Does absence of nasal bone due to trisomy 21 suggest mental retardation in fetus?

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Absent nasal bone is a soft tissue in ultrasound to diagnose Trisomy 21. Absent fetal nasal bone is one of the strongest soft markers in the second-trimester ultrasound screening. Detailed prenatal diagnosis is advisable to rule out abnormal karyotype when absent nasal bone is associated with other ultrasound markers of fetal aneuploidy or structural abnormality. Nearly two-thirds of 15-22-week-old fetuses with Down's syndrome lack a nasal bone according to fetal-medicine specialist. For normal fetuses, the figure is 1%. This makes it unlikely that the test would wrongly diagnose Down's syndrome. Mental retardation cannot be diagnosed in 30 weeks scan. But if only an absent nasal bone is there with no other structural abnormality and NIPT (noninvasive prenatal testing) or amniocentesis is normal then please understand that the risk of isolated NB absence compared with non-isolated NB absence is much lower.