Teenage girls are more likely to become pregnant if their older sister also has had a baby as a teenager, according to a Norwegian and British study. The study included data from more than 42,000 Norwegian children born between 1947 and 1958.

"Our results show that teen motherhood of the older sister has a significant positive impact on the probability that her younger sister will also have a teen birth," the researchers conclude in the report Is Teenage Motherhood Contagious? Evidence from a Natural Experiment, from the ESRC-funded Centre for Market and Public Organisation.

The probability for pregnancy went up from one in five to two in five if a sister had had a baby. The effect is larger for siblings who are close in age, and for women from low income households.

Although more education for an older sister may reduce the chances of the younger sister having a baby, the pregnancy "contagion effect" between sisters is stronger. "This suggests that increasing education for girls will decrease teenage fertility but the strong effect of within-household spillovers also suggests policies aimed directly at decreasing teenage pregnancy may also be needed to reduce teen births," the report concludes.

• CMPO: Is teenage motherhood contagious?
• BBC News: Teenage pregnancy is 'contagious'
• The Telegraph: Teenage pregnancies 'contagious'
• The Guardian: Chance of pregnancy higher for sisters of teenage mums