How Much Does a Baby Weigh at 30 Weeks: Averages & Ranges

At 30 weeks of pregnancy, the average singleton baby weighs roughly 1,300 to 1,500 grams, or about 3 pounds. That number sits at the midpoint of a range that can stretch a few hundred grams in either direction and still be perfectly normal. Where your baby falls within that range depends on a mix of genetics, placental health, maternal nutrition, and even fetal sex, and the number your provider gives you after an ultrasound is itself an estimate with a built-in margin of error.

How Providers Arrive at an Estimated Fetal Weight

No one can put a baby on a scale while it is still in the womb. Instead, fetal weight is calculated from ultrasound measurements of the baby’s head circumference, biparietal diameter (the width of the skull), abdominal circumference, and femur length. These four measurements get plugged into a formula, most commonly the Hadlock equation, to produce an estimated fetal weight, or EFW.¹ The word “estimated” matters here. A systematic review of ultrasound weight predictions found significant error levels across the board, with accuracy declining in late pregnancy partly because it becomes harder to get clear measurements of a larger, more cramped baby.² Operator skill and training also play a role in how close the estimate comes to reality.

A meta-analysis looking at how well ultrasound-based weight projections hold up over time found a mean absolute error of about 240 grams and a mean absolute percentage error of roughly 8%.³ At 30 weeks, when the average weight is in the ballpark of 1,400 grams, an 8% error margin translates to roughly 110 grams in either direction. That means a baby estimated at 1,400 grams could realistically weigh anywhere from about 1,290 to 1,510 grams. The takeaway is that ultrasound weight estimates are useful guides, not precise measurements, and a small discrepancy from the “average” on one scan rarely means something is wrong.

Why the “Average” Number Varies by Source

If you look up fetal weight at 30 weeks on different websites or in different books, you will see slightly different numbers. Part of the reason is that there is no single universal growth chart. The Hadlock reference curves, developed in the 1980s and 1990s from a largely White American population, remain widely used. More recently, the INTERGROWTH-21st project created an international standard drawn from healthy pregnancies across multiple countries. A comparison of these two charts in the third trimester found that they produce somewhat different weight estimates at the same gestational age. At one third-trimester scan, Hadlock yielded a mean EFW of about 1,967 grams while INTERGROWTH-21st came in at roughly 1,862 grams, a gap of over 100 grams, though both correlated strongly with each other.⁴ That particular scan was at a later gestational age than 30 weeks, but the principle holds: which chart your provider uses shifts the number your baby is compared against.

Customized growth charts attempt to solve this by adjusting for the mother’s height, weight, ethnicity, and parity. Research from South Korea found that a customized chart identified a different set of small-for-gestational-age pregnancies than a population-based chart and was substantially better at predicting which of those babies were truly at risk.⁵ In practice, this means the “normal range” at 30 weeks for a baby born to a petite mother differs from the normal range for a baby born to a tall mother, and both can be healthy.

Sex Differences in Fetal Weight

Boys tend to be slightly heavier than girls at every point during the third trimester. A study of fetal growth in a low-risk population found that estimated fetal weight was consistently different between male and female fetuses from 20 weeks onward. By 24 weeks, boys were already about 21 grams heavier than girls at the 50th percentile, and girls reached the 500-gram milestone roughly a day later than boys.⁶ The gap widens as pregnancy progresses. At 30 weeks, the difference is modest enough that it does not change clinical management in most cases, but it does mean that sex-specific growth charts are slightly more accurate than sex-neutral ones. If you know the baby’s sex, your provider may factor that into how they interpret the percentile.

Maternal and Ethnic Factors That Shift the Range

Fetal growth is largely dictated by how well nutrients travel from the mother’s bloodstream through the placenta to the baby. Changes in placental nutrient transporters are linked to both restricted and excessive growth, essentially representing the body’s attempt to match fetal growth rate to the nutrient supply available.⁷ This mechanism helps explain why maternal factors have such a strong influence on how much a baby weighs at any given gestational age.

Maternal weight gain during pregnancy is one of those factors. A sibling-comparison study, which helps isolate the effect of weight gain from family-level genetics, found that for a woman with a BMI of 25, each standard-deviation increase in pregnancy weight gain was associated with about 89 grams of additional birth weight after adjusting for confounders.⁸ Research on twin pregnancies highlighted that weight gain during the second trimester seems to be a particularly strong predictor of fetal growth, especially in women who started pregnancy at a normal weight.⁹

Ethnicity also plays a measurable role. A large U.S. study, the NICHD Fetal Growth Studies, found that estimated fetal weight differed significantly by racial and ethnic group after 20 weeks. When a growth standard derived only from White participants was applied across all groups, as many as 15% of non-White fetuses were erroneously classified as growth-restricted.¹⁰ A separate analysis confirmed that Black, Hispanic, and Asian mothers tended to have fetuses with smaller head circumference, abdominal circumference, and biparietal diameter compared with White mothers, though the researchers cautioned that the reasons behind these differences, whether biological, nutritional, socioeconomic, or some combination, remain poorly understood.¹¹ The practical upshot is that a 30-week weight that looks small on one chart may be perfectly appropriate when the mother’s background is taken into account.

Altitude and Other Environmental Influences

Where you live can affect how much your baby weighs. A review of studies on pregnancies at high altitude found a significant inverse relationship between altitude and birth weight: for every 1,000-meter increase in elevation, birth weight dropped by about 91 grams on average.¹² The mechanism is thought to involve reduced oxygen availability affecting placental development and nutrient transfer. For someone living in a city like Denver (about 1,600 meters) versus a city at sea level, the expected difference would be modest, but populations in the Andes or the Tibetan Plateau see more pronounced effects.

