The third trimester of pregnancy starts at week 28 and lasts until week 40 or whenever you deliver. Most major health organizations agree on this cutoff, though you may occasionally see week 27 mentioned. If you’re counting down, week 28 marks the beginning of your final 12 weeks of a full-term pregnancy.
Why Week 28 Is the Dividing Line
The week 28 threshold isn’t arbitrary. By this point, your baby’s central nervous system can regulate body temperature and trigger rhythmic breathing movements visible on ultrasound. The eyelids can partially open, and the brain is entering a period of rapid growth that continues through delivery. At 28 weeks, the average fetus measures about 10 inches from the crown of the head to the rump and weighs roughly 2¼ pounds.
These milestones matter because they reflect a significant jump in the baby’s ability to survive outside the womb if born early. Babies delivered at 28 weeks still need intensive medical support, but their odds are dramatically better than those born just a few weeks earlier. That transition in viability is part of why the medical community draws the trimester line here.
What Your Body Feels Like Starting Week 28
The third trimester is when pregnancy gets physically harder. Your uterus is now large enough to press against your diaphragm, making deep breaths feel like work. Heartburn often intensifies because the growing uterus pushes stomach acid upward. Swelling in your feet and ankles becomes more noticeable, especially by the end of the day, as your body retains more fluid and your circulation works harder against the extra weight.
You’ll likely start feeling Braxton Hicks contractions if you haven’t already. These are practice contractions: your uterus tightens for 30 seconds to two minutes, then relaxes. They can be surprisingly uncomfortable, but they’re not labor. The key differences are that Braxton Hicks contractions are irregular, tend to be felt in the front of your abdomen rather than radiating through your back, and typically go away when you rest or drink water. True labor contractions grow closer together over time and don’t stop when you change what you’re doing.
Back pain, frequent urination, trouble sleeping, and general fatigue are also standard. Your baby is gaining weight quickly now, and so are you.
How Prenatal Visits Change
Once you hit the third trimester, your appointment schedule picks up. Visits typically shift to every two to four weeks from week 28 through week 35. Starting at week 36, most providers want to see you every one to two weeks until delivery. These more frequent check-ins allow your provider to monitor your blood pressure, check the baby’s position, and catch potential complications like preeclampsia early.
One routine test happens during weeks 36 or 37: screening for Group B Strep, a type of bacteria that can live in the vaginal or rectal area without causing you any symptoms. About 1 in 4 pregnant women carry it. The test is a simple swab, and it’s recommended even if you’re planning a cesarean birth. If the result is positive, you’ll receive antibiotics during labor to prevent passing the bacteria to your baby.
Weight Gain and Calorie Needs
Your calorie needs increase in the third trimester to about 450 extra calories per day compared to your pre-pregnancy diet. That’s roughly the equivalent of a peanut butter sandwich and a glass of milk, not eating for two in any dramatic sense.
Total weight gain targets for the entire pregnancy depend on your pre-pregnancy BMI. For a single pregnancy, the recommendations break down like this:
- Underweight (BMI under 18.5): 28 to 40 pounds total
- Normal weight (BMI 18.5 to 24.9): 25 to 35 pounds total
- Overweight (BMI 25 to 29.9): 15 to 25 pounds total
- Obese (BMI 30 or higher): 11 to 20 pounds total
If you’re carrying twins, the numbers are significantly higher, ranging from 25 pounds on the low end to 62 pounds on the high end depending on your starting BMI. Much of the weight gained in the third trimester is the baby itself, plus the placenta, amniotic fluid, increased blood volume, and fluid retention.
What Your Baby Is Doing in There
The third trimester is primarily about growth and maturation. At week 28, your baby’s lungs are still developing the substance they need to inflate properly after birth. This process continues into the final weeks, which is one reason why babies born even a few weeks early sometimes need breathing support.
Between weeks 28 and 40, your baby will roughly triple in weight. The brain develops rapidly, forming the folds and grooves that increase its surface area. Fat accumulates under the skin, smoothing out the wrinkled appearance and helping with temperature regulation after birth. By the middle of the third trimester, most babies settle into a head-down position in preparation for delivery, though some take their time or stay breech.
You’ll notice the baby’s movements feel different as space gets tighter. The fluttery kicks of the second trimester give way to rolls, stretches, and visible shifts across your belly. The movements should stay consistent in frequency even as they change in character.
Telling Braxton Hicks From Real Labor
Because contractions become a regular feature of the third trimester, knowing what’s normal saves a lot of anxiety. Try timing any contractions that concern you. If they come at irregular intervals (say, 10 minutes apart, then 6, then 15), they’re almost certainly Braxton Hicks. Real labor contractions follow a pattern, growing steadily closer together and more intense over time.
The simple test: lie down, drink a glass of water, and wait. If the contractions ease up or stop, they were practice rounds. If they keep coming regardless of what you do, and especially if they’re accompanied by lower back pain, pelvic pressure, or any fluid leaking, that warrants a call to your provider. Before 37 weeks, regular contractions that don’t stop could signal preterm labor, which is treated differently than labor at full term.