Birth control pills may shrink portions of a woman's brain and affect its function, scientists have warned.

It's possible that the synthetic hormones found in the pill - and possibly the suppression of natural hormones that occurs when women are using the pill - cause these alterations in brain structure and function, according to a new study.

Researchers at the University of California, Los Angeles in a study conducted on 90 women found that two key brain regions, the lateral orbitofrontal cortex and the posterior cigulate cortex, were thinner in women who used oral contraception than in those who did not.

The lateral orbitofrontal cortex plays an important role in emotion regulation and responding to rewards, while the posterior cigulate cortex is involved with inward-directed thought, and shows increased activity when we recall personal memories and plan for the future.

Changes in the lateral orbitofrontal cortex could be responsible for the increased anxiety and depressive symptoms that some women experience when they start taking the pill, researchers said.

"Some women experience negative emotional side effects from taking oral contraceptive pills, although the scientific findings investigating that have been mixed," Nicole Petersen, a neuroscientist at UCLA and the study's lead author, told 'The Huffington Post'.

"So it's possible that this change in the lateral orbitofrontal cortex may be related to the emotional changes that some women experience when using birth control pills," Petersen said.

Researchers are yet to determine if these neurological changes are permanent, or if they only last while a woman is on the pill.

The study was published in the journal Human Brain Mapping.