Does doxycycline taken early in pregnancy cause birth defects?

It has in some cases. Doxycycline is a type of tetracycline. It has been assigned to the pregnancy category d, indicating that animal studies have revealed this can injure the embryo & fetus. Most congenital defects appear to be due to the toxicity of skeletal formation; also injuring tooth development. Some expectant mothers have taken Doxycycline with no ill effects; however, the risk may outweigh the benefit.