The third trimester is the final stage of pregnancy, spanning from week 29 through week 40. These last roughly 12 weeks bring the most rapid fetal growth, the most noticeable physical changes for the pregnant person, and a series of medical screenings designed to prepare for a safe delivery.
When the Third Trimester Starts and Ends
The third trimester begins at week 29 and continues until birth, which typically happens around week 40. That covers approximately three months, though the exact length varies since not every pregnancy reaches 40 weeks. Babies born before 37 weeks are considered preterm, while those arriving between 37 and 42 weeks are considered full term or near term.
How the Baby Grows
At the start of the third trimester (around week 28), the baby weighs roughly 2.25 pounds. From there, growth accelerates dramatically. By week 32, the baby is about 3.75 pounds. By week 38, that number jumps to around 6.5 pounds, and at week 40, the average is about 7.5 pounds, though healthy babies can range from well under to nearly 9 pounds or more.
Several critical developments happen during these weeks. The lungs begin producing surfactant, a substance the baby needs to breathe air after birth. Cartilage that formed earlier in pregnancy gradually hardens into bone. The brain grows rapidly, adding layers of complexity that support functions like temperature regulation and coordinated movement. Fat stores build under the skin, helping the baby maintain body heat once born. By the final weeks, most babies settle into a head-down position in preparation for delivery.
Common Physical Changes
As the baby takes up more space, the effects on your body become harder to ignore. Shortness of breath is one of the most common complaints, caused by the growing uterus pressing upward against the rib cage. This can feel worse when lying on your back. As the baby drops lower into the pelvis closer to delivery, breathing often gets easier, but pelvic pressure and bladder urgency increase. You may find yourself needing to urinate far more frequently than earlier in pregnancy.
Swelling in the feet, ankles, and hands is normal and happens because the body retains more fluid during late pregnancy. Elevating your legs and staying physically active can help manage it. Backaches, heartburn, and trouble sleeping are also typical as the belly grows larger. Braxton Hicks contractions, sometimes called “practice contractions,” may become more frequent. These are irregular tightenings of the uterus that come and go without a predictable pattern.
Sleeping Comfortably
Sleeping on your back during the third trimester can compress a major blood vessel (the inferior vena cava) that returns blood from the lower body to the heart. This reduces blood flow to both you and the baby and can cause dizziness or discomfort. Side sleeping is the recommended position, with the left side being ideal because it maximizes blood flow to the baby and improves kidney function. A pillow between the knees or under the belly can make side sleeping more comfortable as the weeks progress.
Nutrition in the Final Weeks
Calorie needs increase modestly during the third trimester. Most people with a normal pre-pregnancy weight need about 2,400 calories per day, which works out to roughly 300 extra calories compared to pre-pregnancy intake. That’s not a dramatic increase; it’s roughly a handful of nuts with a piece of fruit. The focus should be on nutrient-dense foods rather than simply eating more. Iron, calcium, and protein become especially important as the baby’s bones harden and blood volume continues to rise.
Key Medical Screenings
Prenatal visits become more frequent during the third trimester, shifting from monthly to every two weeks and then weekly as the due date approaches. One important screening is the Group B Strep (GBS) test, performed during week 36 or 37. GBS bacteria naturally come and go in the body, which is why the test is done late in pregnancy, close to delivery. If the result is positive, steps are taken during labor to reduce the risk of passing the bacteria to the newborn.
Your provider will also monitor blood pressure closely. Preeclampsia, a condition involving dangerously high blood pressure, is most likely to develop during the third trimester. Warning signs include sudden swelling of the face and hands, blurred vision or light sensitivity, severe headaches, and intense abdominal pain. These symptoms require immediate medical attention because preeclampsia can escalate quickly and affect both the pregnant person and the baby.
Tracking Fetal Movement
Sometime in the third trimester, your provider may ask you to start doing “kick counts,” a simple way to monitor the baby’s well-being at home. The standard approach is to pick a time when the baby is usually active, sit or lie down, and either count how many movements you feel in one hour or measure how long it takes to feel 10 movements. Ten kicks, rolls, or flutters within one hour is considered typical. If you haven’t felt 10 movements after two hours, it’s worth contacting your provider. A temporary slowdown can happen when the baby is sleeping, but a noticeable, sustained decrease in movement deserves attention.
Telling Real Labor From Practice Contractions
One of the biggest questions in the third trimester is whether contractions mean labor has started. Braxton Hicks contractions are irregular, don’t increase in intensity, and usually stop if you change positions or drink water. True labor contractions follow a regular pattern and get closer together over time. The uterus becomes noticeably hard during each contraction and soft between them. The pain often starts in the back or pelvis and feels similar to strong menstrual cramps.
A common guideline is the 5-1-1 pattern: contractions coming every 5 minutes, lasting 1 minute each, and continuing that way for at least 1 hour. Other signs that labor is approaching include a gush or steady trickle of fluid (your water breaking), a bloody or mucus-like vaginal discharge, and a persistent low backache that doesn’t go away with rest. Not every labor starts the same way, but regular, intensifying contractions are the most reliable signal.