Pregnancy in a woman with down syndrome (47, XX, +21) and a euploid fetus: implications for prenatal diagnosis and counseling

OBJECTIVE: To report a rare pregnancy in a woman with Down syndrome (DS) whose fetus has a normal karyotype, and to discuss implications for prenatal diagnosis and counseling. CASE REPORT: A 24-year-old primigravida with cytogenetically confirmed DS (47, XX, +21) was referred at 17 weeks' gestation. Amniocentesis revealed a euploid fetal karyotype (46, XX) with no pathogenic copy-number variants. A comprehensive fetal anatomical survey was normal, although fetal biometry measurements were at the lower end of the normal range. The pregnancy was complicated by maternal subclinical hypothyroidism, which was effectively managed with levothyroxine. The pregnancy is ongoing with scheduled antenatal follow-up. CONCLUSION: This case underscores that women with DS can conceive and carry fetuses with normal karyotypes and supports a counseling approach that emphasizes individualized risk assessment and the importance of offering invasive prenatal diagnostic testing.