The first trimester runs from week 1 through week 13, the second trimester covers weeks 14 through 27, and the third trimester spans weeks 28 through 40. These divisions, defined by the American College of Obstetricians and Gynecologists (ACOG), mark distinct phases of fetal growth and give your care team a shared timeline for scheduling tests and tracking development.
How Pregnancy Weeks Are Counted
Pregnancy is measured from the first day of your last menstrual period (LMP), not the day of conception. Because ovulation and fertilization typically happen about two weeks after your period starts, you’re technically considered “two weeks pregnant” before conception even occurs. This can feel confusing, but it’s the universal standard doctors use. When your provider says you’re 10 weeks along, they mean 10 weeks from your last period, which is roughly 8 weeks since the embryo actually formed.
A full pregnancy lasts 40 gestational weeks from the LMP. That 40-week span is then divided into three trimesters of roughly 13 to 14 weeks each.
First Trimester: Weeks 1 Through 13
The first trimester is when fertilization happens and every major organ system begins to form. Despite being the shortest-feeling stretch for many people, it’s the most intense period of structural development.
During weeks 1 and 2, your body is preparing for ovulation. Conception occurs around week 3, when sperm and egg meet in a fallopian tube and form a single cell containing 46 chromosomes, 23 from each parent. That cell divides rapidly into a cluster of cells and travels toward the uterus to implant.
By week 5, levels of the hormone HCG rise sharply, signaling your ovaries to stop releasing eggs and ramp up estrogen and progesterone production. These hormonal shifts fuel placenta growth and are responsible for many early symptoms: nausea, breast tenderness, and fatigue. By week 6, the neural tube (which becomes the brain and spinal cord) is closing, the heart begins to form, and tiny limb buds appear. At week 7, the face is taking shape, with early nostrils and the beginnings of retinas visible.
Over the remaining weeks of the first trimester, the embryo transitions to being called a fetus (around week 10), fingers and toes separate, and external genitalia begin to differentiate. By the end of week 13, all major organs are in place, though none are fully mature.
Second Trimester: Weeks 14 Through 27
The second trimester is a period of rapid growth and refinement. Many people find this stretch the most comfortable, as first-trimester nausea often fades while the physical demands of late pregnancy haven’t yet set in.
At week 14, red blood cells start forming in the spleen and the baby’s sex may become visible on ultrasound. Bone development picks up around week 15, with the skeleton becoming clear on imaging. By week 16, the head is upright, the eyes can move slowly, and limb movements are coordinated enough to show up on ultrasound, though they’re still too small for you to feel.
That changes around week 20, the halfway point of pregnancy. This is when many people first feel fetal movement, sometimes called quickening. The baby now has regular sleep-wake cycles and can be startled by loud noises or your movements. A protective coating called vernix begins covering the skin around week 19, and fine downy hair called lanugo appears by week 21 to help hold that coating in place.
By week 22, eyebrows and hair are visible. The reproductive organs are largely formed. Glucose screening for gestational diabetes is typically done between weeks 24 and 28, near the end of this trimester.
Third Trimester: Weeks 28 Through 40
The third trimester is about weight gain, organ maturation, and preparation for life outside the uterus. At week 28, the baby weighs roughly 2¼ pounds and measures about 10 inches from crown to rump. The nervous system can now regulate body temperature and trigger practice breathing movements.
Growth accelerates from here. By week 31, most major development is complete and the focus shifts to putting on weight. At week 32, the baby is around 3¾ pounds. By week 34, that number jumps past 4½ pounds. The skull remains soft and flexible even as other bones harden, which will help during delivery.
In the final weeks, fat fills in under the skin, making the limbs look chubby and helping the baby regulate temperature after birth. By week 36, the skin smooths out. At week 38, the baby may weigh around 6½ pounds, though there’s wide variation: some babies are close to 9 pounds at this point. By the due date at week 40, the average weight is about 7½ pounds with a crown-to-rump length of roughly 14 inches.
What “Full Term” Actually Means
Not all weeks at the end of pregnancy are equal. Medical organizations break down the final stretch into more specific 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
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a week or two before 39 weeks can have less mature lungs and brains than those who reach full term. This is why elective deliveries are generally not scheduled before 39 weeks unless there’s a medical reason.
The Fourth Trimester: First 12 Weeks After Birth
You may hear the phrase “fourth trimester” used to describe the first 12 weeks after delivery. It’s not an official medical trimester, but it recognizes that the postpartum period involves significant physical recovery and emotional adjustment. Maternal health issues during this window often overlap and compound one another, yet most people aren’t scheduled for a follow-up visit until six weeks after birth. If you’re planning ahead, it’s worth knowing that this stretch can feel as physically demanding as pregnancy itself, and building support for it early makes a real difference.