On the World Health Organization growth charts, the median weight for a 6-month-old boy is about 7.9 kg (roughly 17.4 lb), while the median for a 6-month-old girl is about 7.3 kg (roughly 16.1 lb). That gap of around 600 grams is consistent and expected, driven by differences in lean tissue that emerge well before birth. But “average” on a growth chart is just a midpoint on a curve, and the healthy range stretches wide in both directions.
What the WHO Growth Standards Actually Represent
The numbers above come from the WHO Child Growth Standards, which were built from data on healthy, breastfed children in six countries across five continents. The WHO charts are a growth standard rather than a mere reference: they describe how children should grow under optimal conditions, not just how a particular group of children happened to grow. Every infant in the reference population was breastfed for at least 12 months and predominantly breastfed for at least four months.1PubMed. Use of World Health Organization and CDC growth charts for children aged 0-59 months in the United States That distinction matters because feeding patterns shape early weight gain, and using a reference built on mixed-fed or predominantly formula-fed babies shifts what “normal” looks like.
The charts present weight as a distribution. At 6 months a boy at the 3rd percentile might weigh around 6.4 kg, while one at the 97th percentile might weigh close to 9.8 kg. Both are within the expected range. Girls follow a similarly wide spread shifted slightly lower. A single weight reading tells you where your baby falls on the curve at one moment in time; the real information comes from tracking how that position changes over weeks and months.
Why Boys Are Heavier Than Girls This Early
Boys tend to outweigh girls from birth onward, and the gap at 6 months is not just a scaled-up version of the birth difference. Part of the explanation starts in utero: androgen activity during fetal development drives faster weight gain in male fetuses, so boys are already heavier at delivery.2PubMed. Fetal growth: boys before girls After birth, the divergence continues and becomes more specifically about body composition rather than just total weight.
By about five months, male infants have accumulated roughly 410 grams more fat-free mass (muscle, bone, organ tissue) than female infants, gaining about 17 grams more lean tissue per week on average.3PubMed Central. Sex Differences in Infant Body Composition Emerge in the First 5 Months of Life Fat mass, by contrast, is roughly similar between the sexes in absolute terms, though girls carry a higher percentage of their body weight as fat. At 6 months, international body composition data show that boys and girls both have a median fat mass of about 1.9 kg, but boys have a fat-free mass of about 5.6 kg compared with 5.1 kg in girls.4The American Journal of Clinical Nutrition. International Standards for Infant Body Composition So most of the weight gap at this age is lean tissue, not fat.
UK reference data on total body water and fat-free mass in infants confirm the same pattern: fat-free mass increases faster in boys during the first two years, while fat mass trajectories are broadly similar between sexes, with fat accumulation dropping off sharply after the first three months regardless of sex.5European Journal of Clinical Nutrition. Body composition reference charts for UK infants and children aged 6 weeks to 5 years based on measurement of total body water by isotope dilution This means the visible chubbiness of a 6-month-old is not a sex-linked trait. Both boys and girls carry plenty of baby fat at this age; boys just also carry more muscle and bone beneath it.
WHO Charts Versus CDC Charts
If you live in the United States, your pediatrician likely uses WHO charts for children under two and then switches to CDC charts from age two onward, following the CDC’s own recommendation. The CDC charts are a growth reference: they describe how American children actually grew during a specific era, including both breastfed and formula-fed babies. The WHO charts, as noted earlier, were designed as a prescriptive standard based on breastfed infants in healthy environments.6Morbidity and Mortality Weekly Report. Use of World Health Organization and CDC Growth Charts for Children Aged 0–59 Months in the United States
The practical difference shows up right around six months. WHO-standard infants tend to be slightly heavier than the CDC median during the first half of infancy but then cross below it around six months and stay below it until about 32 months, at which point the two medians converge.7The American Journal of Clinical Nutrition. Comparison of the WHO Child Growth Standards and the CDC 2000 Growth Charts In plain terms, the CDC reference population was somewhat heavier, probably because it included more formula-fed infants. A breastfed baby who appears to be “slowing down” on a CDC chart after three or four months is often growing perfectly normally by WHO standards.
