Stages of Pregnancy and Fetal Development Introduction

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Menstrual Phase: The female reproductive cycle (see figure 3) is a monthly cycle that begins with menstuation (shedding of the endometrium, or uterine lining) and ending with either pregnancy or the beginning or another menstruation.

Proliferative Phase: Following the menstual phase when hormone levels drop off and the uterine lining is shed, the proliferative phase begins, and estrogens produced by the ovary begin to act on the female reproductive tissues. The endometrium thickens in preparation for implantation of a fertilized egg. Meanwhile, in the ovary, the cycle of ovulation (see figure 2) is preparing for release of a mature oocyte, or egg. Developing from primary follicle, then a secondary follicle, and finally a mature follicle, the mature egg awaits hormonal signals that cause its release from the ovary. The increase in levels of estrogens produced by the ovary causes the hypothalamus to secrete a hormone GnRH which in turn causes the anterior pituitary gland to secrete large amounts of the hormones FSH (Follicle Stimulating Hormone) and LH (Leutinizeing Hormone).

Ovulation: The LH surge causes the egg to be released from the mature follicle and the ovary itself. It is released into the body cavity where it is (almost always) caught by the fimbriae (finger-like projections) of the fallopian tube where it travels toward the uterus (see figure 1), awaiting fertilization by a sperm.

Secretory Phase: Once the egg is released from the ovary, the mature follicle from which it erupted becomes the corpus luteum, which still secretes small amounts of estrogen. But the secretory phase is dominated by the hormone progesterone, which is produced by the cells of the ovary. The secretion of progesterone causes the enometrium to thicken and prepare for implantation of an embryo.

Fertilization and Implantation: If the egg is not fertilized by a sperm, the corpus luteum regresses, levels of estrogens and progesterones drop off, and the mentrual phase begins the reproductive cycle again. Yet if the egg is fertilized by a sperm while traveling down the fallopian tubes and the embyo implants into the wall of the uterus, the future placental tissues secrete chorionic gonadotropin which maintains the corpus luteum. The corpus luteum of pregnancy, as it is now called, and is responsible for producing the estrogens and progesterone required to maintain the pregnancy for the first few months of the pregnancy. After that point, the placenta itself produces enough estrogesn and progesterone to maintain the pregnancy on its own.(3)