A fetus gains roughly half a pound per week during the third trimester, when the most dramatic weight changes happen. Before that, weekly gains are measured in fractions of an ounce. The total journey from a cluster of cells to an average 7.5-pound newborn follows a predictable curve, but the rate accelerates sharply in the final 12 weeks of pregnancy.
First and Second Trimester Growth
For most of the first trimester, a fetus weighs almost nothing in practical terms. By the end of week 12, it weighs roughly half an ounce (14 grams). Weekly gains during this period are tiny because the body is focused on forming organs, limbs, and the nervous system rather than putting on mass.
During the second trimester, weight picks up more noticeably. By week 20, a fetus typically weighs around 10 to 11 ounces. By week 24, that number jumps to about 1.3 pounds. The weekly gain through the second trimester averages somewhere around 2 to 3 ounces, though it’s not perfectly uniform. Fat deposits begin forming under the skin around week 24, which marks the start of the transition from organ building to rapid weight gain.
Third Trimester: The Fastest Growth Phase
The third trimester is where the numbers become striking. Here’s what average fetal weight looks like at key checkpoints, based on Mayo Clinic data:
- Week 28: about 2.25 pounds (1,000 grams)
- Week 30: nearly 3 pounds (1,300 grams)
- Week 32: about 3.75 pounds (1,700 grams)
- Week 34: more than 4.5 pounds (2,100 grams)
- Week 38: about 6.5 pounds (2,900 grams)
- Week 40: about 7.5 pounds (3,400 grams)
Between weeks 28 and 34, the fetus gains roughly 200 grams (about 7 ounces) per week. That’s close to half a pound every seven days. From weeks 34 to 38, the pace stays high at around 200 grams per week. In the final stretch from week 38 to 40, growth slows slightly to about 250 grams (roughly 8 ounces) over two weeks, as the placenta nears the end of its capacity and the baby runs out of room.
This late-pregnancy surge happens because fat accumulation accelerates dramatically. Fetal fat tissue, which helps regulate body temperature after birth, builds rapidly in the late second and third trimesters. Lipids from the mother’s bloodstream become a key fuel source for this fat storage, which is why maternal nutrition in the final months has an outsized effect on birth weight.
What Affects How Fast a Fetus Gains Weight
These weekly averages are just that: averages. Several factors push individual babies above or below the curve.
Maternal health plays the biggest role. Gestational diabetes increases the amount of glucose crossing the placenta, which triggers the fetus to produce more insulin. That extra insulin acts as a growth hormone, driving fat storage and producing a larger baby. Smoking has the opposite effect, restricting blood flow to the placenta and often resulting in lower birth weight. Maternal malnutrition and high blood pressure can also slow fetal growth by limiting nutrient delivery.
The placenta itself matters enormously. It controls the flow of glucose, amino acids, and fatty acids to the fetus, and its size, blood vessel density, and transport efficiency all influence how much nutrition actually reaches the baby. A placenta that functions well supports steady growth. One that doesn’t, whether from preeclampsia or other complications, can restrict it.
Genetics set the baseline. Family history of larger or smaller body size, along with inherited growth pathways, shapes how a fetus responds to available nutrients. Babies also inherit growth patterns from both parents, which is why birth weight often runs in families.
How Twins Differ From Singletons
If you’re carrying twins, the growth curve looks different. Each twin individually gains weight at a similar rate to a singleton through the early third trimester, but growth slows earlier as space and placental resources are shared. Twins are typically smaller at birth, with each baby averaging around 5 to 5.5 pounds at full term rather than 7.5 pounds. The maternal weight gain target for a twin pregnancy is higher overall (about 35 to 45 pounds total), with a recommended gain of roughly 1.5 pounds per week during the second and third trimesters to support both babies.
How Accurate Are Ultrasound Weight Estimates
If your doctor has estimated your baby’s weight during an ultrasound, it’s worth knowing that these numbers come with a built-in margin of error. The accepted accuracy for ultrasound weight estimates is plus or minus 15%. That means a baby estimated at 6 pounds could actually weigh anywhere from about 5.1 to 6.9 pounds.
The formulas used to calculate weight rely on measurements of the baby’s head, abdomen, and thigh bone. The most accurate formulas, which combine all three measurements, achieve that 15% confidence window. Simpler formulas that use fewer measurements can be off by as much as 22%. Accuracy also drops at the extremes: estimates for unusually large babies carry the highest error rates.
This means week-to-week comparisons from ultrasound can be misleading. A baby who appears to have gained a pound in two weeks may have gained somewhat more or less. The overall growth trend across multiple appointments is more reliable than any single measurement.
Week-by-Week Summary Across Pregnancy
To put the full picture together: in the first trimester, weekly weight gain is negligible, measured in grams. During the second trimester, the fetus gains roughly 2 to 3 ounces per week. In the early third trimester (weeks 28 to 34), that jumps to about 7 ounces (200 grams) per week. The peak growth rate hits around weeks 32 to 36, when gains can reach half a pound or slightly more per week. In the final weeks before delivery, the pace eases off as the baby reaches full size.
Size at birth varies considerably. While 7.5 pounds is a common average at 40 weeks, some babies weigh close to 9 pounds or more, and others are healthy at 6 pounds. The range reflects the combined influence of genetics, placental function, maternal nutrition, and how long the pregnancy lasts.