The luteal phase is the latter phase of the menstrual cycle or the earlier phase of the
estrous cycle. 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.
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, 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, 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 for eight to twelve weeks, after which the placenta takes over this function.
Luteal phase defect Luteal phase defect 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, 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.
LPD can be diagnosed by ultrasound, day 21 progesterone tests, and observing the length
of luteal phase. Behavioral effects
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Rutgers University Department of Animal Sciences. ^ a b c d Losos, Jonathan B.; Raven, Peter
H.; Johnson, George B.; Singer, Susan R.. Biology. New York: McGraw-Hill. pp. 1207–09.
ISBN 0-07-303120-8. ^ a b Weschler, Toni. Taking Charge of Your
Fertility. New York: HarperCollins. pp. 361–2. ISBN 0-06-093764-5.
^ Weschler 2002, pp. 310,326 ^ Weschler 2002, p. 47
^ Wilcox, A. J., Baird, D. D., Weinberg, C. R.. "Time of implantation of the conceptus
and loss of pregnancy". New England Journal of Medicine 340: 1796–1799. doi:10.1056/NEJM199906103402304.
PMID 10362823. ^ Glade B. Curtis. "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, A. M., Croxatto,
H. B.. "Relative contributions of anovulation and luteal phase defect to the reduced pregnancy
rate of breastfeeding women". Fertility and Sterility 58: 498–503. PMID 1521642.