Less-invasive diagnostic approaches for low-birthweight preterm neonates with suspected differences of sex development have not been established. Herein, we describe our diagnostic approaches for a 297-g neonate with ambiguous genitalia. Using a fiberscope, the external genitalia were inspected in an incubator to minimize the risk of hypothermia and infection. Endotracheal aspirate, collected during routine care, was used for genetic testing to avoid anemia and vital signs fluctuations caused by peripheral blood sampling. Array comparative genomic hybridization indicated a 46,XY karyotype. No pathogenic variants of AR and SRD5A2 were found. Endocrinological data could not be evaluated owing to the absence of reference data. Identification and structural evaluation of the internal genitalia and gonads were difficult. On postnatal day 42, the parents assigned their baby's sex as male. Our less-invasive diagnostic approaches of inspection and genetic testing are useful for management, including sex assignment in extremely low-birthweight preterm neonates with ambiguous genitalia.
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