The third trimester of pregnancy starts at week 28 and lasts until you give birth, which is typically around week 39 or 40. That means it covers roughly the final 12 to 13 weeks of a full-term pregnancy. Most major health organizations, including the Cleveland Clinic and ACOG, agree on week 28 as the starting point.
How the Third Trimester Breaks Down
Your third trimester spans weeks 28 through 40, though not all of those weeks carry the same clinical significance. ACOG divides the tail end of pregnancy into more specific categories that hospitals and providers use when discussing delivery timing:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks and beyond
These distinctions matter because babies born even a couple of weeks early can have different outcomes than those born at full term. A baby delivered at 37 weeks is technically in the third trimester and viable, but their lungs and brain are still maturing. The “full term” label at 39 weeks reflects the point where outcomes are generally best.
What’s Happening With Your Baby
At week 28, your baby’s central nervous system reaches a milestone: it can regulate body temperature and trigger rhythmic breathing movements visible on ultrasound. The eyelids can partially open. From here, the final trimester is largely about growth and maturation rather than forming new structures.
The brain develops rapidly throughout these weeks, adding billions of neural connections. Lung maturation is one of the last major processes to complete, which is a key reason why every additional week in the womb matters for babies at risk of preterm delivery. By around 36 to 37 weeks, most babies have turned head-down in preparation for birth. Average weight at 28 weeks is roughly 2 to 2.5 pounds, climbing to about 6.5 to 8 pounds by week 40.
Tracking Fetal Movement
Once you’re in the third trimester, your provider will likely ask you to start paying attention to your baby’s movements. The standard approach is to count kicks, flutters, swishes, or rolls and aim for 10 movements within a two-hour window. Most babies hit that number well within an hour.
Pick a time when your baby tends to be active (often after a meal or in the evening), sit or lie down, and start counting. If two hours pass without 10 movements, contact your provider. A single quiet session doesn’t necessarily signal a problem, since babies sleep in cycles, but a noticeable change in your baby’s usual pattern is worth reporting promptly.
Common Physical Changes
The third trimester brings a distinct set of physical shifts as your body accommodates a rapidly growing baby. Pelvic pressure increases as the baby drops lower, especially in the final weeks. Sleep becomes harder to come by, partly because of the sheer difficulty of finding a comfortable position and partly because of more frequent trips to the bathroom. Swelling in the feet and ankles is common, particularly by the end of the day. Shortness of breath can happen as your uterus pushes upward against your diaphragm, though this often eases in the last few weeks when the baby “drops” into your pelvis.
Braxton Hicks contractions also become more noticeable during this trimester. These feel like a tightening in one area of your abdomen and are irregular, meaning they don’t follow a predictable pattern and they eventually taper off on their own. They’re uncomfortable but not typically painful.
Prenatal Visits and Screening Tests
Your appointment schedule picks up during the third trimester. From week 28 through about week 35, you can expect visits every two to four weeks. Starting at 36 weeks, most providers shift to weekly or biweekly checkups until delivery.
The glucose screening for gestational diabetes usually happens just before the third trimester, between 24 and 28 weeks, so you may have this done right around the transition. The other major test is Group B Strep (GBS) screening, which happens later in the trimester. GBS is a common bacterium that can live in the vagina and rectum without causing you any symptoms, but it can pass to your baby during labor. Without treatment, about 1 to 2 out of every 100 babies born to GBS-positive mothers become infected. If your test comes back positive, you’ll receive antibiotics during labor to reduce that risk.
Recognizing True Labor
Toward the end of the third trimester, distinguishing real contractions from Braxton Hicks becomes a practical concern. True labor contractions start at the top of the uterus and move downward in a coordinated wave. They’re painful, they get stronger over time, and the intervals between them shorten. Braxton Hicks, by contrast, stay focused in one area, don’t intensify, and eventually stop.
A useful guideline is the 5-1-1 rule: if your contractions are coming every 5 minutes, each one lasts at least 1 minute, and this pattern has continued for 1 hour, you’re likely in true labor and should head to your hospital or birth center.
Warning Signs That Need Immediate Attention
Most third-trimester symptoms are normal discomforts, but certain signs require urgent care. According to the CDC, you should seek immediate medical attention for a severe or persistent headache, vision changes like flashing lights or blurriness, extreme swelling of your hands or face, a fever of 100.4°F or higher, severe belly pain that doesn’t resolve, vaginal bleeding or fluid leaking, or a noticeable decrease in your baby’s movement. Chest pain, trouble breathing, and a racing or irregular heartbeat also warrant an immediate call or trip to the hospital.