During the last trimester of pregnancy, a baby roughly triples in weight, going from about 2 to 2.5 pounds at week 28 to an average birth weight of 7 to 8 pounds at week 40. That means your baby puts on approximately 5 pounds in just 12 weeks, with most major organ development already complete and the body focused almost entirely on growing bigger and stronger.
How Fast Babies Grow Week by Week
The average fetus gains about 175 to 200 grams per week during the third trimester, which works out to roughly 6 to 7 ounces. That pace isn’t perfectly steady. Early in the third trimester, weight gain is modest as the baby’s organs continue maturing. By around week 31, most major development is finished, and the body shifts into a rapid weight-gaining phase. The fastest growth typically happens between weeks 32 and 36, when your baby may pack on close to half a pound per week.
Growth slows slightly in the final weeks before birth. By weeks 38 to 40, the baby is running out of room in the uterus and the placenta is reaching the limits of its nutrient-delivery capacity. Still, your baby continues adding weight right up until delivery.
What All That Weight Gain Is For
The pounds a baby adds in the third trimester aren’t just padding. Much of the weight comes from body fat, which serves two critical purposes: insulation and energy. Newborns have a special type of tissue called brown fat that generates large amounts of heat immediately after birth. This is what keeps your baby warm during the transition from the temperature-controlled womb to the outside world. Babies born prematurely, before they’ve had time to build up these fat stores, often struggle to regulate their body temperature.
The third trimester is also when the brain undergoes explosive growth, roughly tripling in weight. The lungs are among the last organs to fully mature, with the final stages of development happening in weeks 34 through 37. This is a key reason why babies born even a few weeks early can have breathing difficulties. The weight gain during this period reflects the energy demands of finishing these vital systems.
How Twins Grow Differently
If you’re carrying twins, the growth curve looks noticeably different. Twin fetuses start falling behind singletons in weight around 26 weeks, and the gap widens as pregnancy progresses. By full term, each twin typically weighs 300 to 350 grams (about 10%) less than a single baby at the same gestational age. The difference is most pronounced in abdominal size, which accounts for over half of the weight gap. This isn’t a sign that something is wrong. It reflects the physical limits of sharing a uterus and a divided nutrient supply. Doctors expect this pattern and use different growth charts for multiples.
Factors That Affect Third Trimester Growth
Several things influence how much weight your baby gains in the final stretch. Genetics plays a baseline role, but maternal health conditions can shift the numbers significantly.
Gestational diabetes is one of the biggest drivers of above-average fetal weight. When a mother’s blood sugar runs high, extra glucose crosses the placenta and stimulates the baby to produce more insulin, which acts as a growth hormone. This can lead to macrosomia, defined as a birth weight of 8 pounds 13 ounces (4,000 grams) or more. Elevated blood fats in the mother can compound the effect, meaning glucose isn’t the only metabolic factor at play.
Maternal weight gain matters too. The Institute of Medicine sets different targets depending on pre-pregnancy weight: women at a healthy weight are advised to gain no more than about 35 pounds total, while the threshold is lower for women who start pregnancy overweight or obese (roughly 25 and 20 pounds, respectively). Exceeding these ranges raises the risk of a larger-than-expected baby.
On the other end, inadequate nutrition, smoking, and high blood pressure can restrict blood flow to the placenta and slow fetal growth. Babies affected by these factors may gain weight more slowly and measure small for their gestational age.
How Your Doctor Tracks Growth
At each prenatal visit during the third trimester, your provider will likely measure fundal height, the distance in centimeters from your pubic bone to the top of your uterus. As a rough rule, this measurement in centimeters should match your gestational age in weeks. A reading more than 3 centimeters above or below the expected number raises a flag for further investigation, usually an ultrasound.
Ultrasound estimates of fetal weight are more precise but still not exact. They work by measuring the baby’s head circumference, abdominal circumference, and thigh bone length, then plugging those numbers into a formula. These estimates carry a margin of error of about 10 to 15%, which means a baby estimated at 7 pounds could realistically weigh anywhere from about 6 to 8 pounds. This is worth keeping in mind if your provider mentions a “big baby” or a “small baby” based on a late-pregnancy scan.
If growth appears to be tracking too fast or too slow, your doctor may order additional monitoring, including more frequent ultrasounds or tests of amniotic fluid levels. In most cases, though, babies follow a predictable curve and arrive within the normal weight range of 5.5 to 10 pounds.