The third trimester starts at 28 weeks and 0 days of pregnancy, as defined by the American College of Obstetricians and Gynecologists (ACOG). That means your second trimester runs through 27 weeks and 6 days, and the moment you hit week 28, you’ve entered the final stretch. From there, the third trimester continues until delivery, typically around 40 weeks and 6 days.
You may see some sources round down and say the third trimester begins at week 27. This usually reflects a simplified division of 40 weeks into three equal chunks, but the official ACOG breakdown is precise: first trimester ends at 13 weeks and 6 days, second trimester covers 14 weeks 0 days through 27 weeks 6 days, and third trimester picks up at 28 weeks 0 days.
How the Trimesters Break Down
A full-term pregnancy is roughly 40 weeks, counted from the first day of your last menstrual period (LMP). The three trimesters aren’t perfectly equal in length:
- First trimester: LMP through 13 weeks 6 days. This is when fertilization happens and all major organs form.
- Second trimester: 14 weeks 0 days through 27 weeks 6 days. A period of rapid growth, when you typically start feeling movement.
- Third trimester: 28 weeks 0 days through 40 weeks 6 days. The baby gains significant weight and organs mature in preparation for life outside the womb.
What’s Happening With Your Baby at Week 28
The transition into the third trimester isn’t just a line on the calendar. By 28 weeks, your baby’s eyelids can partially open, and the central nervous system has developed enough to regulate body temperature. The brain can also trigger rhythmic breathing movements, which are sometimes visible on ultrasound even though the baby isn’t actually breathing air yet.
Over the next several weeks, development accelerates. By week 30, the eyes can open wide. By week 33, the pupils can change size in response to light, and the eyes can detect it. The final weeks of pregnancy are largely about weight gain and organ maturation, particularly the lungs, which need to be ready to handle oxygen independently at birth.
Physical Changes You’ll Notice
The third trimester brings a distinct shift in how pregnancy feels day to day. Your baby is growing fast, and your body is working harder to support that growth.
Braxton Hicks contractions become more noticeable for many people during these weeks. They feel like a mild tightening across the belly, sometimes similar to menstrual cramps. They tend to show up in the afternoon or evening, after physical activity, or after sex. Unlike true labor contractions, they’re irregular, don’t get progressively stronger, and often ease up if you change positions, walk around, or drink water. As you get closer to your due date, they may become more frequent and more intense, but they still won’t follow a steady pattern.
Swelling in the ankles and feet is common as fluid retention increases alongside weight gain. Elevating your legs throughout the day helps. Sleep also gets harder. Between the size of your belly making it difficult to find a comfortable position, more frequent trips to the bathroom, and general discomfort, insomnia is one of the most universal third trimester complaints.
Prenatal Tests in the Third Trimester
A few key screenings happen around the time the third trimester begins. Glucose screening, which checks for gestational diabetes by measuring blood sugar levels, is typically done between 24 and 28 weeks. If you had gestational diabetes in a previous pregnancy or have other risk factors, your provider may run this test earlier.
Later in the third trimester, you’ll be tested for Group B strep (GBS), a common bacterium that can live in the vagina and rectum without causing any symptoms or illness in adults. The concern is that GBS can pass to the baby during labor. A positive result doesn’t mean anything is wrong with you. It simply means your care team will take steps during delivery to protect the baby.
Tracking Your Baby’s Movement
Once you’re in the third trimester, daily kick counts become a useful way to monitor your baby’s wellbeing. The process is simple: pick a time when your baby is usually active (for many people, that’s in the evening), sit with your feet up or lie on your side, and count each movement until you reach 10. Kicks, jabs, pokes, and rolls all count.
The specific number of minutes it takes matters less than the pattern. What you’re looking for is consistency. If your baby usually hits 10 movements in 15 minutes and one evening it takes an hour, that’s a change worth reporting to your provider. The same goes if movements suddenly feel weaker than usual, or if you notice a rapid increase in activity that seems different from the norm. Changes in movement patterns are sometimes the earliest signal that something needs attention.
Braxton Hicks vs. True Labor Contractions
Since Braxton Hicks contractions can start weeks before actual labor and sometimes feel quite painful, knowing the difference matters. False labor contractions are irregular, tend to be felt mostly in the front of the abdomen, and usually stop when you rest or hydrate. True labor contractions come at regular intervals, follow a pattern, and get stronger over time regardless of what you do.
If you’re unsure, time your contractions and note whether they continue after you’ve been resting and drinking water. Contractions that ease up with rest are not true labor. However, certain signs call for an immediate trip to the hospital: your water breaks without contractions starting, you have heavy vaginal bleeding, you experience constant severe pain with no relief between contractions, or you notice your baby is moving less often than usual.