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. Another is the endocannabinoid anandamide AEA, where the lowest plasma AEA level is observed in the luteal phase. The opposite of the luteal phase, the rest of the two weeks, is called the follicular phase.
After ovulation, the pituitary hormones–FSH and LH released from the anterior pituitary 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, estrogen. 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.[medical citation needed] The increased progesterone and BBT will cause at least a modest rise in BMR at the start of the luteal phase and remains in an elevated state until the phase ends. Research has discrepant findings on how much of an increase typically occurs and how much it can vary. Early, small sample studies have found various figures, such as; a 6% higher postovulatory sleep metabolism, a 7% to 15% higher 24 hour expenditure following ovulation, and an increase and a luteal phase BMR by up to 12%. A study by the American Society of Clinical Nutrition found that an experimental group of female volunteers had an 11.5% average increase in 24 hour energy expenditure in the two weeks following ovulation, with a range of 8% to 16%. This group was measured via simultaneously direct and indirect calorimetry and had standardized daily meals and sedentary schedule in order to prevent the increase from being manipulated by change in food intake or activity level. A 2011 study conducted by the Mandya Institute of Medical Sciences found that during a woman’s follicular phase and menstrual cycle is no significant difference in BMR, however the calories burned per hour is significantly higher, up to 18%, during the luteal phase. Increased state anxiety (stress level) also temporarily increased BMR.
Several days after ovulation, the increasing amount of estrogen 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 estrogen surge".[medical citation needed]
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 estrogen. These falling levels of ovarian hormones cause increased levels of FSH, which begins recruiting follicles for the next cycle. Continued drops in levels of estrogen and progesterone trigger the end of the luteal phase: menstruation and the beginning of the next cycle.[unreliable medical source?]
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.[medical citation needed]
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.
For diagnosis of LPD, the main methods are:
- An endometrial biopsy, preferably on the 12th day when there is an expected 14 day luteal phase.
- Vaginal ultrasonography, to detect abnormalities in folliculogenesis.
The characteristics of the rise in basal body temperature after ovulation is not helpful in the diagnosis of luteal phase defect, but there is a high likelihood of luteal phase defect if it lasts less than 11 days. A single progesterone test is unreliable because of high variability across the day, and multiple tests are expensive and inconvenient.
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