Average Length of a 6-Week-Old Infant
The average length of a healthy full-term 6-week-old infant is approximately 54-56 cm (21-22 inches), based on WHO growth standards that should be used for all infants under 24 months. 1, 2
Growth Standards and Measurement Context
WHO growth charts are the recommended standard for all infants aged 0-24 months in the United States, regardless of feeding method, as they represent optimal growth patterns based on healthy, predominantly breastfed infants from six countries. 1, 2
The WHO charts include longitudinal data collected frequently during early infancy when growth is most rapid, making them more accurate than the CDC charts for this age group. 1
Expected Growth Pattern from Birth to 6 Weeks
Healthy term newborns typically measure 48-52 cm at birth (mean approximately 50 cm), with boys averaging slightly longer than girls. 1
During the first 6 weeks of life, infants gain approximately 3-4 cm in length, though this varies based on individual growth trajectories. 2
Measured length may appear to increase by 0.2-0.4 cm over the first 2 days of life as the infant's flexor posture relaxes, which is a measurement artifact rather than true growth. 3
Clinical Measurement Considerations
Length measurement accuracy is critical but often poor in clinical practice—59% of standard practice measurements differ by more than 0.5 cm from gold-standard length-board measurements, and 53% result in different percentile classifications. 4
Proper technique using a length-board is essential for accurate measurement, requiring full extension of the infant's legs and knees, which does not harm hip development despite historical concerns. 4, 5
Serial measurements are more informative than single measurements for assessing growth patterns, as individual infants may track along different percentiles while growing normally. 2, 6
Normal Variation and Clinical Significance
Normal length at 6 weeks typically falls between the 2nd and 98th percentiles on WHO charts, with values outside this range potentially indicating underlying conditions requiring evaluation. 2
Breastfed and formula-fed infants show similar length growth during the first 3 months, unlike weight gain patterns which differ between feeding methods. 2
Most healthy small-for-gestational-age (SGA) infants complete catch-up growth by 2 years of age, with the majority achieving catch-up by 6-12 months, so a 6-week-old SGA infant may still be shorter than average while growing normally. 7