Antiretroviral therapy (ART) in pregnancy substantially reduces the risk of mother-to-child transmission of HIV, but concerns exist about the potential for teratogenic effects. This analysis was undertaken to explore the relation between ART in pregnancy and birth defects in infants born to HIV-infected women in the United Kingdom and Ireland between 1990 and 2003. Comprehensive obstetric and pediatric HIV surveillance is carried out through the National Study of HIV in Pregnancy and Childhood. Congenital abnormalities were reported in 101 of 3172 infants (100 of 3120 pregnancies). There was no statistically significant association between the prevalence of congenital abnormalities and exposure to ART overall: 3.4% (90 of 2657 pregnancies) in exposed pregnancies and 2.2% (10 of 463 pregnancies) in nonexposed pregnancies (P = 0.166); prevalence was similar whether or not exposure occurred in the first trimester: 3.7% (20 of 541 pregnancies) after early exposure and 3.1% (80 of 2579 pregnancies) without early exposure (P = 0.476). There was also no significant association with type of ART in early pregnancy (ie, highly active antiretroviral therapy [HAART] vs. mono- or dual therapy, HAART with protease inhibitor and/or nonnucleoside reverse transcriptase inhibitor). The lack of association was maintained after adjustment for potential confounding factors. These findings are reassuring, but continued monitoring is essential in view of the increasing number of women on therapy at conception and the likely continuing diversity of drug regimens.