What is the recommended dosage of Macrobid (nitrofurantoin) for a pregnant woman with an uncomplicated urinary tract infection?

Macrobid Dosing in Pregnancy

Nitrofurantoin (Macrobid) 100 mg twice daily for 7 days is the recommended regimen for treating uncomplicated urinary tract infections in pregnant women, though it should be avoided in the first trimester and near term (after 38 weeks gestation) due to theoretical risks. 1

Standard Dosing Regimen for Pregnant Women

• The recommended dose is nitrofurantoin 100 mg orally twice daily for 7 days for treatment of asymptomatic bacteriuria or symptomatic UTI in pregnancy. 1
• A 1-day regimen (100 mg twice daily for 1 day) is significantly inferior to the 7-day regimen, with cure rates of only 75.7% versus 86.2%, and should not be used. 1
• The 7-day duration is critical in pregnancy—shorter courses result in lower cure rates, lower birth weights, and earlier gestational age at delivery. 1

Critical Timing Considerations in Pregnancy

• Avoid nitrofurantoin in the first trimester when possible, as this is the period of organogenesis, though retrospective data from 91 pregnancies showed no evidence of fetal toxicity or increased malformations. 2
• Absolutely contraindicated after 38 weeks gestation and during labor/delivery due to theoretical risk of hemolytic anemia in the newborn (though this has not been definitively proven in clinical practice). 3, 2
• The safest window for use is during the second trimester and early third trimester (weeks 14-38). 3

Why Nitrofurantoin Remains Preferred in Pregnancy

• Nitrofurantoin has a 35+ year safety record in pregnancy with no documented R-factor resistance, making it superior to many newer antimicrobials. 3
• Retrospective analysis of 91 pregnancies treated with nitrofurantoin showed no significant difference in fetal death, neonatal death, malformations, prematurity, low birth weight, low Apgar scores, or jaundice compared to the general U.S. population. 2
• No abnormal events in these pregnancies were considered drug-related. 2

Alternative Agents When Nitrofurantoin Cannot Be Used

• Trimethoprim-sulfamethoxazole 160/800 mg twice daily for 3 days can be used in the second trimester only (avoid in first trimester due to neural tube defect risk and in third trimester due to kernicterus risk). 4, 5
• Fosfomycin 3 g single dose is an alternative, though it has slightly lower efficacy than nitrofurantoin (58% vs 70% clinical resolution). 6, 5
• Beta-lactams (amoxicillin-clavulanate, cephalexins) are generally safe throughout pregnancy but have higher resistance rates. 3

Renal Function Precautions

• Check creatinine clearance before prescribing—nitrofurantoin is contraindicated if CrCl <60 mL/min due to inadequate urinary concentrations and increased toxicity risk. 7
• This is particularly important in pregnancy-related renal changes or preeclampsia. 7

Common Pitfalls to Avoid

• Do not use single-dose or 3-day regimens in pregnancy—the 7-day course is essential for adequate cure rates and preventing adverse pregnancy outcomes. 1
• Do not prescribe near term (>38 weeks)—switch to alternative agents if UTI develops close to delivery. 3, 2
• Do not use for pyelonephritis—nitrofurantoin does not achieve adequate renal tissue concentrations; use IV ceftriaxone or gentamicin instead. 7, 5
• Nausea occurs in 3% of patients; taking with food may reduce this side effect. 6