J Pediatr 166 (5): 1246 - 1251, 2015. doi: 10.1016 / j. jpeds.2015.02.015 Use During Pregnancy and Breastfeeding Tetracyclines cross the placenta, enter fetal circulation, accumulate in fetal bones, and, if used during the 2nd or 3rd trimester, may cause permanent discoloration of the fetus's teeth.