(LPD) A luteal phase that is shorter than optimal for a fertilised egg to undergo implantation. Very likely if the luteal phase is shorter than normal (11 to 16 days), but not all luteal phases within this range are innocent. Often caused by a prior defect of the follicular phase or of the development of the ovulating tertiary follicle (the preovulatory follicle) and hence there can be a defect of the egg itself (if ovulation has actually occurred) or there can be a luteinised unruptured follicle -- any of which will either increase the risk of miscarriage or will prevent pregnancy altogether for that cycle. Synonymous with luteal phase insufficiency.

The part of the ovarian cycle between ovulation and the start of a new follicular phase, so dominated by the presence of the corpus luteum and the progesterone it produces. Normally between 11 and 16 days in length. Extended by the action on the corpus luteum of human chorionic gonadotropin (hCG) if implantation of the embryo is successful. Shorter cycles can interfere with implantation (a luteal phase defect). Corresponds with the secretory phase of the menstrual cycle.

(LUF) Occurs when a reasonably mature tertiary follicle (or preovulatory follicle) receives an LH surge that's enough to make it start producing progesterone, but is not enough to cause it to release its egg through ovulation. The result is that the egg is trapped in the follicle, which to a greater or lesser extent then functions like a corpus luteum, though this stage of the follicle's life is often shorter, giving rise to a luteal phase defect (LPD).