The synergistic effect of estrogen and progesterone refers to the fact that progesterone builds on the effects of estrogen to encourage further development of the female genitals and breasts in preparation for pregnancy. Estrogen and progesterone are both hormones secreted by the ovaries and are both steroidal hormones. Estrogen and progesterone have both synergistic and antagonistic effects. 1. Synergistically, estrogen promotes the development of female genitals and breasts, while progesterone, on the basis of estrogen’s action, further promotes the development of female genitals and breasts in preparation for pregnancy. 2. Antagonism, estrogen can promote endometrial proliferation and repair, while progesterone can limit endometrial proliferation, and promote the transformation of the proliferating endometrium into the secretory phase, preventing excessive endometrial proliferation. Estrogen and progesterone also have antagonistic effects on uterine contraction, tubal peristalsis, and sodium and water retention and excretion.