"In our study we focus on high school friends because the later a friendship is formed, the more likely it is that the individual chooses the friends on common future family plans or common family orientations," Nicoletta Balbo told Reuters Health in an email.
Balbo, a researcher at the Carlo F. Dondena Center for Research on Social Dynamics at Bocconi University in Italy, coauthored the study with Nicola Barban, a sociologist at the University of Groningen in the Netherlands.
"We looked at dyads (pairs) of friends to see whether the childbearing of one of the friends in the dyad increases the probability for the other friend to have a child," Balbo said.
The researchers analyzed data from a large U.S. study that has followed thousands of participants, starting when they were adolescents in the 1990s, and doing repeated interviews over the years.
Balbo and Barban focused on 1,170 women, of whom 820 became parents during the study period. About half of the pregnancies were planned and half unintended, according to the women's own reports. Their average age at the time they had their first child was 27.
The researchers found that after one of the women in each friendship pair had a baby, the likelihood that her friend would also have her first baby went up for about two years, and then declined.
Image: Pregnant friends, via Shutterstock