Estrogen is the hormone that stimulates the growth
of the uterine lining (endometrium), causing it
to thicken during the pre-ovulatory phase
of the cycle.
One of progesterone's most important functions is to cause the endometrium to secrete special proteins during the second half of the menstrual cycle, preparing it to receive and nourish an implanted fertilized egg. If implantation does not occur, estrogen and progesterone levels drop, the endometrium breaks down and menstruation occurs.