The third trimester of pregnancy lasts about 12 to 13 weeks, starting at week 28 and ending at week 40 or whenever you deliver. That covers roughly the final three months of pregnancy, or months seven through nine.
Of course, not every pregnancy hits exactly 40 weeks. Some babies arrive early, and some take their time. Here’s what to expect across those final weeks, from how your baby grows to how your body changes and what “full term” actually means.
Week-by-Week Breakdown
Your baby roughly triples in weight during the third trimester. At week 28, the average fetus weighs about 2¼ pounds and measures nearly 10 inches from head to rump. By week 32, that jumps to around 3¾ pounds. At week 34, most babies weigh more than 4½ pounds, and by week 38 the average is about 6½ pounds, though some babies are already close to 9 pounds at that point.
At week 40, the typical newborn weighs about 7½ pounds with a head-to-rump length of around 14 inches. But there’s wide variation in size, and your provider will track your baby’s individual growth curve rather than comparing to a single number.
Beyond gaining weight, these weeks are critical for organ maturation. The lungs are among the last organs to fully develop, which is a major reason why the final weeks of pregnancy matter so much for babies born early. The brain also undergoes rapid growth and wiring throughout the third trimester, building the neural connections your baby will need after birth.
What “Full Term” Actually Means
Not all weeks within the third trimester are equal when it comes to your baby’s readiness for the outside world. Medical organizations define term pregnancy in four categories:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
This distinction matters because babies born even a couple of weeks early can face more breathing and feeding challenges than those born at 39 weeks or later. The 39-week mark is generally considered the sweet spot where the benefits of extra time in the womb level off.
Survival and Outcomes for Early Arrivals
If your baby arrives at the start of the third trimester, the outlook is much better than many people expect. Survival rates for infants born at 28 weeks are between 80 and 90 percent, and only about 10 percent of those babies develop long-term health problems.
The numbers improve quickly from there. At 32 weeks, survival reaches 95 percent with very low risk of complications during infancy and childhood. By 34 weeks, babies generally have the same long-term health outcomes as those born at full term. Each additional week in the womb during the early third trimester dramatically reduces risk, which is why providers work hard to delay delivery when possible.
How Your Body Changes
The third trimester brings the most noticeable physical changes of pregnancy. As your baby grows, the added weight shifts your center of gravity and puts increasing pressure on your lower back and pelvis. Many women develop backaches and pelvic pain from the combination of extra weight and loosening joints (your body releases hormones that relax ligaments in preparation for delivery).
Swelling in the face, hands, and ankles often becomes more noticeable as your due date approaches. Your baby is also pressing directly on your bladder, urethra, and pelvic floor muscles, which leads to more frequent bathroom trips and sometimes leaking urine when you sneeze, cough, or laugh. Shortness of breath is common too, since your growing uterus pushes up against your diaphragm.
These symptoms tend to intensify through weeks 28 to 36 and then shift somewhat in the final weeks. When your baby “drops” lower into your pelvis toward the end of pregnancy, breathing often gets easier, but bladder pressure and pelvic discomfort typically increase.
Prenatal Visits in the Final Weeks
Your appointment schedule picks up during the third trimester. You’ll typically have a prenatal visit every two weeks until week 36, then switch to weekly visits until delivery. These more frequent check-ins allow your provider to monitor blood pressure, your baby’s position, and signs that labor may be approaching.
A few weeks before your due date, you’ll be screened for group B strep, a common bacterial infection that doesn’t usually cause problems for you but can be passed to your baby during delivery. If the test is positive, you’ll receive antibiotics during labor to protect your newborn.
Braxton Hicks vs. Real Labor
Sometime during the third trimester, you’ll likely start feeling Braxton Hicks contractions. These are practice contractions that feel like a tightening focused in one area of your abdomen. They’re uncomfortable but not usually painful, they come at irregular intervals, and they don’t get stronger over time. They also tend to stop if you drink water and rest.
True labor contractions are different in several key ways. They start at the top of the uterus and travel downward in a coordinated wave. They’re painful, they get progressively stronger and longer, and the intervals between them get shorter. Real contractions don’t stop when you change position or hydrate.
A common guideline for knowing when to head to the hospital is the 5-1-1 rule: contractions every 5 minutes, each lasting at least 1 minute, continuing for at least 1 hour. If you’re unsure whether what you’re feeling is the real thing, drinking a few large glasses of water can help you tell. Braxton Hicks contractions typically ease off once you’re rehydrated, while true labor keeps going.