At seven months pregnant (roughly 28 weeks), a baby typically weighs about 1,200 grams, or around 2.6 pounds. Over the course of the seventh month, which spans weeks 28 through 31, that number climbs steadily to about 1,730 grams (3.8 pounds) by week 31. This is the start of a rapid growth phase where your baby will roughly triple in weight before birth.
Week-by-Week Weight During Month Seven
The seventh month of pregnancy covers weeks 28 through 31, and babies gain close to half a pound each week during this stretch. Here’s what the 50th percentile (the statistical middle of the range) looks like:
- Week 28: about 1,209 g (2.7 lb), roughly the size of a large eggplant
- Week 29: about 1,372 g (3.0 lb), comparable to a butternut squash
- Week 30: about 1,546 g (3.4 lb), similar to a large cabbage
- Week 31: about 1,730 g (3.8 lb), close to the size of a coconut
By week 28, average crown-to-heel length is about 25 cm (9.8 inches), though this measurement grows less dramatically than weight over the coming weeks. Most of the energy your baby’s body is spending right now goes toward building fat stores and growing the brain, lungs, and liver.
What Counts as a Normal Range
No two babies grow at exactly the same rate, so doctors look at a range rather than a single number. At 28 weeks, a baby at the 10th percentile weighs around 1,037 grams (2.3 lb), while one at the 90th percentile weighs about 1,380 grams (3.0 lb). By week 31, that spread widens: the 10th percentile sits near 1,485 grams (3.3 lb) and the 90th near 1,976 grams (4.4 lb).
Both ends of that range are considered healthy. What matters more than a single measurement is the growth trend over time. A baby who has been tracking along the 20th percentile throughout pregnancy is likely fine, even though they’re smaller than average. A baby who drops from the 60th percentile to the 15th over a few weeks is more likely to get extra monitoring, because the change in trajectory can signal a problem even when the absolute weight still looks reasonable.
Small for Gestational Age
Babies whose estimated weight falls below the 10th percentile for their gestational age are classified as small for gestational age (SGA). At 28 weeks, that threshold is roughly 1,037 grams. Being SGA doesn’t automatically mean something is wrong. Some babies are simply genetically small, particularly if both parents are petite. But it can also be a sign of restricted blood flow through the placenta, so your care provider will usually schedule additional ultrasounds to track growth more closely.
How Fetal Weight Is Measured
There’s no way to put a baby on a scale while it’s still in the womb. Instead, ultrasound measurements of the head, abdomen, and thigh bone are plugged into a formula that produces an estimated fetal weight. This estimate is reasonably accurate but not perfect. In a large study, the ultrasound estimate fell within 10% of the actual birth weight about 72% of the time. The average error was small (around 2%), but individual readings can be off by more, especially for very small or very large babies. Larger babies tend to have their weight underestimated, while smaller babies are more often overestimated.
If your ultrasound report shows a weight that seems surprisingly high or low, keep that margin of error in mind. A reading of 1,100 grams at 28 weeks could mean the baby actually weighs anywhere from about 990 to 1,210 grams. That’s a wide enough window that one measurement alone rarely justifies alarm.
What Affects Your Baby’s Size
Several factors influence how much your baby weighs at this stage, and some are outside your control.
Maternal height and BMI both have a positive association with fetal growth in the third trimester. Taller parents tend to have longer, heavier babies. A higher pre-pregnancy BMI is also linked to larger fetal size, independent of gestational diabetes. Pre-existing diabetes has one of the strongest effects, pushing fetal growth higher because the baby is exposed to elevated blood sugar, which acts like extra fuel.
Smoking has the opposite effect. It consistently reduces third-trimester fetal growth, likely because it restricts blood flow through the placenta. Even cutting back (rather than quitting entirely) can improve growth outcomes during this period.
Baby’s sex plays a role too. Male fetuses tend to be slightly heavier than female fetuses at the same gestational age, though the difference is modest at 28 weeks and becomes more pronounced closer to term.
Nutrition matters, but in a less dramatic way than many people assume. Severe calorie restriction can limit fetal growth, but for most people eating a reasonably balanced diet, the placenta does an efficient job of extracting what the baby needs. You don’t need to eat large quantities of food to support healthy growth at this stage. Quality, particularly adequate protein, iron, and calcium, matters more than volume.
Why Growth Picks Up Now
The seventh month marks the beginning of the third trimester, and it’s a turning point in how your baby grows. During the first and second trimesters, the body is focused on building organs, forming limbs, and developing the nervous system. By 28 weeks, most of those structures are in place. Now the priority shifts to gaining weight, mainly by laying down fat beneath the skin.
This fat serves two critical purposes. First, it helps regulate body temperature after birth. Second, it provides an energy reserve for the first days of life, before breastfeeding or formula feeding is fully established. A baby born at 28 weeks can survive with intensive medical support, but the extra weeks of fat accumulation that happen between now and full term make a significant difference in how smoothly the transition to life outside the womb goes.
Between weeks 28 and 40, most babies will gain roughly 4.5 to 6.5 pounds. That growth isn’t evenly distributed: it accelerates through weeks 28 to 36 and then slows slightly in the final month as the baby runs out of room.