Is Hydrochlorothiazide Safe During Pregnancy?

What does the evidence say about hydrochlorothiazide during pregnancy?

1. A 2024 European consensus guideline lists methyldopa, nifedipine, and labetalol or metoprolol as first-choice medicines for pregnancy hypertension. It describes diuretics as unsuitable for routine blood-pressure use because of possible uteroplacental perfusion effects, while noting that hydrochlorothiazide can be considered when another indication requires a diuretic. 1
2. A review of diuretic use in pregnancy reports a small randomized chronic-hypertension study in which continued diuretic use reduced normal plasma-volume expansion compared with stopping the medicine, 18% versus 52%, but detected no perinatal-outcome difference. The study was too small to establish safety or equivalence. 2
3. A Cochrane review of five older thiazide trials involving 1,836 women found no clear reduction in preeclampsia, perinatal death, or preterm birth, and more nausea and vomiting. Trial quality was uncertain, confidence intervals were wide, and the review addressed risk reduction rather than continuation for chronic hypertension. 3
4. The hydrochlorothiazide label states that thiazides cross the placenta and lists possible fetal or newborn jaundice, thrombocytopenia, and other adult-like adverse effects. It also directs clinicians to monitor for fluid and electrolyte imbalance, including hyponatremia and hypokalemia. The pregnancy section relies on limited human data and older animal studies. 4
5. In a large U.S. case-control study, most of 28 noncardiac birth defects were not associated with first-trimester antihypertensive exposure, but small-intestinal atresia and anencephaly had elevated adjusted odds. Exposure was self-reported and class-wide, and underlying hypertension could confound results, so these findings are not hydrochlorothiazide-specific. 5

Is hydrochlorothiazide safe in each trimester?

• First trimester. Arrange a medication review as soon as pregnancy is recognized. A 2024 European guideline lists methyldopa, nifedipine, and selected beta-blockers as first-choice options, while reserving hydrochlorothiazide for situations in which a diuretic is required for another indication.
• Second trimester. Normal pregnancy includes substantial plasma-volume expansion. One very small randomized study found less expansion among women who continued a diuretic, without a detected perinatal-outcome difference, so blood pressure, hydration, kidney function, and electrolytes need individualized follow-up.
• Third trimester. The product label notes placental passage and possible newborn jaundice, thrombocytopenia, or electrolyte effects, although older randomized evidence did not confirm a clear excess. Make sure the delivery and newborn teams know about late-pregnancy exposure. Available studies do not establish that thiazides slow labor.