Pregnancy is divided into three trimesters, each roughly three months long, spanning from the first day of your last menstrual period through about 40 weeks. The first trimester covers weeks 1 through 13, the second runs from week 14 through week 27, and the third stretches from week 28 until delivery. Each trimester brings distinct changes for both the developing baby and your body.
First Trimester: Weeks 1 Through 13
The first trimester is when all of the baby’s major organs form. By the end of week 13, the heart is beating, the brain and spinal cord are taking shape, and tiny limb buds have developed into arms and legs with fingers and toes. Despite how small the embryo still is (roughly 3 inches by week 12), this period of organ formation makes it the most critical window for healthy development.
Most of the symptoms you feel in the first trimester are driven by a sharp rise in hormones, especially one called hCG, which peaks toward the end of this trimester. That hormone surge is largely responsible for morning sickness, which typically starts between weeks 4 and 9 and can hit at any time of day. Rising progesterone levels slow your digestion (leading to constipation and heartburn) and can leave you feeling deeply fatigued. Breast tenderness, frequent urination, and sudden food cravings or aversions are also common.
Your first prenatal screening usually happens between weeks 11 and 13. It combines a blood test measuring two proteins with an ultrasound that checks for extra fluid behind the baby’s neck. Together, these help assess the risk of certain chromosomal conditions and heart defects early on.
Second Trimester: Weeks 14 Through 27
Many people call the second trimester the most comfortable stretch of pregnancy. Morning sickness typically fades as hCG levels drop, energy returns, and the baby bump becomes visible without yet being physically burdensome. That doesn’t mean symptoms disappear entirely. You may notice back pain, round ligament pain in the lower abdomen, and skin changes like a dark line running down the belly.
For the baby, this trimester is defined by rapid growth. Bones harden, muscles strengthen, and the nervous system matures enough that you can usually feel the first kicks and flutters somewhere between weeks 16 and 22. By the end of week 27, the baby weighs roughly 2 pounds and can open and close their eyes.
The anatomy scan, one of the most detailed ultrasounds of the entire pregnancy, is performed between weeks 18 and 22. A technician measures and photographs the baby’s brain, heart, spine, kidneys, limbs, and facial features to confirm everything is developing on track. A separate blood screen between weeks 15 and 20 (often called a quad screen) checks protein levels in your blood to flag increased risk for certain birth defects. If you’re being screened for gestational diabetes, that glucose test typically lands toward the end of this trimester as well.
Third Trimester: Weeks 28 Through 40
The third trimester is all about weight gain and organ maturation. By around week 31, the baby has finished most major structural development and shifts into a phase of rapid weight gain, often more than doubling in size before birth. The lungs are among the last organs to fully mature, which is a key reason why the timing of delivery matters so much for premature babies.
For you, the growing uterus creates a new set of physical challenges. Shortness of breath, trouble sleeping, swelling in the feet and ankles, and frequent bathroom trips are par for the course. You may also start feeling Braxton Hicks contractions, brief tightening sensations across your belly. These are practice contractions, not signs of labor, and they tend to be irregular and painless or mildly uncomfortable.
Why the Week Count Matters for Viability
The trimester boundaries aren’t just organizational labels. They correspond to real developmental thresholds that affect a baby’s chances if born early. Before 24 weeks, survival rates are generally below 50 percent. At 24 weeks, that number rises to between 60 and 70 percent. By 28 weeks (the start of the third trimester), survival rates reach 80 to 90 percent. At 32 weeks, a premature baby’s chance of surviving is as high as 95 percent, and babies born after 34 weeks tend to have the same long-term health outcomes as those delivered at full term.
Key Screenings by Trimester
Prenatal care follows a predictable schedule tied to these trimesters, so knowing what’s coming can help you prepare.
- First trimester (weeks 11 to 13): Blood test plus nuchal translucency ultrasound to screen for chromosomal conditions. Noninvasive prenatal testing (NIPT), a blood draw that analyzes fetal DNA circulating in your blood, may also be offered during this window.
- Second trimester (weeks 15 to 22): Quad blood screen for birth defect risk, followed by the anatomy scan ultrasound around week 18 to 22. A glucose screening for gestational diabetes is commonly scheduled near week 24 to 28.
- Third trimester (weeks 28 to 40): More frequent prenatal visits to monitor blood pressure, fetal position, and growth. A Group B Strep swab is typically done between weeks 36 and 37 to check for a common bacterium that could affect the baby during delivery.
How Your Hormones Shift Across All Three
The hormonal landscape changes dramatically from one trimester to the next, and those shifts explain many of the symptoms you experience. hCG rises rapidly in the first trimester, peaks around weeks 11 to 12, then gradually declines for the rest of pregnancy. That decline is a big reason the nausea and exhaustion of early pregnancy often ease up in the second trimester.
Progesterone, on the other hand, climbs steadily throughout all three trimesters. It keeps the uterine lining stable, supports the baby’s growth, and relaxes smooth muscle throughout your body. That relaxation effect is why heartburn and constipation can persist or even worsen as pregnancy progresses. Estrogen also rises continuously, increasing blood flow and helping prepare the breasts for nursing. Together, these hormones drive nearly every pregnancy symptom, from swollen gums to nasal congestion to the “pregnancy glow” that comes from increased blood volume near the skin’s surface.