Average Weight of a 1-Month-Old Baby
The average weight of a 1-month-old baby is approximately 4.5 kg (9.9 lbs) for boys and 4.2 kg (9.3 lbs) for girls, based on WHO growth standards that should be used for all infants under 24 months of age. 1, 2
Growth Standards for Infants Under 24 Months
Use WHO growth charts for all infants under 24 months, regardless of feeding type. 1, 2, 3 The Centers for Disease Control and Prevention (CDC) recommends these charts because they:
- Represent optimal growth standards based on healthy, predominantly breastfed infants 1, 2
- Include longitudinal data collected more frequently during the first months of life when growth is most rapid 2
- Are based on international data from six countries with children living in optimal conditions 1
The CDC charts, in contrast, have limited data for young infants—at 2 months of age, only 72 weight measurements were available (38 boys and 34 girls). 1, 2
Expected Weight Range at 1 Month
Normal weight at 1 month typically falls between the 2.3rd and 97.7th percentiles (labeled as 2nd and 98th percentiles on growth charts). 4, 2, 3 Values outside these ranges may indicate adverse health conditions requiring evaluation. 4, 2
For context on early weight gain patterns:
- Birthweight typically doubles by approximately 4 months of age (mean 119 days or 3.8 months). 5
- Bottle-fed infants double their birthweight earlier (113 days) compared to breastfed infants (124 days). 5
- Boys double their birthweight earlier than girls (111 days vs. 129 days). 5
Important Feeding-Related Growth Patterns
Breastfed and formula-fed infants show different growth trajectories that are both normal. 1
- During the first 3 months: Healthy breastfed infants typically gain weight faster than formula-fed infants. 1
- After 3 months: Breastfed infants gain weight more slowly than formula-fed infants for the remainder of infancy. 1
This difference is critical for clinical interpretation—using WHO charts helps prevent inappropriate recommendations to supplement or wean breastfed infants who are growing normally but more slowly after 3 months. 1
Clinical Monitoring Recommendations
Serial measurements are more informative than single measurements. 4, 3 For infants under 12 months with any growth concerns:
- Monitor weight every 1-2 weeks until adequate nutrition is established 4
- Then monitor monthly during the first year 4
- Plot all measurements on WHO growth charts to visualize trends over time 4, 3
A drop of 40 percentile points represents clinically significant growth faltering requiring intervention before reaching the 2nd percentile. 4
When to Evaluate Further
Values below the 2.3rd percentile (2nd percentile on charts) warrant evaluation for underlying causes. 4, 2, 3 Children identified as having low weight-for-age on WHO charts are more likely to have substantial deficiencies requiring immediate attention. 3
Evaluation should include: