Average Weight of a 4-Month-Old Infant
For a healthy 4-month-old infant, the average weight is approximately 6-7 kg (13.2-15.4 lbs), with boys typically weighing around 7 kg and girls around 6.5 kg, based on WHO growth standards that should be used for all infants under 24 months of age.
Recommended Growth Assessment Approach
Use the WHO growth charts (not CDC charts) for all infants under 24 months to assess weight at 4 months 1. The CDC explicitly recommends the 2006 WHO international growth standard for children aged <24 months, regardless of feeding type, because these charts reflect optimal growth patterns of predominantly breastfed infants 1.
Expected Weight Parameters
Based on the WHO growth standards:
- Boys at 4 months: The median weight falls around 7 kg based on the WHO weight-for-age curves 1
- Girls at 4 months: The median weight falls around 6.5 kg based on the WHO weight-for-age curves 1
Supporting research from the Netherlands (1998) found median weights at 4 months of 7.15 kg for boys and 6.59 kg for girls 2, which aligns closely with WHO standards.
Clinical Application
When assessing a 4-month-old:
- Plot the weight on WHO growth charts available at https://www.cdc.gov/growthcharts 1
- Use the 2nd and 98th percentiles (±2 standard deviations from median) to identify concerning growth patterns, not the traditional 5th and 95th percentiles 1
- Normal range spans approximately 5.5-8.5 kg for boys and 5-7.5 kg for girls at this age
Important Caveats
Breastfed vs. formula-fed infants show different growth patterns 1. Formula-fed infants tend to gain weight more rapidly after 3 months, but this accelerated growth may signal early overweight risk 1. The WHO charts are based on predominantly breastfed infants and represent the optimal growth standard 1.
Weight gain velocity matters more than absolute weight. Average daily weight gain from birth to 4 months is approximately 27.7 g/day for boys and 24.5 g/day for girls 2. Deviations from expected growth velocity warrant investigation for suboptimal environmental or health conditions 1.
Do not use age-based weight estimation formulas (like "age in months + 9 divided by 2") for routine well-child assessments 3. These are emergency estimation tools only and lack the precision needed for growth monitoring.