At 24 weeks of gestation, a fetus typically weighs around 600 grams, or roughly 1.3 pounds. That number sits at approximately the 50th percentile on standard growth charts for singleton pregnancies. But “should” is doing a lot of work in the question, because the range of healthy weights at this stage is surprisingly wide, influenced by the baby’s sex, the number of fetuses, the mother’s health, ethnic background, and the accuracy of the measurement itself.
How Fetal Weight Is Estimated
No one actually weighs a fetus at 24 weeks. The number your provider gives you is an estimate derived from ultrasound measurements of the baby’s head, abdomen, and thigh bone, which are then plugged into a mathematical formula. Several formulas exist, and they do not all agree. A systematic review of ultrasound estimation studies found that the most commonly used approach, the Hadlock formula, produced the most accurate results with the lowest random error, while formulas using only two measurements generated large and inconsistent errors across multiple studies.1PubMed Central. The accuracy of ultrasound estimation of fetal weight in comparison to birth weight: A systematic review Other formulas have their own quirks: some consistently overestimate at higher weights, while others underestimate.2PubMed Central. Comparison of Various Formulas (Hadlock’s I and II, Merz, Shepard, Campbell, Wilkin’s) to Calculate Birth Weight by Antenatal Ultrasound
The practical upshot is that ultrasound weight estimates come with a margin of error. A study of periviable neonates (babies born around the edge of viability, including those at 24 weeks) found the median difference between the estimated weight and actual birth weight was about 9%, and the estimate overshot the real weight three-quarters of the time.3PubMed. Accuracy of Ultrasound Estimated Fetal Weight in Small for Gestational Age and Appropriate for Gestational Age Grown Periviable Neonates For babies who were small for gestational age, the overestimation was even larger, averaging around 16%. So if your ultrasound says 600 grams, the real weight could reasonably be anywhere from the low 500s to the mid-600s. That matters, because at this stage of pregnancy even a 50-gram difference can shift which percentile a baby falls on and how a provider interprets the result.
What the Normal Range Looks Like
Growth charts plot fetal weight across percentiles at each week of pregnancy. The widely used U.S. standard was originally built from tens of thousands of liveborn infants combined with data from earlier gestational ages.4American Journal of Obstetrics and Gynecology. A standard of fetal growth for the united states of America At 24 weeks, the 10th percentile sits roughly around 470 to 500 grams, and the 90th percentile reaches approximately 750 to 780 grams, depending on the population used. A baby between the 10th and 90th percentiles is generally considered appropriate for gestational age.
An important distinction that often gets lost: being statistically small is not the same thing as being growth-restricted. A fetus below the 10th percentile may be constitutionally small and perfectly healthy, while a fetus that measures in the normal range could actually be growing too slowly for its own genetic potential.5PubMed Central. Abnormal Fetal Growth: Small for Gestational Age, Fetal Growth Restriction, Large for Gestational Age: Definitions and Epidemiology That is why providers look at weight trajectories over time rather than fixating on a single measurement. A fetus tracking steadily along the 15th percentile is a very different clinical picture from one that was on the 50th percentile at 20 weeks and has dropped to the 15th by 24.
Boys Versus Girls
Male fetuses tend to be slightly larger than female fetuses, and this difference is detectable well before 24 weeks. A population-based cohort study found that from the second trimester onward, males had larger head circumference and abdominal circumference measurements compared to females.6PubMed Central. Sex-specific differences in fetal and infant growth patterns: a prospective population-based cohort study Another study confirmed that sex-related differences in head measurements appeared as early as the early second trimester, with the differences in abdominal circumference becoming more prominent later in pregnancy.7PubMed. Fetal sex and intrauterine growth patterns Significant differences in estimated fetal weight between males and females have been documented across multiple biometric parameters.8PubMed. Sex-specific antenatal reference growth charts for uncomplicated singleton pregnancies at 15-40 weeks of gestation
The absolute difference in weight is small at 24 weeks, probably in the range of 15 to 30 grams, but it is enough to matter when a baby is being evaluated against a chart that does not account for sex. Some growth charts are sex-specific for this reason. If yours is not, a female fetus that appears to be tracking along the lower end of normal may simply be doing what female fetuses do.
Ethnic and Population Differences
Fetal size varies meaningfully across ethnic groups, and that variation shows up well before 24 weeks. A multi-ethnic population study found that at week 24, South Asian fetuses had a slightly lower estimated weight compared to European fetuses, while Middle Eastern and North African fetuses were slightly heavier. East Asian fetuses were smaller throughout pregnancy.9Early Human Development. Ethnic differences in fetal size and growth in a multi-ethnic population These differences persisted after adjusting for maternal factors like height, weight, and age, suggesting they reflect genuine population-level biological variation rather than simply differences in maternal nutrition or health care access.
