Early preterm (<34 weeks of gestation) small for gestational age infants (<10th percentile birth weight for sex and gestational age) experience high rates of morbidity and mortality, the causes of which are poorly understood. Mounting evidence suggests that genetic disorders contribute. Scarce data exist regarding the prevalence of genetic disorders and their contribution to morbidity and mortality.

This study aimed to determine the proportion of genetic disorders in early preterm small for gestational age infants (with and without congenital anomalies) compared to early preterm appropriate for gestational age infants and the association of genetic disorders with morbidity or mortality.

This is a retrospective cohort study of infants delivered at 23 and 0/7 to 33 and 6/7 weeks of gestation from 2000 to 2020 from the Pediatrix Clinical Data Warehouse. Data included diagnosed genetic disorders and congenital anomalies, baseline characteristics, and morbidity or mortality. We excluded cases of death in the delivery room before neonatal intensive care unit admission, multiple gestations, and cases transferred after birth or before death or discharge.

We identified 223,431 early preterm infants, including 21,180 small for gestational age. Genetic disorders were present in 441 (2.3%) of small for gestational age infants without congenital anomalies, in 194 (10.8%) of small for gestational age infants with congenital anomalies, and in 304 (4.5%) of small for gestational age infants that experienced morbidity or mortality (with or without congenital anomalies). Trisomies 13, 18, and 21 were the most prevalent genetic disorders in these groups, together accounting for 145 small for gestational age infants without congenital anomalies, 117 small for gestational age infants with congenital anomalies, and 166 small for gestational age infants with morbidity or mortality (with or without congenital anomalies). Less prevalent genetic disorders consisted of other aneuploidy (45, X and 47, XXY), copy number variants (13q14 deletion syndrome, cri du chat syndrome, DiGeorge syndrome), and single gene disorders (cystic fibrosis, Fanconi anemia, glucose-6-phosphate dehydrogenase deficiency, hemophilia, hypophosphatasia, sickle cell disease, and thalassemia). Comparatively, genetic disorders were found in 1792 (1.0%) appropriate for gestational age infants without congenital anomalies, in 572 (5.8%) appropriate for gestational age infants with congenital anomalies, and 809 (2.0%) appropriate for gestational age infants that experienced morbidity or mortality (with or without congenital anomalies). Genetic disorders were associated with an adjusted odds ratio (95% confidence interval) of 2.10 (1.89–2.33) of isolated small for gestational age and 12.84 (11.47–14.35) of small for gestational age accompanied by congenital anomalies. Genetic disorders were associated with an adjusted odds ratio of 2.24 (1.83–2.74) of morbidity or mortality.

These findings suggest that genetic disorders are more prevalent in early preterm small for gestational age infants, particularly those with congenital anomalies. These findings also suggest that genetic disorders are associated with increased morbidity and mortality. These associations were primarily driven by trisomies 13, 18, and 21. Genetic diagnoses in this cohort were made through routine clinical care, principally via karyotype, chromosomal microarray, and single gene testing. These findings support evolving clinical guidelines for genetic testing of small for gestational age infants. Our study is limited due to the lack of prospective, genome-wide testing.

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