MONOCHORIONIC TWINS AND

INCREASED NUCHAL TRANSLUCENCY

Risk assessment for Down syndrome in multiple gestation pregnancies has had several limitations until the advent of NT-based screening. Maternal serum screening has not been used widely with multiple gestations because of the potential for discordance between twins and the impact of different placentas on the various analytes. The detection rate for Down syndrome by the second-trimester serum quad test in twins has been estimated at only 47% for a 5% false-positive rate, although this varies depending on whether the pregnancy is monochorionic or dichorionic (1).

A recent report on the association between increased nuchal translucency thickness (at 10-14 weeks) and its association with the development of the twin-twin transfusion syndrome (TTS) has been examined (2).

In 132 monochorionic twin pregnancies, 16 developed TTS prior to 24 weeks:

• Positive predictive value = 38%.
• Negative predictive value = 91%.
• Incidence of TTS = 12%.

The sensitivity of nuchal translucency thickness in twin gestations is reportedly similar to that of single pregnancies (3,4). The higher rate of false positives noted in twins in comparison to singleton pregnancies (9% versus 6%) is probably the result of the higher prevalence of increased nuchal translucency thickness in karyotypically normal fetuses of monochorionic gestations, possibly reflecting cardiac failure associated with twin-twin transfusion (5).

It has been suggested that an early marker that identifies a monochorionic pregnancy as being at high risk for subsequently developing the TTS is a nuchal translucency of one of the fetuses is increased or if there is a discordancy in the translucency by > 1mm. At 15-18 weeks, there is infolding of the inter-twin membrane in 30%, half of which will be complicated by severe TTS at 18-24 weeks.

It has been hypothesized that the increased nuchal translucency is an expression of early hemodynamic imbalance. This is probably based on an association between increased nuchal translucency and congenital cardiac malformation (interpreted as early hemodynamic expression of a cardiac defect in utero). An increased nuchal translucency at 10-14 weeks is associated with a four fold increased risk of developing TTS.

A nuchal translucency above the 95th percentile occurs in only 30% of TTS and is therefore unlikely to significantly enhance the prediction of TTS (6). It is thought that it may be more accurate to examine the intertwin difference in nuchal translucency.

REFERENCES

1. Cuckle H. Down's syndrome screening in twins. J Med Screen 1998;5:3-4.
2. Sebire NJ, D'Ercole C, Hughes K et.al. Increased nuchal translucency thickness at 10-14 weeks gestation as a predictor of severe twin-to-twin transfusion syndrome. Ultrasound Obstet Gynecol 1997;10:86-89.
3. Panyada PP, Snijders RJM, Johnson SP et.al. Screening for fetal trisomies by maternal age and fetal nuchal translucency thickness at 10-14 weeks gestation. Br J Obstet Gynaecol 1995;102:957-962.
4. Panyada PP, Hilbert F, Snijders RJM et.al. Nuchal translucency thickness and crown-rump length in twin pregnancies with chromosomally abnormal fetuses. J Ultrasound Med 1995;14:565-568.
5. Snijders RJM, Johnson S, Sebire NJ et.al. First trimester ultrasound ultrasound screening for chromosomal defects. Ultrasound Obstet Gynecol 1996;7:216-226.
6. Ville Y. Opinion. Monochorionic twin pregnancies: "les liaisons gangereuses". Ultrasound Obstet Gynecol 1997;10:82-85.