This creates a clinical problem. If you apply a growth standard derived predominantly from one ethnic group to everyone, you risk mislabeling healthy babies as growth-restricted. The NICHD Fetal Growth Studies found that using a White-only reference standard erroneously classified as much as 15% of non-White fetuses as growth-restricted.10PubMed Central. Racial/Ethnic Standards for Fetal Growth, the NICHD Fetal Growth Studies Whether growth charts should be population-specific or universal remains debated, but you should know that the “normal” number your provider quotes is drawn from a particular reference population that may or may not match your background.
Twins and Other Multiples
If you are carrying twins, the growth expectations at 24 weeks are different from those for a singleton. A large study producing twin-specific growth charts found that twins go through a period of catch-up growth around 24 weeks, during which they track close to singleton sizes, but then fall progressively behind as pregnancy advances.11PubMed. Fetal Medicine Foundation charts for fetal growth in twins Dichorionic twins (those with separate placentas) were closest to singleton standards at around 23 weeks, while monochorionic twins (sharing a placenta) were already somewhat smaller than singletons at 24 weeks, with their 50th percentile sitting around the 36th percentile of a singleton chart.
Applying singleton growth curves to twins will almost always flag them as too small. Twin-specific charts exist precisely because the growth trajectory of multiples is different by design, not by pathology. That said, a large weight discrepancy between twins, especially monochorionic twins, is taken seriously, because it can signal uneven blood flow between the fetuses.
Maternal Health Factors That Shift Fetal Weight
Several maternal conditions can push fetal weight away from its expected range at 24 weeks, though some have a more pronounced effect later in pregnancy.
Gestational Diabetes
A common worry is that gestational diabetes leads to oversized babies, and that is true, but mostly in the third trimester. Research examining fetal measurements at 22 to 24 weeks found that the size difference between babies of mothers who later developed gestational diabetes and those who did not was tiny, less than 5 grams for estimated fetal weight.12PubMed Central. Gestational Diabetes Mellitus: Predictive Value of Fetal Growth Measurements by Ultrasonography at 22-24 Weeks: A Retrospective Cohort Study of Medical Records The accelerated growth tends to kick in after about 25 weeks, with one study finding that pregnancies affected by gestational diabetes showed a higher weekly growth rate from week 25 onward, resulting in meaningfully higher weight from around week 28.13The Journal of Clinical Endocrinology & Metabolism. Effect of Gestational Diabetes on Fetal Growth Rate and Later Overweight in the Offspring So at 24 weeks specifically, diabetes is unlikely to have visibly affected the baby’s weight yet.
Pre-eclampsia and High Blood Pressure
Pre-eclampsia has the opposite effect: it restricts fetal growth by impairing blood flow through the placenta. The birth weight difference between babies of pre-eclamptic mothers and those with normal blood pressure is largest at earlier gestational ages and narrows as pregnancy progresses. At 28 weeks, the adjusted birth-weight deficit associated with pre-eclampsia was roughly 440 grams, shrinking to about 110 grams by 39 weeks. Doppler screening of the uterine arteries at 22 to 24 weeks can identify women at higher risk for placental insufficiency, with elevated risk ratios for both pre-eclampsia and fetal growth restriction.14Fetal Diagnosis and Therapy. Screening for Placental Insufficiency by Transvaginal Uterine Artery Doppler at 22–24 Weeks of Gestation This means that a baby measuring small at 24 weeks may prompt your provider to investigate placental blood flow more closely.
Smoking and Environmental Tobacco Smoke
Maternal smoking affects fetal growth through multiple pathways, reducing blood flow to the placenta and directly exposing the fetus to toxic chemicals. A study using MRI to measure fetal organ volumes found that smoking-exposed fetuses had lower brain, kidney, and lung volumes.15PLOS ONE. Maternal Smoking during Pregnancy and Fetal Organ Growth: A Magnetic Resonance Imaging Study Even secondhand smoke exposure during pregnancy was associated with smaller fetal head measurements at the 20-to-24-week ultrasound scan.16PubMed. Environmental tobacco smoke exposure among pregnant women: impact on fetal biometry at 20-24 weeks of gestation and newborn child’s birth weight The growth-reducing effects of smoking begin in midpregnancy: reduced femur length is detectable from 18 to 24 weeks, with head and abdominal circumference reductions becoming apparent from 25 weeks onward.17American Journal of Epidemiology. Maternal Smoking and Fetal Growth Characteristics in Different Periods of Pregnancy: The Generation R Study
Genetic Contributions to Growth Restriction
When a fetus is consistently measuring well below the expected range and no maternal cause is apparent, genetic factors sometimes play a role. Chromosomal abnormalities and single-gene conditions can impair fetal growth from early in development. A review of the genetic background of fetal growth restriction noted that the causes can be fetal, placental, or maternal in origin, with chromosomal microarray and newer genetic testing techniques increasingly used to investigate unexplained cases.18PubMed Central. Genetic Background of Fetal Growth Restriction
In practice, genetic causes account for a small fraction of growth restriction cases. A study of isolated early-onset fetal growth restriction found a genetic anomaly in about 3% of cases, with most of those anomalies detected only by advanced postnatal testing rather than standard prenatal screening.19PubMed Central. Placental insufficiency markers to assess the risk of non-chromosomal genetic conditions in early-onset fetal growth restriction Interestingly, certain blood markers of placental function helped distinguish which growth-restricted babies were more likely to have a genetic cause versus a purely placental one. This is a growing area of research that may eventually change how providers evaluate unexpectedly small fetuses.
