Women who were friends in high school have a high likelihood of getting pregnant around the same time.

A study published in the American Sociological Review titled "Does Fertility Behavior Spread among Friends?" found a link between strong social bonds and pregnancy. After female friends graduate, when one gets pregnant, the others have a window of time in which pregnancy is "contagious," and the probability of getting pregnant increases significantly.

"The study shows the contagion is particularly strong within a short window of time: It increases immediately after a high school friend gives birth, reaches a peak about two years later, and then decreases, becoming negligible in the long-run," co-author Nicoletta Balbo, a postdoctoral fellow at Italian Bocconi University said in the press release.

"Overall, this research demonstrates that fertility decisions are not only influenced by individual characteristics and preferences, but also by the social network in which individuals are embedded," she continued.

The authors of the study observed 1,700 American women over a 15-year span, starting from the age of 15 and ending when the women were 30.

This is not the first study to determine that social interaction influences pregnancy. A 2010 study called "Businesses, Buddies and Babies: Social Ties and Fertility at Work," which looked at pregnancy rates among coworkers, found "that there are non-trivial peer effects on the choice of timing in childbearing," and that the closer the coworkers in age and pay scale the stronger the influence they had on each other.

Balbo suggests several reasons for this subconscious pregnancy pact, including a feeling of pressure for friends to keep up with each other, and because "friends are an important learning source. Becoming a parent is a radical change. By observing their friends, people learn how to fulfill this new role."

The bond stemming from being part of the same high school social group is also a factor. A report from the psychological science department at Purdue researched the effects of social learning among children and teenagers, and found that children don't often outwardly pressure each other to engage in certain activities. Instead, for children to behave similarly, "all friends need to do is make that behavior seem exciting and enjoyable."

This childhood social reinforcement seems to carry over to adulthood for some friends. When a woman sees that her friend is happily expecting a child, her own wish to start a family increases, according to the ASA press release.

The attraction of belonging, of being part of the group, is also a reason pregnancy might be contagious: "Synchronizing childbearing with friends may reduce the risk of being left behind by friends who already have a child," the ASA report states.
The study authors stress that this effect is not because of a conscious decision among friends to have children at the same time, but is rather an example of how strongly friends can influence each other.