A fertilized egg starts out smaller than a grain of sand, and by 40 weeks it has grown into a baby who typically weighs somewhere around 3,000 to 4,000 grams (roughly 6.5 to 9 pounds) and measures about 50 centimeters (20 inches) from head to heel. That transformation follows a remarkably predictable curve, which is why doctors can track your baby’s size at every prenatal visit and flag anything that looks off. But the week-by-week numbers you see on pregnancy apps and websites are averages drawn from large studies, and how those numbers are measured, what they actually mean, and how much individual variation is normal all deserve more attention than the cute fruit comparisons suggest.
How Fetal Size Is Measured in the First Trimester
In the earliest weeks, the embryo is too small and curled up for meaningful length or weight measurements. From about weeks 5 through 13, the standard measurement is crown-rump length, or CRL, which is the distance from the top of the head to the bottom of the rump (legs excluded, since the baby is folded up). An early ultrasound typically captures this number, and it is one of the most accurate ways to pin down a due date. Between 9 and 13 weeks, growth by CRL is rapid and follows a tight range, meaning gestational age can be estimated quite precisely from this single measurement.1PubMed Central. International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown–rump length in the first trimester of pregnancy
To give you a rough sense of scale: at 8 weeks, the embryo’s CRL is in the ballpark of 1.5 centimeters, about the size of a raspberry. By 12 weeks it has grown to roughly 5 to 6 centimeters, comparable to a lime. These numbers are averages, and even among perfectly healthy pregnancies, some variation exists. A systematic review of CRL dating charts found considerable differences between published equations used to estimate gestational age from a given CRL, though limiting the analysis to high-quality studies narrowed that spread considerably.2PubMed. Pregnancy dating by fetal crown-rump length: a systematic review of charts
Even tiny measurement errors at this stage ripple forward. A CRL that is off by just 2 millimeters can shift a baby’s estimated weight percentile at the 20-week scan by a full 10 percentile points. Later in pregnancy the impact shrinks, but it does not disappear entirely: the same 2 mm error at first-trimester dating can still shift the estimated weight at 36 weeks by a couple of percentile points.3PubMed. Crown-rump length measurement error: impact on assessment of growth That is one reason why your provider may push for an early dating scan rather than relying solely on last menstrual period math.
Second and Third Trimester Measurements
Once the fetus outgrows the curled-up posture that makes CRL useful, clinicians switch to a set of measurements that capture different parts of the body. The anatomy scan around 18 to 22 weeks, and any growth scans that follow, typically record several values. Head circumference and biparietal diameter (the width of the skull measured ear to ear) track brain and skull growth. Abdominal circumference captures the size of the torso, which reflects liver size and subcutaneous fat. Femur length, the thigh bone measurement, serves as a proxy for overall skeletal growth. The INTERGROWTH-21st project and the WHO Fetal Growth Charts both track these five primary ultrasound measures from 14 weeks through the end of pregnancy.4The Lancet. International standards for fetal growth based on serial ultrasound measurements from the Fetal Growth Longitudinal Study of the INTERGROWTH-21st Project5PLOS Medicine. The World Health Organization Fetal Growth Charts: A Multinational Longitudinal Study of Ultrasound Biometric Measurements and Estimated Fetal Weight
Here is a rough week-by-week guide using averages from large population studies. Keep in mind these are 50th-percentile values, meaning half of healthy babies measure above and half below:
- 14 weeks: About 80 grams, roughly 9 cm head to heel
- 18 weeks: Around 190 grams, about 14 cm
- 20 weeks: About 300 grams (two-thirds of a pound), roughly 25 cm head to heel
- 24 weeks: Around 600 grams, about 30 cm
- 28 weeks: Roughly 1,000 grams (2.2 pounds), about 37 cm
- 32 weeks: About 1,700 grams (3.7 pounds), roughly 42 cm
- 36 weeks: Around 2,600 grams (5.7 pounds), about 47 cm
- 40 weeks: Roughly 3,400 grams (7.5 pounds), about 51 cm
These averages come from standardized growth references, but your baby’s actual measurements at any given week can differ from the 50th percentile by hundreds of grams and still be perfectly normal. What matters more than the absolute number is whether the growth trajectory stays consistent over time.
