Each month most females will release an egg into their ovaries. The FSH or (follicle stimulating hormone) eggs will be begin to grow and mature inside the ovary. When the egg is ready to be released the luteinizing hormone or LH will finally reach the peak level. With the high hormone levels, the egg will burst through the follicle. This sets off a reaction to the hormone estrogen which in turn makes the uterus lining thicker. The lining sets itself up to protect the egg and nourish the egg if it is fertilzed. If the egg doesn't get fertilized, the uterine lining will decrease and shed. This is when a woman gets her menstrual cycle. So the increase in the estrogen is what makes the thickness of the uterine lining.
