The researchers reason that this mutation must have arisen in one of the twins while in the womb.

BBC: Scientists track leukaemia's origins 'back to the womb'

Half had been exposed to up to five ultrasound scans while in the womb, while the rest had had only one ultrasound exposure before birth.

BBC: Pregnant woman undergoing an ultrasound

While in the womb, foetuses are free to somersault in their amniotic sacs, and this constant movement has so far hindered clear images of growing brains.

BBC: Babies' brains to be mapped in the womb and after birth

Hallman was presented with a third option: Doctors could operate on her baby while it was still in the womb.

CNN: Study: Major benefits for spina bifida surgery in the womb