How Is Fetal Weight Estimated
You cannot weigh a baby while it is still inside the uterus, so the weight figure on your ultrasound report is always an estimate. The most widely used formula is the Hadlock equation, which combines abdominal circumference, head circumference, and femur length to produce an estimated fetal weight.6PubMed. Comparison of the Hadlock and INTERGROWTH formulas for calculating estimated fetal weight in a preterm population in France Other formulas exist, including the INTERGROWTH equation (which uses only abdominal and head circumferences), but the Hadlock remains the standard in most clinics worldwide.
No formula is perfectly accurate. A systematic review found that the Hadlock formula produced the lowest levels of random error among the commonly used equations, but errors are still significant, particularly in late pregnancy when getting clean measurements becomes harder due to the baby’s size and position.7PubMed Central. The accuracy of ultrasound estimation of fetal weight in comparison to birth weight: A systematic review A study comparing estimated weights with actual birth weights concluded that conventional two-dimensional ultrasound has essentially reached its accuracy ceiling for fetal weight estimation, and meaningful improvement will require fundamentally new imaging approaches.8PubMed. What are the limits of accuracy in fetal weight estimation with conventional biometry in two-dimensional ultrasound? A novel postpartum study In practical terms, that means the weight printed on your ultrasound report can be off by roughly 10 to 15 percent in either direction. A baby estimated at 3,000 grams might actually weigh 2,550 to 3,450 grams at birth.
Even in unusual situations, the Hadlock formula holds up reasonably well. When tested in fetuses with congenital diaphragmatic hernia, a condition that distorts the shape of the torso, the Hadlock formula predicted birth weight with an error of about 8 percent, which was comparable to a formula specifically designed for that condition.9PubMed Central. Accuracy of Estimated Fetal Weight Assessment in Fetuses with Congenital Diaphragmatic Hernia-Is the Hadlock Formula a Reliable Tool?
What Growth Charts Actually Tell You
When your provider plots your baby’s measurements on a growth chart, they are comparing those numbers against a reference population. The two most prominent international standards are the INTERGROWTH-21st charts and the WHO Fetal Growth Charts. The INTERGROWTH-21st project was specifically designed to create prescriptive standards showing how fetuses grow under optimal conditions, drawing on healthy pregnancies from multiple countries and matching the approach used for postnatal WHO child growth standards.10PubMed. The INTERGROWTH-21(st) fetal growth standards: toward the global integration of pregnancy and pediatric care The charts show percentile curves: the 50th percentile is the middle, the 10th is on the smaller side of normal, and the 90th is on the larger side.
Some providers use customized growth charts instead, which adjust expected fetal size for factors like maternal height, weight, ethnicity, and the baby’s sex. A study comparing these two approaches found that the customized standard identified small-for-gestational-age babies more accurately than the INTERGROWTH-21st standard, with sensitivity of about 39 percent versus 25 percent.11PubMed. Customized fetal growth standard compared with the INTERGROWTH-21st century standard at predicting small-for-gestational-age neonates The debate over which approach is better remains active in obstetrics. Universal charts are simpler and more standardized, but customized charts may do a better job distinguishing a baby who is constitutionally small (because the parents are small) from one whose growth has genuinely slowed.
Boys and Girls Grow Differently Before Birth
Sex differences in fetal size appear surprisingly early. In the first trimester, male embryos already measure slightly longer on CRL. From the second trimester onward, the differences become more specific: males have larger head circumferences and slightly larger abdominal circumferences, but females actually have longer femurs relative to their body size.12PubMed Central. Sex-specific differences in fetal and infant growth patterns: a prospective population-based cohort study A separate study found that the difference in head circumference and biparietal diameter between boys and girls was significant from 20 weeks onward, equating to roughly a 3-day difference in apparent gestational age at 20 to 24 weeks.13PubMed Central. Sex differences in fetal growth and immediate birth outcomes in a low-risk Caucasian population
At birth, boys are on average heavier, longer, and have a larger head circumference. Male sex is also a risk factor for macrosomia, the term for an unusually large baby. In one study, being male roughly doubled the odds of macrosomia compared to being female.14PubMed Central. Risk Factors for Macrosomia These differences are modest enough that most growth charts still use a single set of curves for both sexes, though some providers apply sex-specific references when available.
