Increased nuchal translucency thickness and normal karyotype: time for parental reassurance.

OBJECTIVES: To study the outcome of pregnancy in chromosomally normal fetuses with increased nuchal translucency thickness (NT), with respect to fetal loss, structural defects and genetic syndromes with developmental delay, and to provide information that would be helpful for parental counseling on the residual risk of adverse outcome when ultrasound findings are normal.

METHODS: We reviewed the outcome of all pregnancies presenting at the Academic Medical Centre in Amsterdam with increased NT between January 1994 and March 2005. Fetal karyotyping and two-step ultrasound investigation at 13-18 and 20-24 weeks' gestation were offered in all cases. Particular attention was paid to the relationship between normal karyotype, ultrasound findings at the 20-24-week scan and subsequent pregnancy outcome. An adverse outcome was defined as miscarriage, intrauterine death, termination of pregnancy at parental request or the finding of one or more structural defects or genetic disorders.

RESULTS: A total of 675 fetuses with increased NT, known karyotype and known pregnancy outcome was analyzed. A chromosomal anomaly was detected in 224 (33%) fetuses. In 451 (67%) fetuses, the karyotype was normal. The overall incidence of an adverse pregnancy outcome in this group was 19% and, when analyzed according to the initial degree of increase in NT, the likelihood of an adverse outcome increased with increasing NT, ranging from 8% to 80%. 425 fetuses underwent a detailed second-trimester ultrasound scan. Anomalies were detected, at the time of ultrasound or after birth, in 54 (13%) of these fetuses (17 isolated cardiac defects, 14 other structural defects and 23 genetic disorders). An adverse pregnancy outcome was recorded in 4% of cases in which there were normal findings at the 20-week scan. Seven of these cases were classified as 'potentially amenable' to ultrasound detection. With exclusion of these cases, the chance of a healthy baby, if the 20-week scan was completely normal, was 98%. Genetic syndromes with dysmorphic features and neurodevelopmental delay occurred in seven (1.6%) of the fetuses with normal karyotype. In three of these pregnancies, non-specific suspicious ultrasound findings (nuchal edema, mild pyelectasis, pericardial effusion) were observed at the mid-trimester scan and in two others, subtle cardiac defects were detected after delivery. In the remaining two cases (0.5%) the mid-trimester scan was completely normal and no structural defects were observed after delivery.

CONCLUSION: After exclusion of chromosomal anomalies, one out of five fetuses with increased NT has an adverse pregnancy outcome. The chance of an uneventful pregnancy outcome depends on the initial degree of increase in NT. However, if the detailed ultrasound examination at around 20 weeks is normal, a favorable outcome can be expected with confidence, irrespective of initially increased NT.