I had a friend who was on clomid and
estrogen (clomid helped her get pregnant, but thinned her lining, so the
estrogen helped counter that), then she went on progesterone as soon as she got a bfp.
and found this on healthywomen . org
"During the reproductive years, the pituitary gland in the brain generates hormones that cause a new egg to be released from its follicle each month. As the follicle develops, it produces estrogen, which causes the lining of the uterus to thicken.