A baby is twice as likely to be born bottom first if either or both the parents were themselves breech deliveries, according to a study published ahead of print on bmj.com. The results suggest genes are a contributing factor.

The vast majority of babies are delivered head first. Fewer than one in twenty are delivered the other way round – what is known as a breech delivery. Such deliveries carry significantly greater risks for the baby: they are more likely to die or suffer from health problems.

Factors such as premature delivery and low birth weight are also known risk factors associated with a breech delivery but these only account for up to one in seven of all such breech births. Until now knowledge of whether genes could also be a factor has been lacking.

The researchers from the University of Bergen in Norway looked at data covering all the births in Norway between 1967 and 2004. They studied the information available on men and women and their first born children - a total of 387,555 parent and child units.

They found that men and women who had been delivered full-term in breech had more than twice the risk of breech delivery in their own first pregnancies. Furthermore, babies delivered naturally, not by caesarean, were at the biggest risk of a breech delivery.

The risk was equally strong for male and female parents. This result was emphasized when researchers studied the 35,056 men who had children with two different women. The authors suggest that genes predisposing to a breech delivery are transferred to the foetus which then increases the risk of the mother having a breech delivery.

Breech delivered parents, born prematurely, had no increased risk of breech delivered offspring - the result of the prematurity rather than genes predisposing towards a breech delivery in these cases.

Despite the suggestion of a genetic trait for breech presentation some of the results suggest an environmental factor or interaction, caution the researchers in accompany editorial. Further research is needed before advising mothers of a high risk of breech delivery if their parents had a breech delivery.