Overall, "accidently" continuing oral contraceptives while pregnant does not place patients in a higher risk category or significantly raise the risk of birth defects.
Oral contraceptives in a typical dose
do not function as abortifacients.
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Emergency contraception (EC) is an extremely high and specially timed dose of oral contraceptives intended to prevent
ovulation. EC has not been scientifically proved to prevent implantation of a fertilized egg.
The sites below offer basic information on the mechanism for oral contraceptives, contraindications, etc.
Wikipedia page on Oral Contraceptives
Oral contraceptive - Wikipedia, the free encyclopedia
Mayo Clinic information on Oral Contraceptives
Birth control pill FAQ: From menstruation to menopause — and everything in between - MayoClinic.com
Merck information on contraception
Contraception: Family Planning: Merck Manual Professional