There is also emerging research on environmental chemical exposures. A study of maternal urinary bisphenols and phthalates found sex-specific associations with estimated fetal weight in mid-to-late pregnancy, with some chemicals linked to decreased weight in smaller-than-average female babies and increased weight in larger ones, and the opposite pattern in males.¹³ The effects were relatively small and the clinical significance remains unclear, but the research illustrates how many variables feed into fetal size.

The Balance Between Genes and Environment

An interesting question is how much of the variation in fetal weight at 30 weeks comes from the baby’s genes versus the uterine environment. A twin study that tracked this balance across pregnancy found that the genetic contribution to estimated fetal weight rises steadily from about 17% at the end of the first trimester to a peak of roughly 71% in the late second trimester, then declines slightly to about 66% near term. Meanwhile, the shared environment’s influence moves in the opposite direction, falling from about 54% early on to just 7% near term.¹⁴ At 30 weeks, genetics are already the dominant driver, which helps explain why two mothers with similar diets and lifestyles can still have babies of quite different sizes.

What Happens When Weight Falls Outside the Expected Range

Providers pay close attention when a baby’s estimated weight drops below the 10th percentile for gestational age, a threshold that defines “small for gestational age,” or SGA. Detecting this early matters because SGA babies face higher risks of complications during delivery and afterward. Simple clinical tools like measuring the distance from the pubic bone to the top of the uterus (fundal height) catch only some of these cases. One study comparing fundal height measurement to handheld ultrasound found that fundal height detected about 43% of fetuses with an estimated weight below the 10th percentile, while ultrasound-based abdominal circumference screening caught all of them.¹⁵

When a provider suspects poor growth at 30 weeks, they often look deeper at the placenta and blood flow. Research using advanced ultrasound imaging has shown that pregnancies resulting in SGA babies have significantly lower placental vascularization and abnormal blood flow patterns in the uterine and middle cerebral arteries compared to appropriately grown babies.¹⁶ Additional screening combining uterine artery Doppler and blood pressure measurements at 30 to 34 weeks can improve prediction of which babies will be born too small.¹⁷

Preeclampsia and Fetal Weight

Preeclampsia, a pregnancy complication marked by high blood pressure, can have a substantial effect on how much a baby weighs, and the earlier it develops, the worse the impact. One study found that early-onset preeclampsia was associated with birth weights about 23% lower than expected, while the overall reduction with preeclampsia of any severity was about 5%. The risk of being born small for gestational age was roughly four times higher in preeclamptic pregnancies.¹⁸ Another study looking at the timing of delivery found that the birth-weight gap between preeclamptic and healthy pregnancies was largest when delivery happened early. At 28 weeks, the adjusted weight deficit was about 440 grams. By 33 weeks, it had narrowed to roughly 330 grams, and by 39 weeks it was only about 110 grams.¹⁹ For a 30-week scan, this means that if you have preeclampsia, your provider will be watching fetal growth especially closely because the condition’s impact on weight is most severe during this period.

Diabetes and Excessive Growth

While preeclampsia tends to restrict growth, maternal diabetes can push a baby’s weight in the other direction. Research on pregnancies complicated by pregestational diabetes found that the abdominal circumference of babies who went on to be large for gestational age was already measurably bigger from 18 weeks onward, with the difference growing progressively larger. The maximum divergence from normally growing babies occurred between 30 and 38 weeks.²⁰ This is why blood sugar control during pregnancy is monitored so carefully: excess glucose in the mother’s blood crosses the placenta and can drive the baby to pack on weight at an accelerated rate, particularly during the third trimester when the growth trajectory naturally steepens.

Twins at 30 Weeks

If you are expecting twins, the weight benchmarks at 30 weeks look different. It is well established that twins have divergent growth trajectories starting around 28 to 30 weeks, and they are generally born smaller than singletons.²¹ Applying singleton growth charts to twins tends to overestimate how many are “too small,” which is why twin-specific references exist. At 30 weeks, each twin might weigh around 1,100 to 1,350 grams. The shared space, shared (or individual) placenta, and competition for nutrients all contribute to the smaller sizes. Providers monitoring twins will also watch for discordant growth, where one twin is significantly smaller than the other, which raises different clinical concerns than both twins being symmetrically small.

What a 30-Week Weight Estimate Actually Tells You

A single weight estimate at 30 weeks is less informative than the growth trend over time. A baby estimated at the 20th percentile who has been tracking at the 20th percentile since 24 weeks is in a very different situation than a baby who was at the 50th percentile at 24 weeks and has dropped to the 20th. The first baby is probably constitutionally small. The second baby may be experiencing growth restriction, and the declining trend is what triggers further investigation. Providers usually compare at least two measurements taken a few weeks apart to see whether the baby is maintaining its trajectory on the growth curve.

It is also worth keeping in mind that the third trimester is when fetal weight gain accelerates most dramatically. A baby at 30 weeks weighing about 1,400 grams will roughly double its weight over the final ten weeks if growth proceeds normally. That rapid gain means small differences in timing can make a big difference in the number you see on a scan report. A scan at 29 weeks and 5 days versus 30 weeks and 3 days could produce meaningfully different estimates simply because a few days of rapid growth separates them. If your provider says everything looks on track, a number that seems low compared to what you read online is usually not cause for worry.

For the same reason, fetal weight estimates are best interpreted alongside other ultrasound findings rather than in isolation. Amniotic fluid volume, placental appearance, Doppler flow measurements in the umbilical cord, and the baby’s movement patterns all give the provider context that a single weight number cannot. A 30-week baby at the 8th percentile with normal blood flow and normal fluid is managed differently than one at the same percentile with reduced flow and low fluid. The weight is the starting point of the conversation, not the end of it.