Teenagers are more likely to give birth prematurely and have a smaller baby than women in their 20s, research has suggested.
The researchers found under-17s to be 21% more likely to give birth prematurely with their first pregnancy and 93% more likely to have their second baby early.
Younger mothers and low weight babies were also linked in the study, which was conducted in north-west England and saw 50,000 women take part.
Early birth risk is more pronounced if 14 to 17-year-olds have a second child, the study, which urged better sex education and contraception, found.
It also discovered that rates of teenage pregnancy increased with increasing social deprivation, with more than a third of the teenage mothers coming from the most socially deprived areas.
Researcher Dr Ali Khashan, from University College Cork in Ireland, said the risk of premature birth in the younger mothers could be down to "biological immaturity".
Dr Khashan added: "It is also possible that the increased risk of poor pregnancy outcome in the second teenage pregnancy is related to numerous complicating factors such as greater social deprivation and less prenatal care."