Inadequate progesterone. Luteal phase defect means your ovary is not producing the normal amount of Progesterone during the second half of your menstrual cycle (luteal phase). Then, the lining of the uterus where a pregnancy implants will not thicken normally, your cycle may be short, and you may have trouble getting pregnant. This occurs in 3 to 10% of women.
Good question. There is no clear consensus on even an accurate definition of luteal phase defect; and other causes for infertility should be sought before making this diagnosis. Persistent defects of follicular or corpus luteum function that impact fertility are more common if your cycle length is consistently longer than 35 days. Treatment with a drug that induces ovulation may be a good choice.