Also known as the secretory phase, the luteal phase is the last stage of the menstrual cycle. At the beginning of the average luteal phase, a temporary endocrine structure called the corpus luteum is formed. The corpus luteum secretes progesterone and estrogen, thickening the endometrium in order to prepare it for implantation. The luteal phase ends in either pregnancy if an egg is fertilized, or the decay of the corpus luteum provided that fertilization does not occur.
The corpus luteum continues to grow and produce large quantities of hormones, especially progesterone, after ovulation. If pregnancy does not occur, then the decay of the corpus luteum leads to plummeting progesterone and estrogen levels which trigger the end of the average luteal phase. At this time, the corpus luteum is purged from the body via menstruation, and the cycle continues once again. If an embryo is implanted during the luteal phase then the corpus luteum stays intact, and will continue to preserve high basal body temperatures as well as produce progesterone for 8-12 weeks, after which time the placenta takes over this role.
The average luteal phase has a duration of 14 days, but can range anywhere from 10-16 days. If a woman’s luteal phase falls under a 12-day duration, she may experience difficulties becoming pregnant. A luteal phase that is below 10 days is considered to be a sign of Luteal Phase Defect (LPD), which can also occur when progesterone levels are lower than they normally are during the average luteal phase. A below average luteal phase cannot effectively support a pregnancy due to the fact that the lining of the uterus breaks down, which causes menstrual bleeding and subsequently, premature miscarriage.
There are various potential causes of LPD, and a medical examination can determine if a combination of more than one is impeding a patient from becoming pregnant. Low follicle production is a cause of LPD, as the corpus luteum evolves from a follicle. If the corpus luteum is able to form, its premature decay or inability to sufficiently line the uterus with progesterone could both prevent pregnancy. The most common cause of LPD, however, is low levels of progesterone in the body, which can be diagnosed by a doctor via a progesterone test and is correctable.