Women who become pregnant while on birth control, may continue taking the pill because they do not realize they are pregnant. This situation isn't a cause for alarm, but once pregnancy has made itself known, it's a good idea to get off the pill.

Effects

There aren't any harmful effects to the fetus if you're taking birth control while pregnant, according to the American Congress of Obstetricians and Gynecologists. The symptoms of pregnancy will not be masked by the pill or other hormonal birth control methods.

Pregnancy

Symptoms of pregnancy, which include nausea, vomiting and weight gain won't be masked if you are still on birth control. If pregnant, you won't get your period during the week of inactive pills. This is the time to see a doctor or take a pregnancy test and get off your birth control.

Increased Hormones

During pregnancy there is an increased level of estrogen hormone, which is the hormone found in birth control. This isn't dangerous, but women can do without the extra estrogen.

Nutrition

Early pregnancy nutrition is extremely important. Because women--especially those on the pill--may not realize they are pregnant for some time, they could miss a crucial window during which their baby needs special nutrition such as folic acid. Once you realize you're pregnant, get off the pill and see a doctor about appropriate nutrition.

Other Substances

While hormonal birth control doesn't seem to have a negative or harmful effect on an unborn child, substance abuse such as drugs or alcohol can be detrimental to a fetus. Alcohol use, even just a little bit anytime during the pregnancy can lead to Fetal Alcohol Syndrome or other defects. Drug abuse, including tobacco, can result in miscarriage, fetal stroke, bleeding and other harmful effects, according to UCSF Medical Center.

Source:

The Mayo Clinic

American Congress of Obstetricians and Gynecologists

University of California San Francisco Medical Center