When it comes to making babies, women do the heavy lifting. The parts of the female reproductive system work together in a symphony of function that starts with fertilization and culminates in the birth of a baby.
Ovaries: Egg and Hormone Production
Ovaries, the female reproductive organs, have two main functions: to produce female germ cells (called eggs or oocytes) and hormones (estrogen and progesterone), which regulate the function of the ovaries. A girl is born with all the germ cells she'll ever have. When she undergoes puberty, some of these cells become mature eggs every month and are released from the ovary every month. Hormones produced by glands in the brain are responsible for triggering hormone production by the ovaries and for regulating the cyclic release of eggs. Every month, one or two mature eggs are released from the surface of the ovary and swept up into the Fallopian tubes.
Fallopian Tubes: Site of Fertilization
The Fallopian tubes provide the natural site of fertilization where egg and sperm meet. The tubes connect the surface of the ovary to the uterus. The ovulated egg is swept up into one of the tubes by the feathery ends of the tube called fimbria, which move over the surface of the ovary. Sexual intercourse deposits sperm in the vagina. From the vagina, sperm swim through the cervix into the uterus, then through the uterus and into the Fallopian tubes, where they might encounter an egg to fertilize. The Fallopian tube environment is optimized for sperm survival and fertilization. Sperm can live for several days in the female reproductive tract, waiting for an egg. The egg has a shorter shelf life after ovulation, becoming less fertilizable after 24 hours. Once fertilized, the resulting embryo spends several more days moving through the Fallopian tubes as it begins to divide, producing more cells as it grows. As it completes its journey through the Fallopian tube, the embryo reaches the blastocyst stage of development and becomes ready to implant into the uterus.
Uterus: Embryo Implantation and Development
The blastocyst consists of two types of cells, making up the trophectoderm, or the inner cell mass. The inner cell mass is made up of the cells that are fated to become the actual baby. The trophectoderm cells make the fetal part of the placenta, which will nourish the growing baby. The epithelial cells lining the uterus are normally shed every month with the menstrual flow, unless the woman becomes pregnant. If she is pregnant, the embryo produces a hormone called human chorionic gonadotropin (hCG), which signals the uterine lining to stay intact and flourish. The blastocyst embeds itself into the uterine lining and begins to burrow into the deeper cell layers to establish connections with the maternal blood system for nourishment. The uterus is where the embryo implants, forms a placenta and continues to grow until the uterus expels the baby at term.