Teenage pregnancy is 'contagious' between sisters, researchers in the UK and Norway have claimed.
A study of more than 42,000 Norwegian teenage girls suggested they were more likely to become pregnant if their older sister had a baby as a teenager.
The effect was greatest when the sisters were of a similar age or from a poorer background.
The Family Planning Association said the results may not necessarily apply to the UK but were still interesting.
Factors such as family background and level of education are already known to influence teenage pregnancy. This new research, carried out in conjunction with the University of Bristol, looked at the specific effect sisters have on each other.
"Sisters generally spend more time together than schoolmates or friends and so sisters are likely to be influenced by the behaviour of their siblings," the report said.
They looked at data from children born between 1947 and 1958 to compare families from a similar background from different regions of Norway.
The researchers said the probability of the younger sister having a teenage pregnancy went from one in five to two in five if the elder sister had a baby as a teenager.
Spending more time in school did reduce the probability of a teenage pregnancy, but on a much smaller scale than the effect of a pregnant sister.
One of the researchers, Professor Carol Propper, described this as "the contagious effect of teen motherhood". She said, "Two groups were particularly vulnerable - those in low income households and sisters close in age."
She argued that any "sister effect" would wear off as the age gap increased because the siblings would be on "different life trajectories", whereas those of similar ages would have similar social circles.
The study looked at births rather than conceptions. Professor Propper said an elder sister with a baby could also influence the decision to keep a baby.
"The research says how important family is compared to institutions or mandating children to stay at school for an extra couple of years," she said.
"It is important when thinking about campaigns to affect teen pregnancy. More policies aimed directly at decreasing teenage pregnancy may be needed in order to reduce teen births."
Rebecca Findlay, from the Family Planning Association, said: "Teenage pregnancy is complicated.
"Social and economic deprivation and poor education all impact hugely onto it and we already know that being the daughter of a teenage mother is one of the contributing risk factors towards teenage pregnancy.
"Although it's difficult to draw direct comparisons to this country just yet, this research is of interest."
Jules Hillier, the deputy chief executive of the charity Brook, said: "There are links between low aspirations, deprivation and teenage pregnancy, but there are also a whole range of measures that need to be in place to reduce teenage pregnancy rates such as comprehensive sex and relationships education and easy access to sexual health services."
Source: BBC news online, 9 August 2011