Same deal with children who are immunodeficient being correlated with higher vaccine doses of various diseases. Parents often dont miss these when they get more sick, while most parents with healthy children naturally tend to forget. Also same with women having more trouble having normal kids when older; its a fact women who choose to have kids later tend to abuse their bodies more, which leads to more compromised healthy reproduction. We dont know 100%, but the lack of answers ironically gives more clarification.