The third trimester of pregnancy spans week 29 through week 40. It’s the final stretch, covering roughly the last three months before delivery. During these 12 weeks, your baby goes through critical development, your body prepares for labor, and your prenatal care schedule picks up significantly.
How Pregnancy Trimesters Break Down
A full pregnancy lasts about 40 weeks from the first day of your last period. The first trimester covers weeks 1 through 12, the second runs from week 13 through 28, and the third trimester begins at week 29 and continues through week 40. These divisions aren’t arbitrary. Each trimester corresponds to distinct phases of fetal growth and maternal changes.
Within the third trimester, the final weeks are further classified by medical organizations into more specific categories. “Early term” refers to 37 weeks through 38 weeks and 6 days. “Full term” is 39 weeks through 40 weeks and 6 days. “Late term” covers 41 weeks through 41 weeks and 6 days, and “post-term” means 42 weeks and beyond. These distinctions matter because babies born even a few weeks early can face different health outcomes than those born at full term.
What Your Baby Is Doing in the Third Trimester
The third trimester is when your baby’s organs finish maturing. Lung development is one of the most important milestones during this period. The cells that produce surfactant, a substance that keeps the lungs from collapsing after each breath, begin appearing as early as 20 to 24 weeks. But clinical lung maturity, meaning a baby can breathe without respiratory distress, generally isn’t reached until around 36 weeks of normal gestation. This is one of the key reasons premature births carry higher risks, and why doctors try to delay delivery when possible.
Your baby also gains a significant amount of weight during these weeks, building fat stores that help regulate body temperature after birth. By around 36 weeks, most babies shift into a head-down position in preparation for delivery. If your baby hasn’t turned head-down by 36 or 37 weeks, your provider may attempt a procedure to manually reposition the baby from the outside.
Physical Changes You Can Expect
As your baby grows larger, you’ll notice increasing pressure on your organs. Shortness of breath is common because the uterus pushes up against the rib cage, leaving less room for your lungs to expand. Lying flat on your back can make this worse. Later in the trimester, when the baby drops lower into your pelvis, breathing often gets easier, but bladder pressure increases. Expect more frequent trips to the bathroom.
Swelling in the feet, ankles, and legs is another hallmark of the third trimester. Propping your legs up, wearing compression socks, and avoiding long periods of standing or sitting with crossed legs can help. Some swelling is normal, but sudden or severe swelling, especially in the face or hands, warrants a call to your provider.
Back pain, heartburn, and difficulty sleeping tend to intensify as well. Your center of gravity shifts forward, your joints loosen in preparation for delivery, and finding a comfortable sleeping position becomes harder as your belly grows.
Braxton Hicks vs. True Labor Contractions
Starting in the third trimester, many people experience Braxton Hicks contractions, sometimes called “practice contractions.” These can feel alarming if you’re not sure what they are. The key differences from real labor come down to pattern, intensity, and response to rest.
- Pattern: True labor contractions come at regular intervals and get closer together over time. Braxton Hicks contractions are irregular with no predictable pattern.
- Intensity: Real contractions steadily get stronger. Practice contractions stay weak or may start strong and then fade.
- Location: True labor pain typically starts in the back and moves to the front. Braxton Hicks contractions are usually felt only in the front of the abdomen.
- Response to movement: Real contractions continue regardless of what you do. Braxton Hicks contractions often stop when you walk, rest, change position, or drink water.
If you’re unsure, timing your contractions while resting and hydrating is a reliable way to tell. Contractions that disappear with rest are not true labor.
Prenatal Visits and Screenings
Your appointment schedule increases during the third trimester. From week 29 through about week 35, you’ll typically have visits every two to four weeks. Starting at 36 weeks, that frequency bumps up to every one to two weeks until delivery. These visits monitor your blood pressure, your baby’s position and heart rate, and your overall readiness for labor.
Two specific screenings happen during this period. The CDC recommends a Tdap vaccine (which protects against whooping cough) between weeks 27 and 36 of each pregnancy, ideally on the earlier side. Getting vaccinated during this window allows your body to produce antibodies that pass to the baby before birth, offering protection during the newborn’s first vulnerable months.
Between weeks 36 and 37, you’ll be screened for Group B strep, a type of bacteria that about 1 in 4 pregnant people carry. The test is straightforward: a swab of the vagina and rectum sent to a lab. If the result is positive, you’ll receive antibiotics during labor to prevent passing the bacteria to your baby during delivery.
Why the Last Few Weeks Matter
It can be tempting to think of 37 weeks as “close enough,” but the distinction between early term and full term is clinically meaningful. Babies born at 39 weeks and beyond have lower rates of breathing problems, feeding difficulties, and temperature regulation issues compared to those born at 37 or 38 weeks. The brain also undergoes significant growth in the final weeks of pregnancy. This is why elective deliveries before 39 weeks are discouraged unless there’s a medical reason.
The third trimester is the longest-feeling stretch for many people, but each week brings meaningful progress toward a safer delivery and a healthier baby.