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When a normal karyotype accompanies increased NT.

Diagnosis/definition: An NT measurement of 3.0 to 3.5 mm or greater, between 10 3/7 and 13 6/7 weeks’ gestation is considered increased.
Epidemiology/Incidence: About 5% of fetuses with a normal karyotype will have an increased NT (5% false positive rate).
Risk factors/associations: In fetuses with NT 3.0mm or greater and a normal karyotype, spontaneous fetal loss prior to 20 weeks’ gestation is reported in 2.2% to 10.6% of pregnancies, and 0.5% to 15.8% end in perinatal death. After excluding aneuploidy, about 20% of fetuses with NT of 3.0mm or greater will have adverse outcomes such as miscarriage, intrauterine death, pregnancy termination, structural defects, or genetic disorders. This decreases to about 2% if no fetal malformations are detected by an anatomic fetal survey in the second trimester.
Management:

  • Screening/Work-up: If the karyotype is normal, about 10% of fetuses with NT of 3.0mm or greater will have anomalies, and about half of these are cardiac. The bigger the NT, the more likely a cardiac anomaly is present. Therefore, once the karyotype is determined to be normal, an ultrasound exam of the fetal anatomy, including a fetal echocardiogram, should be performed.
  • Prenatal care: If the ultrasound and fetal echocardiogram are normal, women can be reassured but counseled about the potential for unidentified malformations and genetic syndromes.
  • Antepartum testing: Given the increased risk of fetal death, a follow up ultrasound to assess fetal growth and assessment of fetal well-being may be reasonable.    

Post-partum/breastfeeding: Evaluation of the newborn by an experienced pediatrician or geneticist is recommended to excluded missed malformations and to assess for dysmorphic features.

Last Reaffirmed: Feb 1, 2013



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