Does the study consider the effect of mobile phone "behaviour" rather than "exposure"?  (Presumably they are thinking of microwave radiation).  Women who use their mobile phone frequently before the birth are likely to continue after birth, and the unpredictable and (to an infant) incomprehensible interruptions to their mother's attention are more likely to be a factor affecting their behaviour.  My wife often breaks off a conversation in mid-sentence to answer her mobile phone and it even irritates me (I am 60).