Following ovulation, what remains of the ruptured follicle transforms itself into a structure called the Corpus Luteum (CL) which produces progesterone. The purpose of progesterone is to prepare the uterus to accept and support an early pregnancy until it is able to sustain itself at around 8-10 weeks of gestation.

The life span of the CL is predetermined to be 12-13 days, unless rescued by a signal from the early pregnancy. If no pregnancy occurs, the CL stops making progesterone and menses ensues 1-2 days later. For most women the length of the second half of the menstrual cycle (the luteal phase) is constant at 14 days. A small percentage of infertile women (3-4%) have a shortened luteal phase. This may result in the loss of pregnancy support before the budding pregnancy has a chance to signal the ovary that it is there. The lining of the uterus (the endometrium) has a specific appearance that changes throughout the menstrual cycle, such that a biopsy of the lining a few days prior to expected menstruation, can accurately date endometrial development. A 3 or more day difference between endometrial dating by biopsy and cycle day as determined by the start of the next menstrual period is indicative of a luteal phase defect (LPD). Sequential mid luteal progesterone levels < 10 ng/dl can also be used to diagnose a LPD. Luteal phase defect can be treated with Clomiphene Citrate, Progesterone supplementation or hCG injection PROGESTERONE LEVELS Normal Progesterone (P) levels rise sharply after ovulation, peaking a week after ovulation. Because P secretion occurs in a pulsatile fashion (around every 90 minutes), single low levels are often found in the course of a normal luteal phase and cannot be used as a predictor of luteal defect or poor fertility potential. In pregnancy, P is secreted by the corpus luteum (of pregnancy) until about 10 weeks of gestation. Because of the large variation in individual P levels, the predictive value of single P measurement in early pregnancy is limited. Usually levels above 20 ng/ml indicate a normal pregnancy while levels below 5 ng/ml suggest a potential problem. It is also important to point that persistent low P levels are usually the result, rather than the cause, of poor outcome and that unfortunately, as a result, P supplementation does not prevent the inevitable.