 30 to 39.9): 11 to 20 pounds
For twin pregnancies, the recommended totals are significantly higher, ranging from 25 to 62 pounds depending on starting BMI. Most weight gain happens in the second and third trimesters, when the baby, placenta, and amniotic fluid are growing fastest. First-trimester gain is typically modest, often just a few pounds.
The Fourth Trimester: After Delivery
You may also hear the term “fourth trimester,” which refers to the 12 weeks after delivery. It’s not an official stage of pregnancy, but the concept highlights how significant the postpartum period is for both the parent and the newborn. The body is recovering from delivery while adapting to breastfeeding, sleep deprivation, and hormonal shifts.
Physical changes during this time are substantial. More than one in four women experience moderate or severe urinary incontinence in the first year after birth. Up to 10% develop postpartum depression, and a similar percentage develop thyroid problems. Constipation affects up to 17% of women in the first six weeks. Most women still weigh about 12 pounds more than their pre-pregnancy weight at the six-month mark, which is normal. The risk of blood clots is five times higher in the postpartum period than during pregnancy itself, and that elevated risk persists for the full 12 weeks after delivery.
Women who had gestational diabetes face continued risk as well: 5% to 10% will have type 2 diabetes after delivery, making follow-up blood sugar testing important in the months that follow.