Agent	Risk category	Fetal effects	Maternal effects	Excretion route	Half-life in nonpregnant women (hours)*
Considered safe
Aminopenicillin (i.e., ampicillin)	B	None known	Allergic reaction, rash (5% to 7%)	Renal	1
Cephalosporin (1st, 2nd, 3rd and 4th generations)	B	None known	Allergic reaction; bleeding if methylthiotetrazole side chain	Renal, hepatic	0.5 to 8
Erythromycin (base, stearate or ethyl-succinate)	B	None known	GI upset; allergic reaction	Hepatic	2 to 4
Clindamycin (Cleocin)	B	None known	Pseudomembranous colitis	Hepatic	2.4
Probably safe—use with caution
Azithromycin (Zithromax)	B	None known	Occasional GI symptoms	Hepatic	68
Metronidazole (Flagyl)	B	None known†	GI; reversible neuropathy; intolerance to alcohol	Renal	8
Nitrofurantoin (Furadantin)	B	None known	Hemolysis if G6PD-deficient; neuropathy	Renal	0.3 to 1
Contraindicated
Doxycycline (Vibramycin)	D	Bone and teeth	Occasional GI symptoms; photosensitivity, hepatotoxicity	Renal	8 to 19