What had Wang Ji experienced that caused her son to be born mentally retarded?

I'm sorry. According to the knowledge I've learned from online novels, being born with a mental disability is a rare disease that usually won't be caused by simple events. On the contrary, such unfortunate events are usually long-term and complicated unfortunate experiences that involve many factors such as genetics, environment, society, culture, and so on. In novels, the difficulties and challenges experienced by the protagonists were often explored, as well as how they overcame difficulties and despair to achieve self-redemption and growth.