What is the expected weight gain rate for a healthy, full‑term 4‑month‑old infant receiving adequate breast milk or formula?

Expected Weight Gain for a 4-Month-Old Infant

A healthy 4-month-old infant should gain approximately 20-30 grams per day (140-210 grams per week), with breastfed infants typically gaining at the lower end of this range after 3 months of age. 1, 2

Growth Velocity by Feeding Method

Breastfed Infants

• Breastfed infants gain weight more rapidly than formula-fed infants during the first 2-3 months, then gain weight more slowly from approximately 3 months onward—this is a normal physiologic pattern. 1, 2
• After 3 months of age, WHO growth curves show a slower rate of weight gain for breastfed infants compared to CDC charts, reflecting the natural deceleration in weight velocity that occurs with exclusive breastfeeding. 1
• The average weight gain for exclusively breastfed infants at 4 months is approximately 17-20 g/kg/day, which translates to roughly 100-140 grams per week for a typical 6-7 kg infant. 3

Formula-Fed Infants

• Formula-fed infants gain weight and length more rapidly than breastfed infants from about 2 months of age through the end of the first year. 2
• Formula-fed infants typically gain 25-30 grams per day (175-210 grams per week) at 4 months, maintaining a higher growth velocity than their breastfed counterparts. 2, 4
• The higher protein intake in formula (which exceeds requirements after 1-2 months) is primarily responsible for the accelerated weight gain, along with higher levels of insulin-like growth factor-1 (IGF-1) and insulin. 2

Clinical Monitoring Recommendations

Normal Growth Patterns

• Consistent percentile tracking is the key indicator of healthy growth—when an infant maintains their growth along the same percentile curve over time, this represents normal constitutional growth, not pathology. 5
• Growth velocity over time using multiple data points is far more informative than isolated measurements for assessing whether an infant is experiencing healthy growth or pathologic decline. 5

When to Be Concerned

• A drop of 40 percentile points (e.g., from the 50th to the 10th percentile) represents clinically significant growth faltering that requires intervention before reaching the 2nd percentile threshold. 5, 6
• Values below the 2.3rd percentile (2 standard deviations below the median) warrant evaluation for chronic malnutrition, underlying medical conditions, or inadequate caloric intake. 5
• Serial measurements every 2-4 weeks are recommended when growth faltering is suspected to track weight trajectory and response to interventions. 6

Appropriate Growth Chart Selection

• WHO growth charts should be used for all infants under 24 months, regardless of feeding method, as they reflect optimal growth patterns among predominantly breastfed infants. 1, 5
• Using CDC charts for infants younger than 24 months tends to over-identify underweight children because the CDC reference includes a mixed-feeding population with higher formula use. 5
• The WHO reference population was 100% breastfed for 12 months and predominantly breastfed for at least 4 months, making these charts the appropriate standard for assessing infant growth. 1

Common Pitfalls to Avoid

• Do not misinterpret the normal deceleration in weight gain that occurs in breastfed infants after 3 months as pathologic growth faltering. Infants who continue exclusive breastfeeding at 3-4 months tend to maintain their percentile position on WHO charts while their percentiles on CDC charts often decline. 1, 6
• Do not initiate formula supplementation or recommend weaning without first assessing and optimizing breastfeeding technique and maternal milk supply. 6
• Clinicians unaware of the normal growth pattern of exclusively breastfed infants may inappropriately recommend supplementation or weaning when infants are actually growing normally. 1