Interesting, because I read a metastudy last year (a year ago this week, to be exact) which evaluated thousands of studies regarding the relationship between hormonal changes in pregnancy and permanent changes in neural functioning in brain regions related to memory. It found overwhelming evidence of small but statistically significant improvements in memory between women who were multiparous compared with women who were primiparous or nulliparous. There was evidence of improvements with each pregnancy that could be measured both in memory tests and brain imaging studies.
So I'm really not surprised to find that hormones, including birth control pills, can affect memory!