Background: Tuberculosis prevention using isoniazid (IPT) is recommended for people with HIV, yet data on the safety of first-trimester pregnancy exposure are limited.

Methods: Planned secondary analysis in a TB prevention trial of adverse pregnancy outcomes among participants assigned to 9-month IPT who became pregnant during (IPT-exposed) or after (unexposed) IPT. Regression models compared binary outcomes of a composite adverse outcome (any non-live birth, excluding induced abortion); preterm delivery <37 weeks; and low birth weight <2500 g) among exposure groups. Models were adjusted for latent TB infection, maternal age, CD4 count and antiretroviral therapy (ART).

Results: 128 participants had a known pregnancy outcome; 39 IPT-exposed and 89 unexposed. At pregnancy outcome, ART use was lower in IPT-exposed (79%) than unexposed women (98%). Overall, 29 pregnancies ended in a composite adverse outcome (25 spontaneous abortions, 2 stillbirths and 2 ectopic pregnancies), 15 preterm deliveries, and 10 infants with low birth weight. IPT was associated with the composite adverse outcome adjusting for covariates at enrollment (aRR 1.98; 95% CI 1.15, 3.41), but the effect was attenuated when adjusted for covariates at pregnancy outcome (aRR 1.47; 95% CI 0.84, 2.55); IPT was not associated with preterm delivery (RR 0.87; 95% CI 0.32-2.42) or low birth weight (RR 1.01; 95% CI 0.29, 3.56).

Conclusion: First-trimester IPT exposure was associated with nearly two-fold increased risk of fetal demise, mostly spontaneous abortion, though the association was attenuated when adjusted for covariates proximal to pregnancy outcome including ART use. Further study is needed to inform TB prevention guidelines.

Keywords: HIV infection; TB preventive therapy; adverse pregnancy outcome; isoniazid; pregnancy.