One more reason to give your child a sibling: a younger brother or sister is good for their health.

According to a study conducted at the University of Michigan, becoming a big bro or sis before first grade might lower a child's risk of becoming obese.

The birth of a sibling, particularly when the child is between 2 and 4 years old, was associated with a healthier BMI in that older son or daughter. Meanwhile, only children the same age are nearly three times more likely to be obese once they hit elementary school.

What's more, the benefits only seem to come into play for the older sibling. The younger sibling doesn't see the same positive effect on their body mass index.

"Research suggests that having younger siblings — compared with having older or no siblings — is associated with a lower risk of being overweight," Dr. Julie Lumeng, a pediatrician at the university, said of the study, which was published in the journal Pediatrics [1].

Researchers arrived at this conclusion after studying nearly 700 children across the US, but they are short on explanations.

Lumeng hypothesized that the positive outcomes could be because children may engage in more "active play" once a younger child is born — they are less likely to have more sedentary time to themselves. She said it might also be attributed to parents changing the way they feed their child when a new sibling is born, as kids happen to develop long-lasting eating habits around the same time, at approximately 3 years old.

Still, those are just theories.

"We need to further study how having a sibling may impact even subtle changes such as mealtime behaviors and physical activity," she said. "Childhood obesity rates continue to be a great cause of concern. If the birth of a sibling changes behaviors within a family in ways that protect against obesity, these may be patterns other families can try to create in their own homes."