Background Infants with critical congenital heart disease ( CCHD ) are more likely to be small for gestational age (GA). It is unclear how this affects mortality. The authors investigated the effect of birth weight Z score on 1‐year mortality separately in preterm (GA <37 weeks), early‐term (GA 37–38 weeks), and full‐term (GA 39–42 weeks) infants with CCHD . Methods and Results Live‐born infants with CCHD and GA 22 to 42 weeks born in California 2007–2012 were included in the analysis. The primary predictor was Z score for birth weight and the primary outcome was 1‐year mortality. Multivariable logistic regression was used. Results are presented as adjusted odds ratios and 95% confidence intervals ( CIs ). The authors identified 6903 infants with CCHD . For preterm and full‐term infants, only a Z score for birth weight <−2 was associated with increased mortality compared with the reference group ( Z score 0–0.5, adjusted odds ratio, 2.15 [95% CI , 1.1–4.21] and adjusted odds ratio, 3.93 [95% CI , 2.32–6.68], respectively). In contrast, in early‐term infants, the adjusted odds ratios for Z scores <−2, −2 to −1, and −1 to −0.5 were 3.42 (95% CI , 1.93–6.04), 1.78 (95% CI , 1.12–2.83), and 2.03 (95% CI , 1.27–3.23), respectively, versus the reference group. Conclusions GA seems to modify the effect of birth weight Z score on mortality in infants with CCHD . In preterm and full‐term infants, only the most severe small‐for‐GA infants ( Z score <−2) were at increased risk for mortality, while, in early‐term infants, the risk extended to mild to moderate small‐for‐GA infants ( Z score <−0.5). This information helps to identify high‐risk infants and is useful for surgical planning.