Early postnatal BMI adaptation is regulated during a fixed time period and mainly depends on maternal BMI.

In the present study, we investigated whether there are critical time periods which influence the course of BMI during the first 6 years of life. From 5,433 children who participated in preschool examinations those 212 children were selected who crossed the BMI percentiles as a result of an extreme postnatal BMI rise (from <10th to 90th percentile) or fall (from >90th to <10th percentile) or who have persistently low or high BMI both at birth and at the age of 6 years. Forty children with a BMI close to the 50th percentile both at birth and age 6 years were selected to serve as controls.