Body mass index in adolescence and number of children in adulthood

Body weight is associated with reproduction and related behaviors, but it is unknown whether it has significance for fertility differences in the general population. We examined whether adolescent body mass index (BMI; kg/m) predicted the number of children in adulthood 21 years later.

The participants were 1298 Finnish women and men (ages 12, 15, and 18 years at baseline) followed in a prospective population-based cohort study (the Cardiovascular Risk in Young Finns) from year 1980 to 2001.

There was an inverted J-shaped association between BMI and the number of children, such that underweight adolescents had 10-16% fewer children in adulthood, overweight adolescents 4-8% fewer, and obese adolescents 32-38% fewer than individuals with normal adolescent weight. This association was similar in women and men, and independent of age, education, urbanicity of residence, and timing of menarche (in women). Adolescents with low or high BMI were less likely to have lived with a partner in adulthood, which partly accounted for their decreased number of children. The influence of adolescent BMI was independent of adulthood BMI in women but not in men. Age at menarche also predicted the number of children, such that women with early or late menarche had more children than those with average age at menarche.

Underweight and especially obesity may have a negative impact on fertility in the general population. The increasing prevalence of obesity in children and adolescents may represent a concern for future reproductive health.