If it baby announcements seem to come all at once from a close group of friends, research shows there may be a reason: Pregnancy can be contagious.

“A friend's childbearing positively influences an individual's risk of becoming a parent,” concluded the authors of a 2014 study published in the journal American Sociological Association.

For the study, the researchers analyzed data on 1,720 women who participated in the National Longitudinal Study of Adolescent Health (ADD Health) in the United States from the mid-1990s to mid-2000s. Tracking female participants who were at least 15 years old in 1995 with home interviews throughout the next decade, the researchers saw that roughly half of the women had a child by the time the final interviews were conducted in 2008 or 2009.

During the interviews, the women noted up to 10 “friendship ties,” which gave the researchers insight into the patterns among groups of friends as they aged.

Focusing specifically on pairs of high school friends though later years, the researchers found there is a strong “contagion” element for planned pregnancy.

The researchers note:

“We found this effect to be short-term and inverse U-shaped: an individual’s risk of childbearing starts increasing after a friend’s childbearing, reaches a peak around two years later, then decrease.”

That backs up a 2011 study from Germany that found significantly stronger peer effects on fertility than sibling effects. According to that study, the “risk” for a woman to get pregnant increases with every friend she has whose given birth within the past three years.

Calling this the “fertility influence,” the researchers of the 2014 study suggest three theories for what may be at play:

Social learning: Women may be more inclined to embark on motherhood when they see a close friend navigate it successfully.

Social influence: Women may not want to feel “left behind” if their friends are collectively stepping into motherhood.

Cost-sharing: From more of a logistical standpoint, there are some financial perks if two friends know they can coordinate on activities and childcare.

It isn’t just the timing of pregnancies that friends influence, but also the number of children they have—with a 2014 report in the journal Demographic Research noting extensive historical evidence on the correlation between friendships and birth rates. The researchers say, “Depending on the context, social mechanisms may act for or against having a large number of children.”

Interestingly, both the 2011 and 2014 studies found the contagion element doesn’t extend to siblings. (Other than in the case of the Kardashians, apparently.) The authors of the 2014 study suggest this is because, “In today’s individualized societies, friends may be equally or more important than siblings and other family members.” They add the facts that friends are chosen seems to have a particularly strong effect on their influence versus the influence of siblings.

Beyond the science, most of us can think of some anecdotal examples of the “pregnancy bug” among peers. There are these childhood friends from who keep expecting babies on the exact same day. Or there was the bride whose all five bridesmaids were due within one month of each other.

It’s even true for me: Despite weddings spaced out through five years, my two best friends from high school and I all welcomed our first babies just months apart. Unfortunately we all live in different states now, so no cost-sharing benefits for us.