The Menstrual Cycle

By; Caroline Jenkins

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The Menstrual Cycle: LH, FSH, progesterone, estrogen

The pituitary gland releases gonadotropins, LH and FSH. The ovaries pick up the FSH and LH which cause the eggs to mature. This stimulates the release of estrogen and progesterone from the ovaries. Depending on how much estrogen and progesterone is released will determine how much more LH and FSH the pituitary gland releases. The feedback system is responsive to many environmental changes influences.

Endometrial lining

Estrogen causes the lining to grow and thicken to prepare the uterus for pregnancy. After ovulation, when one of the eggs is released from the ovaries, progesterone begins to increase. Progesterone prepares the endometrium to receive and nourish a fertilized egg. If pregnancy does not occur, estrogen and progesterone levels decease. The decrease in progesterone triggers menstruation, the shedding on the endometrial lining. Once the lining is shed, the process starts all over again.

Follicular Development

Increasing FSH levels bring about the maturation of the ovarian follicle. The follicular cells produce increasing levels of estrogen. The sharp increase in LH causes ovulation. Once the ovum leaves the follicle, the remaining cells turn into the corpus luteum because of LH. Increasing levels of progesterone and estrogen are produced in the corpus luteum. When estrogen and progesterone are high, FSH and LH are low. This creates a negative feedback loop.

Pregnancy and Hormones

HCG is the key hormone present during pregnancy. It's produced by what ultimately becomes the placenta. It basically tells the woman there is a life growing inside of her. Progesterone continues to be produced until about 10 weeks when it is taken over by the placenta. Estrogen plays a key role in the development of the fetus but is taken over by the placenta at about 10 weeks.

Pregnancy and the Endometrial Lining

During pregnancy, the endometrial lining should thicken. The endometrium is the layer of the uterus that thickens in preparation for the implantation of the fertilized embryo.