The chart you use changes how many babies get flagged for being too light or too heavy. Using WHO charts, fewer U.S. children are classified as underweight after six months (under 3%, versus 7 to 11% on the CDC reference), and fewer infants under 12 months are classified as having high weight-for-length.6Morbidity and Mortality Weekly Report. Use of World Health Organization and CDC Growth Charts for Children Aged 0–59 Months in the United States So the choice of chart is not just a bureaucratic detail. It can determine whether your baby gets labeled as a concern or given a clean bill of health.
Factors That Shift an Individual Baby’s Weight
The median is a population summary. Individual babies deviate from it for a long list of reasons, some of which matter clinically and some of which are just part of normal variation.
Feeding method. Breastfed babies often gain weight rapidly in the first three months and then decelerate relative to formula-fed babies. By six months, formula-fed infants tend to have a higher weight-for-length score than exclusively breastfed infants.8PubMed Central. Growth Indices of Exclusively Breastfed Until 6 Months Age and Formula-Fed Infants in Southwest of Iran Meanwhile, longer duration of breast milk feeding is associated with somewhat lower weight outcomes across the first year.9PubMed Central. Associations between breast milk feeding, introduction of solid foods, and weight gain in the first 12 months of life None of this means breastfed babies are underweight. It means the typical growth curve for a breastfed baby looks different from that of a formula-fed baby, and the WHO charts were specifically designed to account for this.
Prematurity and birth weight. Babies born early or at low birth weight often spend months catching up to their peers. Among preterm infants born under 1,000 grams, underweight at a corrected age of six months is more common and hospital stays tend to be longer.10PubMed Central. Growth parameters of NICU admitted low birth weight preterm neonates at corrected ages of 6 and 12 month When tracking a premature baby, clinicians plot weight at the corrected age (chronological age minus weeks of prematurity), not the calendar age. A six-month-old who was born six weeks early is plotted as a four-and-a-half-month-old, which makes a big difference in where they land on the curve.
Maternal health during pregnancy. Gestational diabetes can nudge growth in both directions during the first year. One multi-ethnic study found that children exposed to gestational diabetes actually had slower BMI growth in the first six months but then gained weight faster from six months onward, eventually tracking toward higher BMI by preschool age.11PubMed. Body mass index trajectories up to preschool age in a multi-ethnic population; relations with maternal gestational diabetes, BMI and gestational weight gain Maternal pre-pregnancy obesity, separately, is linked to higher BMI at birth and persistently higher BMI throughout early childhood.12PubMed Central. Gestational diabetes and Offspring’s growth from birth to 6 years old
Population-level body build. Even among healthy, well-nourished infants, ancestry-linked differences in body proportions appear very early. A multi-country comparison found that South African infants carried more body fat than Australian or Indian infants in the first three months, while Australian boys had more lean mass during the same window. By 3 to 24 months, patterns shifted again, with Brazilian infants showing the highest fat-free mass among the groups studied.13European Journal of Clinical Nutrition. Infant growth and body composition from birth to 24 months: are infants developing the same? A broader analysis of 46 lower-income countries found that parental body size explained more variation in infant weight-for-height than climate or latitude did, and that these population differences appeared to originate in utero rather than from postnatal feeding.14PubMed. Population level differences in adult body mass emerge in infancy and early childhood: evidence from a global sample of low and lower-income countries In other words, a healthy baby from a naturally leaner population may land below a WHO median built from a pooled sample without anything being wrong.
Growth Trajectory Matters More Than a Single Number
Pediatricians care less about where a baby falls on the chart at any single visit and more about the shape of the growth curve over time. A baby consistently tracking the 15th percentile is not a concern. A baby who was at the 50th percentile at two months and drops to the 10th percentile by six months might be. The American Academy of Pediatrics recently published a clinical practice guideline defining “faltering weight” as any one of three patterns: weight-for-length below the 5th percentile, weight gain velocity below the 2.3rd percentile for age, or a decline of one or more standard deviations in weight, weight-for-length, or BMI.15PubMed. Clinical Practice Guideline for Diagnosis and Management of Faltering Weight
The flipside also matters. Rapid upward movement across percentiles in early infancy is linked to higher odds of obesity later in childhood. Children who crossed upward through two or more weight-for-length percentile bands in the first six months had about twice the odds of being obese at age five compared with children whose trajectories were more stable.16Archives of Pediatrics & Adolescent Medicine. Crossing Growth Percentiles in Infancy and Risk of Obesity in Childhood Another study found that each increment in weight-for-length z-score at six months was associated with progressively higher BMI and skinfold thickness at age three, with the predicted obesity rate reaching 40% among infants in the highest quartile at both birth and six months.17PubMed Central. Weight status in the first 6 months of life and obesity at 3 years of age
This is not a reason to restrict an infant’s intake. Healthy babies regulate their own feeding well, and the point is one of monitoring, not intervention. If a baby is rapidly climbing percentiles, it prompts a conversation with the pediatrician about feeding practices and portion cues rather than any kind of calorie-counting.
