This study confirmed the association between the number of children, birth order, and later BMI in a nationally-representative sample of children and adolescents. The current findings showed that the odds of underweight was higher in the third children compared with the first children; this result remained significant even after adjusting for confounding factors. The second children were more likely to be overweight compared with the first children, but this association lost its significance after controlling for confounding factors. A previous study found that last-born children showed lower odds of obesity than only children (5). A study in Sweden showed that birth order was inversely associated with BMI; first-borns had a BMI of 0.8% and 1.1% higher than the second born and third born children, respectively (27). In contrast, some studies have reported that the oldest children were significantly associated with overweight or increased adiposity (13, 28).
This association can be justified in light of the fact that lower-birth-order infants are more likely to be smaller at birth than later-born infants (15, 29, 30). The former group is tend to undergo catch-up growth, a pattern associated with obesity risk (14). Khong et al. (31) suggested that pregnancy leads to permanent anatomical changes in the spiral arteries that can be associated to the number of previous pregnancies. This finding suggests that birth weight can be increased with increasing parity. Some studies purpose that differences in fetal nutrition and changes in this in successive pregnancies, reflected in e.g. birth weight (the Developmental Origins Hypothesis), have been put forward (12, 14). Another study suggested that psychosocial origin may explain the associations. Interaction patterns between family members may be mentioned as the underlying mechanism explaining the association of birth order and weight status (32).
Also we found that children within three- to four-children families had higher OR of underweight in comparison to those within one- or two-child families, as well as children within five-child families or more compared to smaller families. Moreover, those children and adolescents with more than two siblings were less likely to be obese than their single peers or peers with one sibling; however, its significance was attenuated after controlling for potential confounding factors. A study found that children who had no siblings had the highest risk of obesity through 8th grade (33). Another study reported that singletons were 1.30 times more likely to be overweight than their peers with at least one sibling (34). A longitudinal study conducted in the USA showed that only children had higher mean BMI compared to those who had at least two brothers, even after adjustment for possible confounding factors (8). In contrast, a study did not show any association between the number of siblings and overweight in Chinese adolescents (35).
The presence of siblings may provide greater opportunity for games and other physical activities (36). Hallal et al. (36) suggested that a higher number of siblings, regardless of the activity level of each individual sibling, promotes active lifestyles in the long run. Moreover, boys without any sibling have been shown to spend more time watching television than those with siblings (37). Siblings may also be a stimulus for child-to-child interactions, cooperative play, or activities that increase the time each child devotes to physical activity (38).
Previous studies have presented that the number of siblings may influence children’s weight status through effects on parental time and attention allotted to children and on the nature and extent of interactions among siblings (39, 40). A previous study showed that single children had significantly higher intakes of many nutrients per 1000 kcal compared to children with sibling(s) (41), which might be explained by the fact that a mother with one child is more concerned with persuading her child to eat and grow than is a mother with several children (42). Therefore, additional siblings may decrease parental attention and availability of food for each child, resulting in the reduction of the odds for overweight (43). Research has also suggested that children in single-child families have considerably greater access to foods and thus intake more nutrients than children with siblings (41). One reason is that a mother with an only child tries to pursuade the child to eat compared a mother with more than one child (42). Accordingly, as the number of children increases there will be less food for each child, reducing the OR for overweight, especially in poor families (8).
This study was conducted on a large representative sample of Iranian children and adolescents; however, it has some limitations. Regarding the cross-sectional nature of the study, casual associations could not be provided. Moreover, we missed the pubertal status of the study participants, which might largely affect the body size interpretation. Furthermore, psychopathological variables and pathological eating behaviors were not considered in the present study.