The luteal phase (or secretory 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.
Hormonal events 
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, 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.
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".
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.
The average length of the human luteal phase is fourteen days (2 weeks). Between ten and sixteen days is considered normal, although 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). hCG 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) 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 (e.g. 9 days instead of 14) and follicular nonresponsiveness 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 placenta is self-sufficient. Some people have seen normalisation with high dose B6.
ULFS can be treated by high dose hCG at ovulation, or by IVF.
Diagnostics are by ultrasound, day 21 progesterone test, and length of luteal phase.
- Bagnell, C. 2005. "Animal Reproduction". Rutgers University Department of Animal Sciences.
- Losos, Jonathan B.; Raven, Peter H.; Johnson, George B.; Singer, Susan R. (2002). Biology. New York: McGraw-Hill. pp. 1207–09. ISBN 0-07-303120-8.
- Weschler, Toni (2002). Taking Charge of Your Fertility (Revised ed.). New York: HarperCollins. pp. 361–2. ISBN 0-06-093764-5.
- Weschler 2002, pp. 310,326
- Weschler 2002, p. 47
- Wilcox AJ, Baird DD, Weinberg CR (1999). "Time of implantation of the conceptus and loss of pregnancy". New England Journal of Medicine 340 (23): 1796–1799. doi:10.1056/NEJM199906103402304. PMID 10362823.
- Glade B. Curtis (1999). "Week 4". Your Pregnancy Week by Week. Element Books. ISBN 1-86204-396-5. Retrieved 2008-09-07.
- Díaz S, Cárdenas H, Brandeis A, Miranda P, Salvatierra AM, Croxatto HB (September 1992). "Relative contributions of anovulation and luteal phase defect to the reduced pregnancy rate of breastfeeding women". Fertil. Steril. 58 (3): 498–503. PMID 1521642.