Background: It has been hypothesized that body weight predicts the number of children that a person will have: obese and underweight persons are hypothesized to have fewer children than do their normal-weight counterparts.
Objective: We aimed to prospectively examine the association between body weight in young adulthood and achieved fertility in later life.
Design: A representative national sample of 12 073 American young adults (aged 17-24 y in 1981) were followed through 2004 (19 survey waves of the National Longitudinal Survey of Youth).
Results: Obese young women and men were less likely to have their first child by the age of 47 y than were their normal-weight counterparts [relative risk (RR) = 0.69; 95% CI: 0.61, 0.78 in women; RR = 0.75; 95% CI: 0.66, 0.84 in men). Obesity also predicted a lower probability of having more than one child, particularly for women. These associations were partly explained by a lower probability that obese participants will marry. Underweight men were less likely to have the first, second, third, and fourth child than were normal-weight men (RRs = 0.75-0.88; 95% CIs: 0.61, 0.95). These associations were largely explained by the lower marriage probability of underweight men. Obese women and men and underweight men were less likely to have as many children in adulthood as they had desired as young adults.
Conclusions: Obesity may be an important risk factor for lower fertility because of its social and possibly biological effect on reproductive behavior. Further data are needed to assess whether this association holds in more recent cohorts.