It cites graduated driver licensing systems in which teens' abilities to drive are expanded over time from the initial stages, when driving is restricted to low-risk conditions.

CNN: Fatal wrecks underscore risks for young drivers

However, risk of on-board transmission, the researchers noted, is mainly restricted to within two rows of the passenger carrying the infection.

BBC: In-flight health issues 'on rise'

He said the demo version could only self-park in an area equipped with sensors and restricted to other robotic cars to avoid the risk of collision.

BBC: Nissan car drives and parks itself at Ceatec

The risk was further increased if the woman's baby had restricted growth or died in the womb, or if the woman had pre-existing risk factors for cardiovascular disease, such as tobacco smoking.

BBC: NEWS | Health | Placenta clues to heart disease