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The aim of this study was to investigate whether specific nucleoside reverse transcriptase inhibitor (NRTI) backbones are associated with risk of adverse pregnancy outcomes among pregnant women starting antiretroviral therapy (ART).Seven observational studies across eight European countries of pregnancies in HIV-positive women.Individual-level data were pooled on singleton pregnancies conceived off-ART in which a single combination ART regimen was initiated at least 2 weeks before delivery, and ending in a live birth in 2008–2014. Preterm delivery (PTD) was defined as less than 37 gestational weeks and small-for-gestational-age (SGA) as less than 10th percentile according to INTERGROWTH standards. Poisson regression models were fitted to investigate associations between NRTI backbones and PTD/SGA.Out of 7193 pregnancies, 45% (3207) were in UK/Ireland, 44% (3134) in Ukraine. 10% (722/7193) of deliveries were preterm and 11.1% (785/7089) of newborns SGA. The most common NRTI backbones were zidovudine (ZDV)-lamivudine (3TC) (71%), tenofovir (TDF)-XTC (16%) and abacavir (ABC)-3TC (10%) with TDF-containing backbone use increasing over time. Overall, 77% of regimens contained ritonavir-boosted lopinavir (LPV/r). There was no association between NRTI backbone and PTD in main adjusted analyses [adjusted prevalence ratios (aPRs) 0.97 (95% confidence interval, 95% CI 0.73–1.28] for ABC-3TC and aPR 1.06 (95% CI 0.83–1.35) for TDF-XTC, both vs. ZDV-3TC) or in 4720 pregnancies on LPV/r [aPR 1.03 (95% CI 0.74–1.43) for ABC-3TC and aPR 1.16 [0.85-1.57] for TDF-XTC, both vs. ZDV-3TC]. Infants exposed to ABC-3TC or TDF-XTC in utero were less likely to be SGA than those exposed to ZDV-3TC [aPR 0.72 (95% CI 0.53–0.97) and aPR 0.70 (95% CI 0.53–0.93), respectively].Results support the safety of TDF-XTC backbones initiated in pregnancy with respect to gestation length and birthweight.