Is the luteal phase measured by the number of days with temps above the coverline or by the number of days from ovulation to the first day of menstruation?
The luteal phase starts right after ovulation.
Menstrual cycle could be divided in preovulatory and postovulatory phases as long as the ovary is concerned. Luteal phase starts midcycle, i.e. after ovulation and ends with the end of the lunar cycle.
Luteal phase starts after egg is released from the ovarian follicle by ovulation. The empty follicle soon gets transformed into an endocrine gland, which is now called a corpus luteum . This structure starts secretion of progesterone.
During luteal phase, progesterone protects a thickened endometrial layer, so that an implantation may take place in case of a possible fertilisation.
Luteal phase will end when there is no fertilisation of egg and there is no feedback in the form of the human chorionic gonadotropin (HCG) hormone. Regression of luteum leads to drastic decline in the level of progesterone, hence uterine endometrium could not be maintained in a thickened condition anymore. Sloughing off of endometrium leads to menstrual bleeding, i.e. it marks the beginning of another cycle.