The risk of tooth abnormalities is greatest from the middle to the end of pregnancy. Because several safe alternative antibiotics are available, tetracyclines are avoided during pregnancy. Taking antibiotics such as streptomycin or kanamycin during pregnancy may damage the fetus' inner ear, possibly resulting in deafness. Chloramphenicol doesn't harm the fetus but can cause a serious illness in the newborn called gray baby syndrome. Ciprofloxacin should not be taken during pregnancy because it has been shown to cause joint abnormalities in animals.
Penicillins appear to be safe.
Most sulfonamide antibiotics taken late in pregnancy may cause the newborn to develop jaundice, which can result in brain damage. However, one sulfonamide antibiotic, sulfasalazine, is much less likely to cause this problem. Significant birth defects occur in up to one fourth of babies exposed to the drug during the first 3 months of pregnancy. Also, abnormal bleeding may occur in both the woman and fetus. If a pregnant woman is at risk of developing blood clots, heparin is a much safer alternative.