Every month, during the childbearing years, a woman's body prepares an egg for possible fertilization. A woman's entire supply of eggs actually develops when she is a fetus. A newborn girl has approximately 2 million immature eggs at birth. By the time puberty arrives, the number has been reduced to about 300,000 to 400,000 eggs.
Each month the hypothalamus of the brain tells the pituitary gland to secrete follicle-stimulating hormone (FSH) which causes the ovaries to produce a mature egg. When the egg is at the proper stage, the pituitary gland secretes the next hormone, lutenizing hormone (LH), which causes the ovary to release the egg. In the mean time, the hormones estrogen and progesterone are causing the lining of the uterus to thicken and prepare for a possible pregnancy.
The egg travels down into the fallopian tube. Sperm must reach the mature egg while it is in the fallopian tube in order for fertilization to occur. Once an egg is released from the ovary it only lives about one day, making timing crucial. If the egg and the sperm are united, the fertilized egg (zygote) must continue down the fallopian tube and into the uterus, finally implanting itself into the uterine lining. The fertilized egg must then divide and develop normally to become a healthy fetus.
If fertilization does not occur, the uterine lining is sloughed off in the monthly menstrual cycle. The complete cycle repeats itself roughly every 28-32 days.
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