The third trimester of pregnancy starts at week 28 and lasts until week 40, or whenever you deliver. That puts the beginning of the third trimester at roughly seven months pregnant. The American College of Obstetricians and Gynecologists defines it precisely as 28 weeks and 0 days through 40 weeks and 6 days.
Why You’ll See Both Week 27 and Week 28
If you’ve been Googling this, you’ve probably noticed some sources say 27 weeks and others say 28. The confusion comes from slight differences in how organizations divide 40 weeks into three chunks. Most major health organizations, including ACOG and the Cleveland Clinic, place the cutoff at week 28. Some older references or pregnancy apps round down to 27. In practice, the difference rarely matters for your care, but 28 weeks is the most widely accepted starting point.
Weeks to Months: A Quick Breakdown
Pregnancy math is notoriously confusing because months don’t divide evenly into weeks. Here’s a rough guide for the third trimester:
- Weeks 28 to 31: Month 7
- Weeks 32 to 35: Month 8
- Weeks 36 to 40: Month 9
These are approximations. Your provider tracks everything by weeks, not months, because weeks are more precise.
What Your Baby Is Doing at Week 28
The third trimester is when your baby’s organs mature enough to function outside the womb. At week 28, the eyelids can partially open for the first time. The central nervous system has developed enough to regulate body temperature on its own. Your baby also begins practicing breathing movements, which show up on ultrasound even though the lungs are still filling with fluid, not air.
From here, development accelerates. By week 30, the eyes can open wide. By week 33, the pupils respond to light, changing size the way they will after birth. Weight gain picks up dramatically during this trimester as the baby adds layers of fat and the organs finish their final stages of preparation.
Physical Changes You Can Expect
The transition into the third trimester brings a noticeable shift in how pregnancy feels. Your uterus is now well above your belly button, and the growing baby puts increasing pressure on your bladder, lungs, and lower back. Many people notice more frequent Braxton Hicks contractions starting around this time. These are irregular, painless tightenings of the uterus that come and go without a pattern. They’re a normal part of late pregnancy, not a sign of labor.
Shortness of breath, heartburn, trouble sleeping, and swelling in the ankles and feet all tend to intensify as the third trimester progresses. Back pain becomes more common as your center of gravity shifts forward. None of these are fun, but they’re expected parts of the final stretch.
Signs of Preterm Labor to Watch For
Because the third trimester starts well before your due date, it’s worth knowing what preterm labor looks like. Any baby born before 37 weeks is considered preterm. The key warning signs include:
- Regular contractions: Tightening in your belly that comes at consistent intervals, unlike the random pattern of Braxton Hicks
- Low, dull backache: Persistent and different from the usual pregnancy back pain
- Pelvic pressure: A feeling of heaviness or pressure low in the pelvis
- Vaginal changes: Spotting, light bleeding, a gush of fluid, or an ongoing trickle of watery or mucus-like discharge
- Mild cramping: Similar to menstrual cramps, sometimes with or without diarrhea
If you notice any combination of these, especially contractions that don’t stop when you rest or change positions, contact your provider right away. Early intervention can make a significant difference in outcomes for preterm babies.
What “28 Weeks” Means for Viability
Week 28 isn’t an arbitrary cutoff. It’s a meaningful milestone in fetal survival. Babies born at 28 weeks have a survival rate above 90% in hospitals with neonatal intensive care units, though they typically need weeks of specialized support. This is one reason providers pay closer attention to monitoring from the third trimester onward, with more frequent appointments and, in some cases, additional ultrasounds or fetal movement tracking.
You’ll likely shift from monthly prenatal visits to every two weeks starting around week 28, then weekly visits from week 36 until delivery. Your provider may also begin checking your baby’s position and discussing your birth plan during these later appointments.