A menstrual cycle is considered short if it lasts less than 21 days. In the case of a short cycle, the length of the luteal phase (the number of days between ovulation and the start of your next period), is the important number to watch. If your luteal phase is shorter than ten days, your endometrium may not have enough time to create favorable conditions for a successful implantation of the embryo in the uterus. If you've been trying to get pregnant for some time, and your cycles are shorter than 21 days, do talk to your doctor.