Women who smoke during pregnancy increase their risk of miscarriage, premature delivery and cot death.

BBC: Helpline for pregnant smokers

With multiple births increasing the risk of premature delivery, the possibility of a caesarean and the chance deformities and disabilities, UK clinics were last year directed not to return more than two embryos to a patient's womb.

BBC: IVF under the microscope

So it is unlikely that the research, published in the journal Neuropsychology, will radically change the way that obstetricians and paediatricians approach delivery of premature babies, and their care in the first hours after birth.

BBC: Brain risk from birth oxygen shortage