The third trimester of pregnancy spans from week 28 through week 40 (or whenever delivery occurs), covering the final three months. This is the stretch where the baby gains most of its weight, organs finish maturing, and your body prepares for labor. It’s often the most physically demanding phase of pregnancy, but it’s also when the finish line comes into view.
When It Starts and How Long It Lasts
Pregnancy is divided into three trimesters of roughly 12 weeks each. The third trimester begins at 28 weeks and zero days and continues through 40 weeks and six days. Since babies arrive on their own schedule, this trimester could last anywhere from 10 to 14 weeks depending on when labor begins or when delivery is medically indicated.
How Your Baby Develops
By the start of the third trimester, your baby’s major organs and structures are already formed. The remaining weeks are about refining those systems and putting on weight. Around week 28, the baby’s central nervous system is mature enough to regulate body temperature and trigger rhythmic breathing movements, which you might notice on an ultrasound. By week 30, red blood cells begin forming in the bone marrow rather than in other tissues, a sign of a maturing circulatory system.
Week 31 marks a turning point: most major development is complete, and rapid weight gain kicks in. Fat layers build under the skin over the coming weeks, smoothing out wrinkled skin and giving the limbs a rounder, chubbier appearance by week 36. This fat isn’t cosmetic. It’s essential for keeping the baby warm after birth. By week 33, the pupils can change size in response to light. At week 39, the baby is still adding fat across its entire body in the final push before delivery.
Throughout these weeks, the lungs are undergoing critical maturation. The air passages that will carry oxygen in and out are developing the lining they need to function independently. This is one reason premature birth carries more risk the earlier it happens: lung readiness is one of the last pieces to fall into place.
How Your Baby Positions for Birth
Most babies settle into a head-down (vertex) position between 32 and 36 weeks. Before that, they move freely and may flip between positions multiple times. Some babies take until the very end of 36 weeks to turn head-down, and it’s even possible for a baby to rotate into position after that point, though it becomes less likely as space gets tighter.
When the baby drops lower into the pelvis, a process sometimes called “lightening,” you may notice it’s easier to breathe but harder to walk comfortably. This shift can happen weeks before labor or not until labor itself begins.
What Your Body Goes Through
The third trimester is physically intense. Your uterus is now large enough to press against your diaphragm, which can make deep breaths feel harder to come by. The added weight puts pressure on your lower back, hips, and bladder, making frequent bathroom trips a near-constant reality. Swelling in the feet and ankles is common as your body retains more fluid and blood volume peaks.
Many people experience Braxton Hicks contractions during this trimester. These feel like a tightening or mild cramping in the abdomen, but they’re irregular, don’t intensify over time, and typically stop if you change positions or take a walk. They’re your uterus practicing for labor, not a sign that labor has started. Sleep often becomes more difficult as finding a comfortable position gets harder, and heartburn may worsen as the baby pushes up against your stomach.
Prenatal Visits and Testing
Appointments become more frequent in the third trimester. You’ll typically have checkups every two to four weeks starting at week 28, then shift to every one to two weeks from week 36 onward. These visits usually include blood pressure checks, urine tests, measuring your belly to track growth, and checking the baby’s heart rate and position.
One key test happens during week 36 or 37: screening for Group B Strep (GBS), a type of bacteria that can live in the birth canal without causing you any symptoms. About 25% of pregnant people carry it. The screening involves a simple swab, and if the result is positive, antibiotics are given during labor to protect the baby. This screening is recommended even if a cesarean birth is planned.
Nutrition in the Final Months
Your calorie needs increase modestly during the third trimester, to about 2,400 calories per day for most people with a normal pre-pregnancy weight, roughly 300 extra calories above your baseline. That’s about the equivalent of a yogurt with fruit and granola, not the “eating for two” feast that popular culture suggests.
What matters more than quantity is quality. Your baby needs calcium for bone growth, iron for blood supply (which also protects you from anemia), and fats for brain development. Protein supports tissue building across the board. Focus on vegetables for iron and folic acid, fruit for vitamins and fiber, dairy or fortified alternatives for calcium, and fish, beans, eggs, or lean meats for protein and B vitamins. Refined sugar and processed carbohydrates provide calories without the nutrients your baby is drawing from your reserves.
Braxton Hicks vs. Real Labor
Knowing the difference between practice contractions and actual labor is one of the most practical things you can learn in the third trimester. Braxton Hicks contractions are irregular, vary in length, and don’t get progressively stronger or closer together. You can walk, talk, and go about your day through them. If you shift positions or take a short walk, they usually ease up or stop entirely.
Real labor contractions follow a pattern. They come at increasingly regular intervals, last between 30 and 90 seconds each, and gradually grow more intense. Changing positions or walking doesn’t relieve them. Over time, talking through a contraction becomes difficult. If your contractions are consistently getting stronger, longer, and closer together, that’s the signal labor has begun.
Warning Signs That Need Immediate Attention
Most third-trimester symptoms are uncomfortable but harmless. A few, however, signal potentially serious complications like preeclampsia, a condition involving dangerously high blood pressure that can develop in the second half of pregnancy. Contact your provider or go to an emergency room if you experience a severe headache that doesn’t respond to rest or hydration, sudden visual disturbances like blurred vision or seeing spots, severe pain in the upper abdomen, rapid or unusual swelling in the face or hands, or severe shortness of breath. These symptoms can escalate quickly, and early evaluation makes a significant difference in outcomes for both parent and baby.