Metolazone has been assigned to pregnancy category B by the FDA. Animal studies have failed to reveal evidence of teratogenicity. There are no controlled studies in human pregnancy; however, it is known that metolazone crosses the placental barrier and appears in cord blood. Metolazone should only be given during pregnancy when need has been clearly established.

Although thiazide and thiazide-type diuretics were considered nonteratogenic when used to treat toxemia in the past, many experts now consider them to be contraindicated in pregnancy, except for some patients with heart disease, since they do not prevent or treat toxemia and they may decrease placental perfusion.

An increased risk of malformations was associated with the use of some thiazide diuretics in the first trimester in an evaluation of 50,282 mother-child pairs in the Collaborative Perinatal Project. In other large studies diuretics were associated with labor induction problems and uterine inertia. Unusual side effects of thiazide diuretics include hypoglycemia, neonatal thrombocytopenia and hemolytic anemia, electrolyte imbalances, and rare cases of acute hemorrhagic pancreatitis.

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Data reveal that related drugs, such as chlorothiazide, chlorthalidone, and hydrochlorothiazide are excreted into human milk in low concentrations. The highest ratio of milk to maternal plasma drug level has been reported with hydrochlorothiazide (1:4), which represented a clinically insignificant amount of drug to the nursing infant. Rare cases of thrombocytopenia have been reported in nursing infants whose mothers were taking chlorothiazide.

Metolazone is excreted into human milk. The manufacturer recommends that due to the potential for serious adverse reactions in nursing infants, a decision should be made to discontinue nursing or discontinue the drug, taking into account the importance of the drug to the mother.

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