Any infections or toxic substances that can cross the placenta while the mother is pregnant can affect this baby's future childbearing. However, this will not be known until she is ready to have a child of her own. An example of this is DES exposure which has affected a whole generation of women. Girls are beginning menstruation at an earlier age than a century ago due to better nutrition. There is a set amount of body fat that is needed for menstruation to begin and because girls weigh more today at an earlier age, they are menstruating earlier.
Every period starts a cycle that usually produces a single egg that can be fertilized.
Cycle lengths vary from 21 - 35 days, but are usually 28 days in length from the beginning of one period to the beginning of the next period. These hormones are then released into the bloodstream, where they are conveyed to the ovaries. Stress, grief, trips or anything that disrupts a woman's life can keep FSH and LH from being released, thus causing lack of a period. FSH and LH reach the ovaries and signal them to produce the female hormone, estrogen. Estrogen signals an egg to begin to ripen.