During the third trimester (weeks 28 through 40), a baby roughly triples in weight and adds several inches in length. Most babies enter the third trimester weighing around 2 to 2.5 pounds and measuring about 14 to 15 inches long. By full term, the average newborn weighs 6.5 to 8 pounds and measures 19 to 21 inches. That rapid gain, roughly half a pound per week in the final stretch, makes the third trimester the most dramatic growth phase of pregnancy.
Weight Gain Week by Week
Growth isn’t perfectly steady across the trimester. In the early weeks (28 to 32), the baby is adding fat stores and filling out, gaining around half a pound per week. From weeks 32 to 36, the pace picks up as the baby puts on the most weight of any period in pregnancy. In the final weeks (36 to 40), weight gain slows slightly as the baby settles into position for delivery. Here’s a rough week-by-week picture of average fetal weight:
- Week 28: about 2.2 pounds
- Week 30: about 3 pounds
- Week 32: about 3.7 pounds
- Week 34: about 4.7 pounds
- Week 36: about 5.8 pounds
- Week 38: about 6.8 pounds
- Week 40: about 7.5 pounds
These are 50th-percentile estimates, so healthy babies can fall a pound or more above or below at any point. The important thing is a consistent upward trend rather than hitting an exact number.
Length and Body Proportions
Length increases are less dramatic than weight but still significant. A baby grows from roughly 14 to 15 inches at 28 weeks to 19 to 21 inches at birth, adding about 5 to 6 inches over the trimester. Early in the third trimester, the baby’s head is still relatively large compared to the body. By the final weeks, the body catches up, fat rounds out the limbs and cheeks, and the baby starts to look like the newborn you’ll meet at delivery.
That fat accumulation is not just cosmetic. The layer of fat beneath the skin helps the baby regulate body temperature after birth. Babies born early in the third trimester often struggle with temperature control precisely because they haven’t had time to build these reserves.
Brain Growth in the Final Weeks
The brain undergoes a fourfold increase in size during the third trimester. That’s not just getting bigger. The brain’s surface area expands dramatically as it develops the characteristic folds and grooves that allow more neural tissue to pack into the skull. These folds begin forming in earnest around weeks 28 to 30, and by full term the brain has developed most of the complex surface architecture it will carry into infancy.
This rapid brain development is one reason why every additional week of pregnancy matters for babies born early. A baby at 34 weeks has a significantly less mature brain than one at 38 weeks, even though the weight difference may only be a couple of pounds.
Lung Maturity and Breathing Readiness
The lungs are one of the last organs to fully mature. The cells that produce surfactant, a slippery substance that keeps the tiny air sacs in the lungs from collapsing, develop between 24 and 34 weeks. But clinically, the lungs aren’t considered mature until about 35 weeks of gestation. This is why premature babies often need breathing support: they simply don’t have enough surfactant to breathe efficiently on their own.
By 36 to 37 weeks, most babies have enough surfactant to handle breathing independently, though the lungs continue refining their function right up until delivery.
Sensory Development
Around week 26, the pathway connecting the eyes to the brain becomes established, and the eyelids open for the first time. From this point on, the baby can perceive changes in light. Shine a flashlight on your belly in the third trimester, and the baby may respond by turning toward or away from it.
Hearing is already well developed by the start of the third trimester. Babies in the womb respond to familiar voices, music, and sudden loud sounds. By the final weeks, a baby can distinguish its mother’s voice from other voices, which is why newborns show a clear preference for that voice from their very first day.
How Movements Change
As the baby grows and runs out of room, you’ll notice the character of movements shift. The flips and somersaults of the second trimester give way to rolling, stretching, and pushing. Women in late pregnancy often describe these movements as “strong and powerful” but also “slow” or “stretching,” like the baby is moving in slow motion. Many feel large, whole-body movements rather than quick kicks.
The type of movement changes, but the frequency and intensity should not normally decrease. A baby that suddenly becomes much less active is worth paying attention to. The old advice that babies “slow down” before labor is outdated. What changes is how the movements feel, not how often they happen.
Nutrient Transfer Ramps Up
The placenta works harder during the third trimester than at any other point. Calcium transport across the placenta increases significantly in the final weeks to support the baby’s rapidly hardening bones and developing teeth. Iron transfer also peaks in the third trimester, building up stores the baby will rely on for the first several months of life. This is why iron-deficiency anemia is more common in late pregnancy and why iron intake matters most in these final months.
Antibodies also cross the placenta in increasing amounts during the third trimester, giving the baby a temporary immune boost that provides protection for the first few months after birth. Babies born several weeks early miss out on some of this antibody transfer, which is one reason they can be more vulnerable to infections.
Tracking Growth From the Outside
Your healthcare provider tracks the baby’s growth in part by measuring fundal height, the distance from your pubic bone to the top of your uterus. Between weeks 20 and 36, the measurement in centimeters roughly matches your gestational week, plus or minus 2 centimeters. So at 32 weeks, a fundal height of 30 to 34 centimeters falls within normal range.
After about 36 weeks, this measurement can actually decrease. That’s because the baby drops lower into the pelvis to prepare for labor, bringing the top of the uterus down from its highest point near the breastbone. This “dropping” is normal and doesn’t mean the baby has stopped growing. If your provider has concerns about growth at any point, an ultrasound can give a more precise estimate of the baby’s size.