The birth control pill is roughly 99 percent effective when taken properly. However, women who use the pill still get pregnant 1 percent of the time. Generally speaking, if a woman keeps taking her birth control pill while she's pregnant, the result is that nothing harmful will happen to her baby, though she should visit her doctor once she discovers she's pregnant to make sure the unborn child is developing in a healthy way. Negative side effects on the health of the mother taking birth control pills while pregnant include more acute pregnancy symptoms like headaches and nausea. As a result, it's best for a woman to stop taking the pill as soon as she discovers she's pregnant.
Hormones
The birth control pill operates by changing the dosages of hormones in a woman's body. The hormones estrogen and progesterone are both tied to ovulation, which is the process of dropping eggs to be fertilized. If an egg is already fertilized (i.e., the woman is pregnant), then taking the birth control pill will have no effect on the egg. The hormones in the pill only prevent the egg from dropping, and once it's already fertilized, the only thing the pill will do is increase the levels of estrogen and progesterone in the woman's body. Though concentrated forms birth control pills can be used as an emergency contraception method, they only work within days of fertilization. By the time a woman finds out she is pregnant, normal birth control concentrations will not act in the same way.
Estrogen
In the past, it was recommended that women who get pregnant should take more estrogen to prevent miscarriages. However, artificially raising the levels of estrogen in pregnant women has been linked to the later development of cancer. Since birth control pills raise a woman's estrogen, they shouldn't be taken once a woman discovers she's pregnant. The birth control pill only regulates normal hormones in the mother's body.
