Just the other day, I heard this '一孕傻三年'. Does anyone have an inkling as to why Chinese people think this?
Since I don't have enough reputation to comment
It's not only in Chinese culture, it's called "baby brain" http://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/expert-answers/baby-brain/faq-20057896
It means that pregnancy makes a woman sutpid and forgetful. In western culture, there is an equivalnent words "baby brain" which means the pregancy or early motherhood can harm a woman's memory and ability to think. As a local Chinese han bridge teacher, many of my colleagues who give birth to baby often told me “一孕傻三年” when they forget something and do something in wrong way.
Even though baby brain or "momnesia" is often reported by pregnant women, webMD states: "Pregnancy does not change a woman's brain even though some women don't feel as sharp as usual when they're pregnant." Even this news article says US researchers claim "momnesia" is a sort of made-up excuse for common forgetfulness. You can read them more since it seems to give other reasons why baby brain actually occurs.
In any case, I've been living in China for 5 years and have heard that Chinese phrase come up around pregnant Chinese mothers. Right now I'm visiting my family in the US, and every time my brother's pregnant wife randomly tweaks out over forgetting something or being slow, I remember what those Chinese said and I blurt out 一孕傻三年. I think she's just under lots of stress and bodily changes, and stress can cause lots of problems on its own.