IVF and Assisted Reproduction
If you conceived through IVF or similar techniques, your baby’s growth trajectory at 24 weeks may look slightly different from population averages. A prospective study comparing frozen-thawed and fresh embryo transfers found that both IVF groups had higher estimated fetal weight than expected for natural conceptions during the midtrimester, with the difference being largest around this stage of pregnancy and gradually decreasing later on.20PubMed. Greater estimated fetal weight and birth weight in IVF/ICSI pregnancy after frozen-thawed vs fresh blastocyst transfer: prospective cohort study with novel unified modeling methodology Frozen-thawed embryo transfers showed slightly higher weight scores than fresh transfers throughout. The reasons are not fully understood but may relate to differences in early placental development or the hormonal environment at the time of implantation. If your baby is measuring a touch above the 50th percentile at 24 weeks after IVF, this pattern may partly explain it.
Why 24 Weeks Gets Special Attention
Twenty-four weeks marks a milestone in obstetric and neonatal medicine because it sits at the boundary of viability, the point at which a baby born prematurely has a meaningful chance of surviving. Across a large U.S. national database, roughly 72% of infants born at a completed 24 weeks survived, compared to only about 24% for those born before 24 weeks.21PubMed Central. Survival of infants born at periviable gestation: The US national database More recent data from over 600 U.S. hospitals showed survival ranging from about 25% at 22 weeks up to 82% at 25 weeks, with survival without severe complications ranging from about 6% at 22 weeks to 43% at 25 weeks.22Pediatrics. Survival of Infants Born at 22 to 25 Weeks’ Gestation Receiving Care in the NICU: 2020–2022
Weight matters at delivery, not just gestational age. Among babies born at 24 weeks, survival rates differ substantially depending on how much the baby weighs. A population-based study found that for European infants known to be alive at the onset of labor, predicted survival at 24 weeks ranged from about 9% at birth weights of 250 to 499 grams to 21% at 1,000 to 1,249 grams.23PubMed. Prediction of survival for preterm births by weight and gestational age: retrospective population based study Those numbers reflect an older era of neonatal care, and survival has improved considerably since then, but the basic relationship holds: bigger babies at the same gestational age do better.
This is part of why your provider monitors fetal weight. If something appears to be going wrong at 24 weeks, knowing the baby’s estimated weight helps the clinical team plan interventions, including antenatal corticosteroids to mature the baby’s lungs in case early delivery becomes necessary. Corticosteroid administration before delivery has been shown to lower neonatal death rates at each week of gestation from 21 through 27 weeks. At 24 weeks specifically, one large study found that babies who received a complete course of corticosteroids before birth had lower mortality than unexposed babies.24American Journal of Obstetrics and Gynecology. Mortality and pulmonary outcomes of extremely preterm infants exposed to antenatal corticosteroids Another study of extremely premature neonates born between 21 and 24 weeks found the neonatal death rate was about 33% with corticosteroid exposure versus 56% without it.25PubMed. The Effects of Antenatal Corticosteroids on Extremely Premature Neonates Born between 21 and 24 Weeks
When a 24-Week Weight Estimate Should Prompt Action
A single measurement at 24 weeks is just a snapshot. Providers become more concerned when the estimated weight falls below the 10th percentile, when it has dropped across percentiles since a previous scan, or when other signs of trouble accompany it, such as abnormal blood flow patterns in the umbilical cord or uterine arteries. Uterine artery Doppler screening at 22 to 24 weeks can flag women at elevated risk for placental insufficiency, carrying a roughly fourfold increased risk of fetal growth restriction when abnormal.14Fetal Diagnosis and Therapy. Screening for Placental Insufficiency by Transvaginal Uterine Artery Doppler at 22–24 Weeks of Gestation
On the other end, a baby measuring well above the 90th percentile at 24 weeks is less commonly a concern at this stage. As noted with gestational diabetes, most accelerated growth driven by maternal metabolic conditions kicks in later. A large 24-week measurement is more often a reflection of constitutional factors, meaning the baby’s own genetic size, or an overestimation by the ultrasound formula.
If your provider says the baby is measuring a bit above or below the 50th percentile and is not recommending further testing, that is usually a sign that everything falls within the expected range. The roughly 600-gram average is a midpoint, not a target, and healthy babies at 24 weeks routinely weigh anywhere from the upper 400s to the mid-700s. The trajectory matters more than any single number.