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Ovulation in Phases : Days 15-28 are the Luteal Phase
During the Luteal Phase, the burst follicle that released the egg during ovulation develops into a small yellow structure or cyst called the corpus luteum. It secretes hormones that cause the uterine lining or endometrium to thicken and be able to nourish a fertilized egg if it were to implant. If the egg is fertilized by a man’s sperm, the resulting embryo would travel down the fallopian tube and implant in the uterus. This happens around the 20th day of the menstrual cycle.
If the egg is not fertilized and implanted, the LH (luteinizing hormone) and FSH (follicle stimulating hormone) levels will decrease, and as a result the corpus luteum will deteriorate.
Around day 26 of your cycle, the uterus will begin the process to shed its lining causing menstruation to begin.
In most women, the luteal phase is fairly consistent in its length - usually 12-14 days, regardless of the total length of a your menstrual cycle. However, 1 out of 10 women may experience a shorter luteal phase due to a decreased level of progesterone and an inadequate uterine lining. Progesterone supplements can correct this inadequacy.
If pregnancy occurs, the corpus luteum will sustain the uterine lining and pregnancy until the placenta develops.