Serum levels of many drugs are lower in pregnancy compared with the nonpregnant state. Their renal clearance is higher and the maternal intravascular volume is expanded. Most antibiotics are safe for use in pregnancy and lactation. Tetracyclines cause fetal tooth discoloration and inhibition of bone growth if used in the second and third trimesters. Aminoglycosides carry a small risk of fetal ototoxicity. Quinolones may cause arthropathies in children, and so are currently not recommended for use in pregnancy. Most drugs are secreted into breast milk in very small amounts, not enough to have any therapeutic effect. The only antibiotics which may be of concern in nursing infants are sulfonamides and quinolones, and possibly metronidazole.