You’ve probably heard the joke that pregnancy is contagious, but now there’s research to back the theory up.

According to a study by Italy’s University of Milan and The Netherlands’ University of Groningen (GrōniNGən), women tend to become pregnant within two years of their high school friends’ pregnancies.

Researchers analyzed data from interviews with more than 1,700 American women who were followed from their teenage years to their early 30s. The data showed a noticeable trend: pregnancies increased in the two years following the women’s friends’ pregnancies, then they declined after the two-year mark.

The study, titled “Does fertility spread among friends?”, specifically looked at the percentage of planned pregnancies that correlated with high school classmates’ pregnancies.

So why do women follow suit when their friends have babies?

The study’s authors have their theories. First, women may have babies around the same time as their friends because people compare themselves to their friends. Also friends serve as a significant learning resource. There are advantages to having children at the same time— not just for the playdates, but also to share experiences and stresses related to pregnancy and childrearing.

The study is the first to look at the correlation of pregnancies among friends.

The research is another confirmation that friends have an increasing role in each others’ lives. Previous studies have shown that friends influence other behaviors, such as smoking, drinking and how much a person exercises.

The average participant had her first child at age 27.

The researchers found it impressive that high school friends had such a strong impact on each others’ life decisions long after graduation. So it turns out your friends in high school may just be your friends for life.