"In addition, it shows 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."
The study tracked more than 1,700 American women from 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 average age for a woman in the study to have her first child was 27.
Giving possible reasons for motherhood being contagious, the researchers suggest:
"First, people compare themselves to their friends," said Ms Balbo.
"Being surrounded by friends who are new parents makes people feel pressure to have kids as well."
Secondly, she suggests friends are an important learning source for how to be a mother.
Lastly, having children at the same time as friends means experiences can be shared - reducing 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," added Balbo.
The study, which appears in the American Sociological Review, relies on data from the US.
- Daily Mail