Merck Manual

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Effect of Reproductive Treatment on the Fetus or Neonate in Animals


Theresa Beachler

, DVM, PhD, DACT, Veterinary Diagnostic and Production Animal Medicine, College of Veterinary Medicine, Iowa State University

Reviewed/Revised Aug 2021 | Modified Nov 2022

An important component of reproductive pharmacology encompasses the effect of treatment on the fetus or neonate of medication administered to pregnant dams or lactating dams that are nursing. Many factors influence the ability of a drug to cross the placenta, including placental architecture of the particular species. In general, drugs that are lipid-soluble, nonionized, and of low molecular weight can be expected to cross the placenta readily.

The potential effects of pharmacotherapeutics on pregnancy and neonatal development are being increasingly studied in domestic animals; frequently, however, inferences must be made from studies in other species or humans. Many drugs are known to be teratogenic during pregnancy; these should be avoided. Among antimicrobials, aminoglycosides are associated with nephrotoxicity and ototoxicity in the fetus, fluoroquinolones may affect developing cartilage, and tetracyclines affect bone and tooth development. Teratogenicity has been associated with use of the antifungal agents griseofulvin and ketoconazole in pregnant animals. All cancer chemotherapeutic agents are potentially harmful to developing fetuses while glucocorticoids may induce palatoschisis or other defects in puppies, in addition to pregnancy loss.

Any administration of medication to lactating animals requires consideration of the excretion of the drug or its metabolites in milk and of the effects on suckling neonates. Milk produced for human consumption must be free of potentially harmful residues, and all relevant laws and regulations regarding usage and appropriate withdrawal times should be followed.

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