1p deletion syndrome: A prenatal diagnosis characterized by an abnormal 1st trimester combined screening test, yet a normal NIPT result.

OBJECTIVE: To present a case with prenatal diagnosis and cytogenetic characterization of 1p36 deletion syndrome whose first trimester combined testing is abnormal but a normal NIPT result.

CASE REPORT: A 33-year-old had an abnormal 1st trimester fetal aneuploidy screening result, but no trisomies 13, 18, 21 were detected by the noninvasive prenatal testing. Amniocentesis was performed after ultrasound showed fetal ventriculomegaly and echogenic bowel. The final conventional cytogenetics revealed a karyotype of 46, XX, del(1)(p36).

CONCLUSION: Every prenatal genetic screening test and diagnostic procedure has its benefit and risk. NIPT offers better sensitivity and specificity for trisomies 13, 18, and 21. Even so, for primary population screening, NIPT provides lower detection rate than sequential screening if considering detection of all chromosomal abnormalities. Diagnostic testing should be offered rather than cell-free DNA screening to pregnant women if a fetal structural anomaly is identified on ultrasound examination.