Having children may be contagious among female high school friends during early adulthood, according to a recent study.

Researchers found that fertility decisions are not just only influenced by individual characteristics and preferences, but also by the social network in which individuals are embedded.

"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," Nicoletta Balbo, co-author of the study and a postdoctoral fellow at the Carlo F. Dondena Centre for Research on Social Dynamics at Bocconi University in Italy, said in a statement.

She added that the findings demonstrate that high school friends impact our lives well after graduation.

"We know that friends influence each other on many behaviors, such as smoking, drinking, and how much we exercise," Balbo said. "Several sociological theories have proposed social influence as an important factor for shaping fertility choices, and a limited number of studies have demonstrated that such a connection exists among relatives and co-workers. But we believe our study is the first to show this type of connection among friends."

For the study, researchers tracked more than 1,700 American women when they were at least 15-years-old through approximately age 30. The researchers looked only at the impact of female high school friends on the birth of first children and their findings only held true for planned pregnancies. The median age at first birth for women in the study was just over 27-years-old.

Researchers found three possible explanations for why having a high school friend who has a baby increase the likelihood that a woman will have a baby of her own

First, people compare themselves to their friends. Being surrounded by friends who are new parents makes people feel pressure to have kids as well. Second, 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. Lastly, having children at the same time as friends may bring about many advantages - friends can share the childbearing experience and thus reduce the stresses associated with pregnancy and childrearing.

" It's also easier for people to remain friends when they are experiencing parenthood at the same time," Balbo said.

The findings were recently published in the journal the American Sociological Review.