These synthetic hormones mimic the effects of a woman's natural estrogen and progesterone produced by the ovaries. This constant, steady level of hormones feeds back to the pituitary gland, inhibiting its normal cyclic production of hormones that stimulate the ovaries to ovulate. Thus, if a woman does not ovulate, she cannot become pregnant. Many pill users experience some nuisance side effects such as nausea, spotting or even bleeding between periods. Mood changes, weight gain, breast fullness or tenderness, headaches, and decreased sex drive may also occur with pill use.
Side effects can vary from one woman to another and from one pill to another.
Most side effects resolve on their own in a matter of days to a few months. Serious side effects are rare, but they do occur. Also, birth control pills are expensive. Thus if your period starts on a Tuesday, you start the birth control pills that day. The advantage to the Sunday start cycle is that you will never have your period on the week-end.