Pregnancy and Embryonic Development
The ovarian hormones estrogen and progesterone regulate the female sexual cycle, on one hand they exert a negative feedback on the pituitary thereby controlling the release of gonadotropins and on the other hand modify the reproductive tract to maintain pregnancy.In the beginning the rising level of the estrogens suppresses the release of FSH and stimulates the pituitary to secrete LH. Which in turn bring in about ovulation and development of corpus luteum.During later stages the level of progesterone increases more than estrogens. Thus, under the influence of progesterone, uterine endometrium gets modified for implanting the fertilized egg.Functions of the ovarian hormone, Although estrogen in the beginning and progesterone during later stages, influence almost all the regions of the reproductive tract, their action on the uterus is of great significance because it is the site where an embryo develops.Progesterone acting synergistically with estrogen causes considerable swelling of the endometrium as a result of general increase in stored nutrients, glandular and vascular supply. At the end of sexual cycle endometrium doubles its thickness reaching up to 4 to 6 mm. The fertilized egg will get implanted in the endometrium for further development during which it supplies nutrients to the growing embryo and later developing placental connection with it.The finger like projections on the trophoblast called chorionivilli and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.The placenta facilitates the supply of O2 and nutrients to the embryo and also removal of CO2 and excretory waste materials produced by the embryo.The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo.Placenta also acts as an endocrine tissue and produces several hormones like Human Chorionic Gonadotropin (HCG), human placental lactogen, estrogen and progestin, etc in the later phase of pregnancy a hormone called relaxin is also secreted by the ovary.

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