London: Ever wondered why new mothers wake up in the middle of the night when their baby stirs while the dads usually carry on fast asleep? The reason lies in the phase of attaining motherhood, suggests a new study.

According to researchers at two California universities, pregnancy changes a woman's brain to allow her to cope with the demands of motherhood and be more attuned to her baby's needs, as reported.

Expectant mothers experience massive hormonal fluctuations during pregnancy -- more than at any other time in their lives, the researchers add.

'There may be a cost but the benefit is a more sensitive, effective mother,' says psychologist Laura Glynn.

The research is published in the December issue of Current Directions in Psychological Science, a journal of the Association for Psychological Science.