The third trimester of pregnancy starts at week 28 and lasts until week 40, or whenever you give birth. Most major health organizations agree on this 28-week mark, though you may occasionally see sources that round down to 27 weeks. If your provider uses a slightly different cutoff, that’s normal, but 28 weeks is the most widely recognized starting point.
How the Trimesters Break Down
A full-term pregnancy spans about 40 weeks, divided into three roughly equal phases. The first trimester covers weeks 1 through 12, the second runs from week 13 through 27, and the third picks up at week 28 and continues through delivery. That gives you about 12 to 13 weeks in the final stretch, though babies arrive on their own schedule.
What Your Baby Is Doing in the Third Trimester
At 28 weeks, your baby is roughly 10 inches long (measured from the top of the head to the tailbone) and weighs about 2¼ pounds. By week 40, those numbers jump to around 14 inches and 7½ pounds. That weight gain isn’t just size for the sake of size. Your baby is building fat stores under the skin that will help regulate body temperature after birth.
The third trimester is when organs finish maturing so they can function independently. The lungs, in particular, are forming the air passages that will carry oxygen in and out after delivery. The brain is growing rapidly, and your baby’s movements become more coordinated. You’ll likely feel stronger, more deliberate kicks and rolls rather than the fluttery sensations of the second trimester.
Physical Changes You Can Expect
As your baby gains weight quickly, your body adjusts in ways that are hard to ignore. Pelvic pressure increases as the baby moves lower, especially in the final weeks. Swelling in the feet and ankles becomes more common because your body is retaining extra fluid and your growing uterus puts pressure on the veins that return blood from your legs. Heartburn and shortness of breath tend to pick up as your uterus pushes against your stomach and diaphragm.
Braxton Hicks contractions often start or become more noticeable during the third trimester. These feel like a tightening in one area of your abdomen. They’re uncomfortable but not usually painful, they come at irregular intervals, and they taper off on their own. They’re your uterus practicing, not a sign that labor has started.
Braxton Hicks vs. Real Contractions
Knowing the difference matters as you get closer to your due date. Real labor contractions are coordinated. They start at the top of the uterus and move downward through the entire muscle. Braxton Hicks tend to stay focused in one spot. Beyond that, the pattern tells you a lot:
- Braxton Hicks: irregular timing, no increase in intensity, and they eventually stop.
- Real labor: intervals get shorter, contractions get stronger and last longer, and they don’t go away.
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.
Tracking Your Baby’s Movements
Starting in the third trimester, paying attention to your baby’s movement pattern becomes important. Babies tend to be most active in the evening. To do a kick count, sit with your feet up or lie on your side and count each movement (kicks, jabs, rolls) until you reach 10. Over time, you’ll learn what’s normal for your baby.
What matters most isn’t a single number but changes in pattern. If your baby suddenly takes much longer to reach 10 movements than usual, or if the movements feel noticeably weaker, contact your provider right away. A change in movement, whether a decrease or a rapid increase, can be an early signal that something needs attention.
Key Screenings and Vaccines
Your prenatal visit schedule typically increases in the third trimester, often shifting from monthly to every two weeks and then weekly as you approach your due date. Two things commonly happen during this window.
The Tdap vaccine, which protects against whooping cough, is recommended between weeks 27 and 36, ideally on the earlier side of that range. Getting it during this window allows your body to produce antibodies that cross the placenta and give your baby some protection in the first few months of life before they can be vaccinated themselves. Only one dose is needed per pregnancy.
Group B Strep testing is also standard in late pregnancy, typically performed between weeks 36 and 37. This is a simple swab test to check whether you carry a type of bacteria that’s harmless to you but could pose a risk to your baby during delivery.
Warning Signs to Take Seriously
Most third-trimester discomfort is normal, but certain symptoms require immediate attention. The CDC identifies several urgent maternal warning signs that apply throughout pregnancy but are especially relevant in the final trimester:
- Severe headache: one that won’t go away, gets worse, or comes with blurred vision or dizziness.
- Vision changes: flashes of light, bright spots, blind spots, or sudden blurry or double vision.
- Extreme swelling: swelling in the hands or face severe enough to make it hard to bend your fingers or open your eyes.
- Trouble breathing: sudden shortness of breath, chest tightness, or difficulty breathing while lying flat.
- Severe belly pain: sharp, sudden, or persistent abdominal pain that doesn’t ease up.
- Vaginal bleeding or fluid leaking: bleeding heavier than spotting, or fluid leaking from the vagina.
Several of these, particularly the combination of headache, vision changes, and facial swelling, can point to preeclampsia, a blood pressure condition that develops in pregnancy and can escalate quickly. Fever of 100.4°F or higher, a fast or irregular heartbeat, and severe swelling or pain in one leg also warrant a call to your provider without delay.