Fetal development occurs in three stages: fertilization/implantation, embryonic period, and the fetal period.
Fertilization and implantation lasts from conception until about 17 days’ gestation. During this time, exposure to toxins will typically result in death followed by spontaneous abortion.
The embryonic stage starts at the end of implantation and lasts through about 55 days’ gestation. Organogenesis occurs during this period and represents a crucial time when teratogenesis is likely to occur.
During the fetal stage, the fetus has a higher barrier of resistance to the effects of teratogens. However, reductions in cell size or number can occur leading to intrauterine growth retardation.
See Table 57-1 for a classification of drugs used in pregnancy.
Beta-lactams (with the exception of imipenem-cilastatin) are pregnancy category B and should be considered first line whenever possible.
Macrolides, clindamycin, daptomycin, and fosfomycin are also pregnancy category B.
Nitrofurantoin, while pregnancy category B, should be avoided near term due to possible hemolytic anemia.
While trimethoprim-sulfamethoxazole has been teratogenic to rats and its use should be cautioned, a number of observational studies support its safety and should be used if necessary.
Chloramphenicol should be avoided near term due to the potential for “gray baby syndrome.”
Table 57-1 Drug Classifications
Category B
Category C
Category D/X
Antibacterial
Azithromycin Beta-lactams Clindamycin Daptomycin Erythromycin Fosfomycin Metronidazolea Nitrofurantoin Polymyxin Quinupristin-Dalfopristinb
Bacitracin Chloramphenicolc Clarithromycin Colistin Fluoroquinolones Linezolid Sulfamethoxazole Sulfadiazined Telavancine Telithromycin Trimethoprim Vancomycin Methenamine
Aminoglycosides Neomycin Tetracyclines Tigecycline
Antiviral
Famciclovir Telbivudine Tenofovir Valacyclovir
Acyclovir Cidofovir Entecavir Foscarnet Ganciclovir Lamivudine Oseltamivir Rimantadine Valganciclovir Zanamivir
Ribavirin—X
Antifungal
Amphotericin Nystatin Terbinafine
Echinocandins Fluconazole Flucytosine Itraconazole Posaconazole
Voriconazole
Antiparasitic
Nitazoxanide Permethrin
Albendazole Atovaquone Chloroquine Dapsone Ivermectin Mebendazole Mefloquine Primaquinef Pyrimethamineg Quinidine Quinine Tinidazole
Antituberculosis
Ethambutol Rifabutin
Isoniazid Rifampin Rifapentine Pyrazinamideh
Streptomycin
a Contraindicated during first trimester.
b Manufacturer does not recommend.
c Should not be used near delivery due to gray baby syndrome.
d Contraindicated near delivery due to risk of kernicterus.
e May cause fetal harm; avoid during pregnancy.
f Risk of hemolytic anemia if fetus is G6PD deficient, defer treatment until after delivery.
g Supplement with folinic acid 5 mg daily if used.
h CDC does not recommend during pregnancy.Related posts:
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