Other health related problems could also cause poor egg health, low ovarian reserve, or abnormal immunological responses, which can affect conception. Stress could also play a role. We all know that menstrual cycles can be altered during times of extreme duress- and this can be emotional, physical, or environmental stressors. In these instances, the first steps should be to avoid life stressors, maintain a healthy weight, routinely exercise, avoid smoking, and reduce alcohol intake, all of which may be contributing to unexplained infertility issues.
Sometimes problems getting pregnant for a second or subsequent time are related to a complication that occurred in a prior pregnancy or prior to delivery (damage to the uterus, for instance). But most often, secondary infertility is caused by the same factors that would cause primary infertility — issues like advanced age, obesity, ovulation problems and so on.