Q: Are short menstrual cycles a problem?

A: It depends what you mean by "too short".  You first should determine if and when during the cycle ovulation happens. If the menstrual cycle is shorter than 21 days, then this is called "polymenorrhea". Polymenorrhea usually reflects irregularity of hormonal events coming from the ovaries, and it often means that you don't ovulate or you don't ovulate regularly. Polymenorrhea decreases your chance of conception because you don't ovulate, and even if you do ovulate, it happens very early in the cycle. In a 20-day cycle, ovulation can occur around CD 6, and if fertilization occurs, the endometrium, the lining of the uterus, is not ready yet for the implantation of the fertilized egg.