The third trimester of pregnancy starts at week 28 and lasts until you give birth, which is typically around week 39 or 40. Most major health organizations agree on this 28-week starting point. These final 12 to 13 weeks bring the most rapid fetal growth, noticeable physical changes for you, and a shift to more frequent prenatal visits as delivery approaches.
When the Third Trimester Starts and Ends
Pregnancy is divided into three roughly equal trimesters. The first covers weeks 1 through 12, the second spans weeks 13 through 27, and the third picks up at week 28 and continues until birth. While some sources place the cutoff a week earlier or later, 28 weeks is the most widely accepted starting point.
Not all weeks within the third trimester carry the same clinical weight. The National Institutes of Health and the American College of Obstetricians and Gynecologists break 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 couple of weeks early can face different health outcomes than those born at full term. The 39-week mark is considered the point at which most babies are fully ready for life outside the womb.
How Your Baby Grows in the Final Weeks
The third trimester is when your baby gains most of its weight and length. By week 32, a fetus measures roughly 42 centimeters (about 16.7 inches) from head to heel. Over the remaining weeks, babies typically double or even triple their weight, going from around 2 to 2.5 pounds at week 28 to an average of 6 to 9 pounds at birth.
Brain development accelerates dramatically during this period. The brain increases roughly fourfold in size during the third trimester, developing critical structures and connections that will be relied on for life. This neurological growth spurt is so energy-intensive that the body redirects resources specifically to fuel it. It’s one of the key reasons every additional week in the womb matters for babies who might otherwise be born early.
Lung maturation is the other major milestone. The lungs are among the last organs to fully develop, producing a substance that keeps the air sacs open after birth. This process isn’t complete until the late 30s in weeks, which is why premature babies often need breathing support.
Physical Changes You Can Expect
As your baby grows rapidly, your body adjusts in ways that are hard to miss. Some symptoms are simply uncomfortable, while others are worth paying attention to.
Braxton Hicks contractions are one of the most common third-trimester experiences. These are mild tightenings of the uterus that come and go irregularly. They tend to show up more in the afternoon or evening, after physical activity, or after sex, and they become more frequent as your due date nears. They’re not labor, but they can feel convincing enough to cause a false alarm.
Swelling in the feet, ankles, and hands is typical and caused by increased blood volume and fluid retention. Propping your legs up, staying active, and wearing compression socks can help. Sudden or severe swelling, especially in the face, is different and worth flagging to your provider (more on that below).
Pelvic pressure and frequent urination increase as the baby drops lower into the pelvis in the final weeks. This shift puts direct pressure on your bladder, which means more bathroom trips and possible leaking when you laugh, cough, or sneeze. It can also create a feeling of heaviness low in the abdomen.
Other common experiences include back pain, heartburn, shortness of breath (as the uterus pushes against the diaphragm), and difficulty sleeping. These tend to intensify as the weeks progress.
Weight Gain in the Third Trimester
For someone who started pregnancy at a normal weight, the general guideline is to gain about 1 pound per week during the third trimester. For those who started pregnancy overweight or obese, the recommended rate is closer to half a pound per week. These are averages, and individual variation is normal. Your provider tracks your weight at each visit to make sure the overall trend is on track rather than focusing on any single week.
Prenatal Visits and Screening
Your appointment schedule picks up during the third trimester. From week 28 to week 36, you’ll typically see your provider every two weeks. After week 36, visits shift to once a week until delivery. These appointments check your blood pressure, measure your belly, track fetal movement and heart rate, and monitor for complications.
One specific test happens during this period: Group B strep screening, performed at week 36 or 37. Your provider takes a simple swab from the vagina and rectum and sends it to a lab. About 1 in 4 pregnant women carry this bacteria, which is harmless to you but can be dangerous to a newborn during delivery. A positive result means you’ll receive antibiotics during labor to protect the baby.
Warning Signs to Watch For
Preeclampsia is one of the most serious third-trimester complications. It involves high blood pressure that develops after 20 weeks and can affect the kidneys, liver, and brain. Severe cases involve blood pressure readings of 160/110 or higher. The symptoms to know include:
- Swelling of the face or hands
- A headache that won’t go away
- Seeing spots or changes in vision
- Pain in the upper abdomen or shoulder
- Nausea and vomiting (new, in the second half of pregnancy)
- Sudden weight gain
- Difficulty breathing
Any combination of these symptoms warrants immediate contact with your provider. Preeclampsia can escalate quickly, and early detection makes a significant difference in outcomes for both you and the baby.
Braxton Hicks vs. Real Labor
Telling practice contractions from real labor is one of the biggest questions in the third trimester. The core difference is coordination. Real contractions start at the top of the uterus and move downward in a wave. Braxton Hicks contractions tend to feel like a tightening focused in one area and don’t travel through the whole uterus.
Braxton Hicks contractions are irregular, don’t get closer together over time, don’t increase in strength, and eventually taper off and stop. Real labor contractions do the opposite: they get closer together, last longer, and grow stronger.
A useful benchmark is the 5-1-1 rule. If contractions are coming every 5 minutes, each one lasts at least 1 minute, and this pattern has continued for 1 hour, that’s a strong signal of true labor. Braxton Hicks will almost never hold to this pattern. Changing positions, drinking water, or lying down will usually ease false contractions but won’t stop real ones.