Until now, only widespread anecdotal evidence from mothers-to-be supported claims that pregnancy actually caused forgetfulness. Now, a team in Australia has replicated the effects in the laboratory.

"There have been plenty of epidemiological studies showing a link between pregnancy and memory loss, but now we have some pretty good evidence of the physiological basis for this," lead researcher Anna Baron said.

The team has based their finding on an analysis of mice -- they treated the rodents for 17 to 50 days with a hormone palette that mimicked the effects of both pregnancy and menopause.

The researchers used human chorionic gonadotropin (hCG) and luteinising hormone, both of which are sharply spiked in pregnancy.

Levels of these hormones also rise with ageing and have been shown to increase the production of toxic beta amyloid protein, which form the brain plaques that cause Alzheimer's.

The team discovered changes in the mice's working, or short-term, memory, and motor skills. They also examined anxiety levels, which can be an predictor of Alzheimer's. Of the two hormones, hCG produced by far more profound results.

"The higher the hCG, the more marked the effect. Lower doses showed a trending effect (on motor skills and memory) but higher does -- those in keeping with the spike of pregnancy -- showed a marked effect.

"The doses over a longer period showed impairment in memory and changes in motor skills that have parallels with Alzheimer's," the sciencewa.net.au quoted Barron as saying.