The Doubling Rule of Thumb
You may have heard that a baby should double their birth weight by about four to five months. That rough guideline has shifted over time. A study tracking birth-weight doubling times found that they had gotten somewhat later compared with data from the mid-1970s, likely reflecting changes in feeding practices and the shift toward breastfeeding.18PubMed. Birth weight doubling and tripling times: an updated look at the effects of birth weight, sex, race and type of feeding Smaller babies at birth tend to double earlier, and larger babies later, so a baby born at 4 kg is not expected to hit 8 kg on the same timeline as one born at 2.8 kg. Treating the doubling milestone as a fixed deadline creates unnecessary anxiety. It is a rough pattern, not a diagnostic criterion.
Getting an Accurate Weight
Infant weights are sensitive to surprisingly small sources of error. A study of repeated weighings found that biological variation and measurement imprecision together are large enough to make weight changes unreliable if measurements are taken less than two weeks apart for babies under nine months. The recommendation: do not weigh babies at intervals shorter than a fortnight, because any change you detect will be smaller than the error in the measurements.19PubMed. A study of errors that can occur when weighing infants
Scale type matters, too. In-bed incubator scales and freestanding scales can produce slightly different readings. In testing with simulated infants, freestanding scales consistently read about 12 grams below the calibrated standard, a difference that is trivial for a full-term 7 kg baby but becomes proportionally larger for very low-birth-weight infants.20Advances in Neonatal Care. ACCURACY OF WEIGHING SIMULATED INFANTS WITH IN-BED AND FREESTANDING SCALES WHILE CONNECTED AND DISCONNECTED TO A VENTILATOR For a healthy 6-month-old weighed at the pediatrician’s office, the practical advice is straightforward: weigh the baby undressed (or in a dry diaper only), use the same scale each visit if possible, and do not read too much into a single number. The trend across visits is what counts.
Secular Trends in Infant Weight
If you compare today’s 6-month-olds with babies from the mid-twentieth century, you might expect them to be heavier, given rising adult obesity rates. The reality is more nuanced. Data spanning eight decades from the Fels Longitudinal Study showed that infants born after 1970 were about 450 grams heavier and 1.4 cm longer at birth than those born before 1970, but they then grew more slowly during the first year, so the trajectories converged by around 12 months.21PubMed Central. Eighty year trends in infant weight and length growth: the Fels Longitudinal Study In other words, modern babies start bigger but gain less in their first year than babies did generations ago. The shift likely reflects changes in prenatal nutrition, maternal health, and infant feeding practices rather than anything genetic.
This finding has a reassuring implication for parents comparing notes with older relatives. When a grandmother says her babies were a certain weight at six months, the growth environment was different enough that the comparison is not very informative. The WHO standards, updated and internationally validated, are a far better benchmark than family lore.
Birth Weight Percentile as a Prognostic Signal
Where a baby starts on the weight curve at birth influences what comes next. A 2026 analysis across multiple cohorts found that infants at higher birth weight percentiles, even those below the formal large-for-gestational-age threshold, were at elevated risk for adverse growth patterns and overweight in childhood. The authors noted that traditional groupings (small for gestational age, appropriate for gestational age, and large for gestational age) remain practical tools for detecting growth differences, but that finer birth-weight categories could add prognostic value.22JAMA Network Open. Birth Weight Percentiles and Infant and Child Growth Dynamics For parents, the takeaway is that birth weight is not destiny, but it does set the initial trajectory. Infants born at the higher end of normal may benefit from closer monitoring even when they are not formally classified as large.