Genetics and the Maternal Environment
How much of your baby’s size is determined by genes and how much by the environment inside the uterus? The answer shifts depending on when during pregnancy you ask. A twin study found that shared environment (mostly the uterine environment) explained over half of the variation in fetal size measurements during the first trimester, while by the late second trimester, fetal genetics dominated, accounting for more than half to as much as 72 percent of the variation depending on which body measurement was analyzed.15Scientific Reports. Genetic and Environmental Influences on Fetal Growth Vary during Sensitive Periods in Pregnancy That finding suggests the first trimester is a window where environmental interventions like nutrition could have an outsized effect on early growth.
Both the baby’s own genes and the mother’s genes contribute to birth weight, but they do so through different channels. A meta-analysis found that a higher fetal genetic score for birth weight was associated with about 98 grams more birth weight per standard deviation increase. Maternal genetics independently contributed about 42 extra grams per standard deviation, acting through the intrauterine environment rather than being directly inherited by the baby.16PubMed Central. Genetic and maternal environmental contributions to estimated fetal weight at 20 weeks gestation compared with birthweight A study using IVF families (where the mother is not always genetically related to the child) confirmed that maternal height influences infant birth weight and head circumference even when the mother and baby share no genes, pointing to a physical constraint imposed by the mother’s body size on fetal growth.17PubMed Central. Estimating the relative contributions of maternal genetic, paternal genetic and intrauterine factors to offspring birth weight and head circumference
The placenta is the mediator between maternal supply and fetal demand. It actively adjusts its nutrient transport depending on what is available. When maternal nutrition is poor or placental blood flow is restricted, the activity of nutrient transporters, especially for amino acids, decreases in late pregnancy, effectively slowing fetal growth to match what the mother’s body can provide.18PubMed Central. Placental transport in response to altered maternal nutrition In the opposite direction, conditions like gestational diabetes can ramp up nutrient delivery and produce a larger-than-expected baby. Reduced placental nutrient transporter activity has been documented across several conditions linked to delivering a growth-restricted infant.19Frontiers in Physiology. Placental Nutrient Transport and Intrauterine Growth Restriction
When Growth Falls Outside the Expected Range
Fetal growth restriction, where the baby is not growing as expected for its gestational age, is broadly divided into early-onset and late-onset forms. Early-onset growth restriction, diagnosed before 32 weeks, tends to involve more serious problems with placental implantation and higher rates of complications. Late-onset restriction, appearing at or after 32 weeks, usually involves milder placental insufficiency and carries lower risk, though it still needs monitoring.20PubMed. Fetal growth restriction: current knowledge Doppler ultrasound, which measures blood flow in the umbilical cord and fetal blood vessels, is critical for distinguishing a baby who is constitutionally small from one whose growth has been restricted by a failing placenta.
On the large end, macrosomia generally refers to a birth weight above 4,000 grams (about 8 pounds 13 ounces), though some definitions use 4,500 grams. The strongest risk factor identified in one study was having previously delivered a macrosomic baby, which increased the odds more than thirteenfold. Maternal weight gain of 16 kilograms (about 35 pounds) or more during pregnancy increased the odds tenfold.14PubMed Central. Risk Factors for Macrosomia Having a male baby, a father with a high BMI, and going past your due date all contribute smaller but still meaningful increases in risk.
