Mothers who have another child with a new partner are more likely to give birth early, suggests research.
Very premature babies face health challenges
The report, by Norwegian researchers, found that the risk of a premature baby doubled if a new father was involved.
The chance of the child dying as a result was also increased in these circumstances, said the study in the British Medical Journal.
Scientists said that it might happen because women with more than one partner tended to smoke and drink more.
There is an increase in remarriage in Western society, and many more women have children fathered by different partners.
Premature birth puts the child at risk, not only of death, but also of long-term disability.
Twice the risk
The study was carried out by a research team from the Norwegian University of Science and Technology, who examined the records of more than 485,000 women.
Of these, 31,683 had changed partner between the births of their children - the rest had had at least two births, but with the same man.
They found that the risk of the baby arriving prior to 37 weeks, in general, doubled for the women with new partners.
Among women classed as "well-educated", the risk increased four-fold.
Although it seems unlikely, it is possible that a biological explanation does exist for the extra risk.
Other studies have suggested that women who have children with a different partner face a slightly increased risk of pre-eclampsia, a high-blood pressure condition which can force doctors to deliver babies early because both mother and child are at risk.
Some doctors say that the unfamiliar sperm from the new partner can trigger an immune reaction which leads to pre-eclampsia - and suggest a six-month delay before attempting for a child with the new partner.
However, the study also found that women with different partners tended to be much more likely to be smokers.
This is a well-known risk factor for premature birth - especially in older women, which would explain at least in part why the second child would be born earlier than the first.
Professor Gordon Smith, a consultant in Obstetrics and Gynaecology at the Rosie Hospital in Cambridge, told BBC News Online that no firm conclusions could be drawn from the study.
"It's very hard to tell from the data that has been presented here. It is possible that smoking is one reason.
"Women who had children by more than one partner were much more likely to be smokers.
"However, it's possible that there may be a biological determinant at work here, but is not certain."