What causes anitbodies in a pregnancy?

Probably OK. Blood tests in pregnancy include a check for any antibodies that could hurt your baby. Typically these would be antibodies against blood groups that you might have gotten from a previous pregnancy or, less likely, from a blood transfusion. Most that are found are not harmful to your baby. Discuss it with your doctor but don't worry if it's not an antibody that is known to cause harm.
Your baby. As the mother, you will be introduced to new "antigens"- or cells or material from your baby that are foreign to your immune system. Your body then responds by producing "antibodies" as a natural immune response. Some of these then may adversely affect you and/or your baby. This typically happens with blood cells.