How Twin Growth Differs from Singleton Growth
If you are carrying twins, the size expectations change. Twins track closely with singletons through the first half of pregnancy, but growth trajectories begin to diverge in a meaningful way around the middle of the second trimester. Abdominal measurements in twins were significantly smaller than in singletons starting at 15 weeks, with consistently larger differences appearing from 25 weeks onward.21JAMA Pediatrics. Fetal Body Composition in Twins and Singletons By 37 weeks, the average optimal weight for a twin fetus was about 389 grams less than for a singleton with the same maternal characteristics.22PubMed. Customized growth charts for twin vs singleton pregnancies and their ability to identify small for gestational age-associated risk of adverse perinatal outcome
This matters clinically because applying singleton growth charts to twin pregnancies leads to massive overdiagnosis of growth problems. When singleton standards were applied to twins, 44 percent were classified as small for gestational age, compared to only about 13 percent when twin-specific charts were used. The twins flagged as small only by singleton standards (but not by twin standards) had no increased risk of stillbirth, meaning those flags were essentially false alarms.22PubMed. Customized growth charts for twin vs singleton pregnancies and their ability to identify small for gestational age-associated risk of adverse perinatal outcome Twin-specific charts also produced stronger associations between diagnosed growth restriction and actual adverse outcomes like preterm birth, suggesting they better separate truly at-risk babies from those who are simply smaller because they are sharing space.23PubMed Central. The Association Between Hypertension in Pregnancy and Preterm Birth with Fetal Growth Restriction in Singleton and Twin Pregnancy: Use of Twin Versus Singleton Charts
What Happens When Pregnancy Goes Past the Due Date
Babies do not stop growing at 40 weeks. Post-term pregnancies (those extending beyond 42 weeks) carry a higher incidence of macrosomia, roughly double the rate seen in term deliveries.24Obstetrics, Gynaecology & Reproductive Medicine. Prolonged pregnancy That extra size is itself a complication, making labor longer and delivery more difficult. But size is only half the story. About 20 percent of post-term fetuses develop what is called dysmaturity syndrome, which paradoxically resembles chronic growth restriction: thin, wrinkled skin, a malnourished appearance, and long nails. This happens because the placenta begins to age and lose efficiency, reducing nutrient and oxygen delivery even as the pregnancy continues.25PubMed Central. Postterm pregnancy So post-term babies can be either unusually large or show signs of wasting, depending on how well the placenta holds up.
Beyond the Tape Measure
Before ultrasound became routine, and still today in many settings, fundal height was the primary tool for tracking fetal growth. That measurement, the distance from the pubic bone to the top of the uterus in centimeters, roughly corresponds to the number of weeks of gestation from about 20 weeks onward (so at 30 weeks you would expect something close to 30 cm). It remains an inexpensive screening method for fetal growth problems.26PubMed. Fetal growth screening by fundal height measurement Fundal height is far less precise than ultrasound and can be thrown off by factors like the amount of amniotic fluid, the mother’s body type, and whether the baby is lying transversely, but in settings without access to routine ultrasound, it is the frontline tool for catching babies that are growing too slowly or too quickly.
Ultrasound itself keeps evolving. Head measurements taken by MRI, for instance, systematically overestimate head circumference by about 14 mm compared to ultrasound, partly because skull ossification in late pregnancy can distort the ultrasound image. Researchers have developed correction models that translate MRI values back to ultrasound-equivalent ranges, which improves diagnostic accuracy substantially.27Frontiers in Radiology. Multimodal assessment of fetal head circumference and biparietal diameter: systematic differences between MRI and US measurements and development of an US-equivalent prediction regression model MRI is not used for routine growth monitoring, but it increasingly plays a role when ultrasound images are poor or when brain abnormalities need clarification.
Why Human Babies Are Born So Fat
One of the most distinctive features of human fetal growth, and one that rarely comes up in pregnancy apps, is fat deposition. Human newborns are dramatically fatter than the babies of other primates. Both the brain and body fat each make up roughly 11 to 14 percent of a human newborn’s weight, a combination that appears to be unique among land mammals.28PubMed. Survival of the fattest: fat babies were the key to evolution of the large human brain That fat is not just insulation. It serves as a fuel reserve (fatty acids stored in triglycerides), a source of precursors for ketone bodies that help build brain lipids, and a store of long-chain polyunsaturated fatty acids essential for brain development.
Compared to other primates, humans invest a relatively small proportion of daily maternal energy into fetal tissue, but our high-quality diets and long gestation allow us to produce babies with both a larger brain-to-body-weight ratio and more body fat at birth than almost any other primate.29PubMed. Comparative energetics of primate fetal growth Much of the third trimester’s rapid weight gain is driven by this fat accumulation, which is why babies born very prematurely look so strikingly lean compared to full-term infants. The composition of maternal weight gain during pregnancy matters here: gains in lean tissue and body water were correlated with higher birth weight, while gains in maternal fat mass were not, suggesting that what the mother’s body directs toward building the baby and the placenta matters more for fetal size than what gets stored as maternal fat reserves.