A peer-reviewed study recently published in the journal PLOS One finds that having a grandmother who had her first kid as a teen is a strong predictor for whether a child will underperform in school—even for a child whose own mother gave birth as an adult, not a teenager. That particular child is, in fact, 39 percent more likely to place in the bottom 10th percentile of scores measuring whether kindergartners are ready for school than a classmate whose grandmother and mother became parents as adults. (The study defines an adult mother as one who was at least 20—an age cutoff often used to define adolescent pregnancy—when she gave birth to her first child.)

The study, co-written by several public-health scholars, including Elizabeth Wall-Wieler, a postdoctoral research fellow at Stanford University’s School of Medicine, analyzed census-like data on 11,300 children in the Canadian province of Manitoba who were born between 2000 and 2009. The researchers then paired the data with statistics on those kids’ “school readiness” —their physical independence, social behaviors, basic numerical knowledge, and ability to communicate effectively, for example—as scored by Manitoba’s kindergarten teachers on a detailed questionnaire.

They chose to focus on Manitoba because of its comprehensive population data and detailed statistics on school readiness, which made it a uniquely apt place to investigate the multigenerational effects of teen pregnancy on academic performance. According to Wall-Wieler, this study is the first ever to probe that question.

Predictably, the study found that school readiness was highest among children for whom neither mother nor grandmother was an adolescent mom, and was lowest among children at the opposite end of the spectrum. Among children of non-adolescent mothers and non-adolescent grandmothers, fewer than a quarter (24 percent) were deemed unready for school in kindergarten. The same was true, meanwhile, of nearly half—46 percent—of the children whose mothers and grandmothers had kids in adolescence.

But what’s especially noteworthy, again, is that the correlation between a child’s school readiness and the age at which her grandma became a mother seems to persist even when the child’s own mother was not a teen mom. Among kids whose mothers but not grandmothers were adolescent moms, 41 percent were not ready for school in kindergarten; for kids whose grandmothers but not mothers were adolescent moms, the rate was 34 percent. Wall-Wieler attributes this phenomenon to the insidious and persistent consequences of poverty, and to the inability of education systems to usher many kids out of that trap.

“What this research really demonstrates is the value of supporting young mothers when they have children in adolescence,” Wall-Wieler says. This support “doesn’t just improve their own lives—it also improves the lives of their children and the lives of their grandchildren.”