Blastocyst quality and congenital malformation risk in singleton births after frozen embryo transfer.

The relationship between blastocyst morphological quality and the risk of congenital malformations in assisted reproductive technology (ART) remains poorly understood, limiting clinical decision-making for embryo selection. We conducted a retrospective cohort study of 3986 frozen embryo transfer cycles (January 2014-June 2023) to evaluate whether blastocyst morphological quality influences the risk of congenital malformations. Blastocysts were classified according to Gardner's grading system, and 1:2 propensity score matching was applied to control for maternal age, BMI, infertility characteristics, and other potential confounders. After matching, 1743 singleton births were analyzed (1162 good-quality vs. 581 poor-quality blastocysts). Baseline characteristics were well balanced between groups. The risk of congenital malformations was similar between good- and poor-quality groups (aOR 1.14, 95% CI 0.54-2.41, P = 0.7310; 1.72% vs. 2.07%), with no significant between-group differences in any ICD-10 organ-specific categories (all P > 0.10). Secondary outcomes showed no significant differences: preterm birth (aOR 0.80, 95% CI 0.57-1.12, P = 0.1976), low birth weight (aOR 1.26, 95% CI 0.72-2.19, P = 0.4172), other neonatal outcomes, and obstetric complications (aOR 0.89, 95% CI 0.64-1.22, P = 0.4629). These findings indicate that blastocyst morphological quality does not influence the risk of congenital malformations, supporting the use of morphologically poor blastocysts when high-quality alternatives are unavailable, which may reduce unnecessary discarding of embryos, alleviate patient anxiety, and improve treatment accessibility.