If you are currently pregnant, taking an EC will not affect your pregnancy. These pills work by stopping an egg from being released from the ovary, stopping fertilization of the egg or stopping a fertilized egg from attaching to the wall of the uterus. They do not make you abort a fetus if you are already pregnant and do not cause birth defects to a developing fetus.

Sources:

"Birth Control," 2011, Staff Writer, Family Health Services

"Emergency Contraception: Dispelling the Myths and Misperceptions," 2010, Elizabeth Westley and Anna Glasier, World Health Organization