The luteal phase is the second phase of the menstrual cycle, during which the body prepares itself for pregnancy. If a woman does not become pregnant, the luteal phase ends with the shedding of the uterine lining. If a pregnancy occurs, hormones secreted by the developing fetus will prevent the shedding of the lining. Women who are trying to get pregnant are sometimes very interested in tracking their luteal phases, along with the menstrual cycle in general.
This phase starts when ovulation ends, and the ovarian follicle which produced an egg during ovulation begins to develop a structure called the corpus luteum or “yellow body.” This structure produces progesterone, which is used to thicken the endometrium and prepare it for implantation of an embryo. If no embryo implants, the corpeus luteum gradually withers, allowing the lining to thin and slough off in a menstrual period. A woman's basal body temperature also rises during the luteal phase, turning her into a literal incubator.
There is a small window for pregnancy to occur; the egg lasts less than a day after ovulation occurs, and a fertilized embryo must implant before the corpus luteum starts to atrophy. Delays in fertilization and implantation can result in a miscarriage, in which an embryo is shed along with the uterine lining because it did not implant early enough to stop the luteal phase. Women who are trying to avoid pregnancy should be aware that sperm can last up to seven days in the Fallopian tubes while it waits for the emergence of an egg to fertilize.
Classically, the luteal phase lasts between 12 and 16 days, with 14 days being the most common. A luteal phase of at least 10 days is needed for pregnancy to occur, and women who are struggling with fertility issues sometimes find that they can't get pregnant due to a so-called “luteal phase defect” which can be treated by a doctor. Ovulation problems can be diagnosed with the use of ovulation charting to keep track of ovulation, and blood tests which look for levels of specific hormones to indicate the stage of a woman's cycle.
The length of a menstrual period is primarily determined by the time at which ovulation occurs. Ovulation can occur early or late for a variety of reasons, ranging from stress to medical problems. The luteal phase tends to be very regular in length, unless a woman is experiencing a medical issue, which makes it easy for women to determine when their periods will arrive if they are able to pinpoint the time of ovulation.