The luteal phase is the latter phase of the menstrual cycle (in humans and a few other animals) or the earlier phase of the estrous cycle (in other placental mammals). It begins with the formation of the corpus luteum and ends in either pregnancy or luteolysis. The main hormone associated with this stage is progesterone, which is significantly higher during the luteal phase than other phases of the cycle. The opposite of the luteal phase, the rest of the four weeks, is called the follicular phase.
After ovulation, the anterior pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum. It continues to grow for some time after ovulation and produces significant amounts of hormones, particularly progesterone, and, to a lesser extent, oestrogen. Progesterone plays a vital role in making the endometrium receptive to implantation of the blastocyst and supportive of the early pregnancy; it also has the side effect of raising the woman's basal body temperature.
Several days after ovulation, the increasing amount of oestrogen produced by the corpus luteum may cause one or two days of fertile cervical mucus, lower basal body temperatures, or both. This is known as a "secondary oestrogen surge".
The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. With continued low levels of FSH and LH, the corpus luteum will atrophy. The death of the corpus luteum results in falling levels of progesterone and oestrogen. These falling levels of ovarian hormones cause increased levels of FSH, which begins recruiting follicles for the next cycle. Continued drops in levels of oestrogen and progesterone trigger the end of the luteal phase: menstruation and the beginning of the next cycle.
The human luteal phase lasts between ten and sixteen days, the average being fourteen days. Luteal phases of less than twelve days may make it more difficult to achieve pregnancy. While luteal phase length varies significantly from woman to woman, for the same woman the length will be fairly consistent from cycle to cycle.
The loss of the corpus luteum can be prevented by implantation of an embryo: after implantation, human embryos produce human chorionic gonadotropin (hCG), which is structurally similar to LH and can preserve the corpus luteum. Because the hormone is unique to the embryo, most pregnancy tests look for the presence of hCG. If implantation occurs, the corpus luteum will continue to produce progesterone (and maintain high basal body temperatures) for eight to twelve weeks, after which the placenta takes over this function.
Luteal phase defect
Luteal phase defect (LPD), or luteal insufficiency, occurs when the luteal phase is shorter than normal, progesterone levels during the luteal phase are below normal, or both. LPD is believed to interfere with the implantation of embryos.The lactational amenorrhea method of birth control works primarily by preventing ovulation, but is also known to cause LPD.
LPD is a spectrum. There is unruptured luteinized follicle syndrome (ULFS), short luteal phases that are, for example, nine days long, and follicular non-responsiveness to hCG. The second two varieties can be stabilized by taking high-dose progesterone suppositories or injections until one gets a positive pregnancy test, and then continuing for another 8–10 weeks until the placenta is self-sufficient. Some people have seen normalization with a high dose of vitamin B6.
ULFS can be treated by a high dose of hCG at ovulation, or by IVF.
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