In the female menstrual cycle, what changes take place in the uterus on days 15 through day 28?
Uterus enters secretory phase after ovulation-- ovulation generally takes place on 14th day of a 28-day menstrual cycle.
Menstrual cycle is divided in distinct pre-ovulatory and post-ovulatory phases. In first 14 days of preovulatory phase, ovary secretes oestrogen. This hormone helps the endometrial lining of uterus to undergo proliferation and thickening.
After ovulation, from 15th day onwards, ovary starts secreting progesterone. This hormone will help the uterine thickening to remain intact.
Endometrial glands also start secreting nourishing fluid to make the uterus more welcoming for an embryo (as there is possibility of fertilisation of the egg in fallopian tube). Thus the uterus is said to be in secretory phase from 15th day onward.
When there is no fertilisation, there is no implantation of embryo, no subsequent secretion of human chorionic gonadotropin(HCG) from uterine lining.
[HCG provides positive feedback for progesterone secretion to ovary. In absence of HCG, there will be fast decline of progesterone secretion from ovary. ]
This will lead to regression of endometrial glands. There will be beginning of degeneration of endometrial thickening from 24th day onwards (in absence of fertilisation and implantation), upto the beginning of a new cycle (which is generally upto 28th day).
A new cycle starts when degenerated endometrial tissue slough off and comes out as menstrual secretion, this is regarded as 1st day of next new cycle.