Harris Birthright Research Centre for Fetal Medicine, King's College Hospital Medical School, London
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Abstract |
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Key words: monochorionic twins/nuchal translucency/twin-to-twin transfusion syndrome
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Introduction |
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Accumulation of fluid subcutaneously behind the fetal neck at 1014 weeks gestation, visualized by ultrasonography as nuchal translucency thickness (NT), is a marker of chromosomal abnormalities, cardiac defects and a wide range of genetic syndromes (Snijders et al., 1998; Souka et al., 1998
; Hyett et al., 1999
). There is also evidence that increased NT may be a marker for the subsequent development of TTS. Thus, in a study of 132 monochorionic twin pregnancies, including 16 that subsequently developed severe TTS, at 1014 weeks gestation, the prevalence of fetal NT above the 95th centile of the normal range and the inter-twin difference in NT were significantly higher in the TTS than in the non-TTS group (Sebire et al., 1997b
).
This extended series of 303 monochorionic twin pregnancies examined at 1014 weeks examines further the possible association of fetal NT in the early prediction of TTS.
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Materials and methods |
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A computer search was carried out to identify all monochorionic diamniotic twin pregnancies with two live fetuses at the 1014 week ultrasound examination in which pregnancy outcome was available with an estimated date of delivery before June 1999. For the purposes of this study, severe TTS was defined by the ultrasonographic features of anhydramnios and non-visible bladder in the donor fetus in combination with polyhydramnios and a dilated bladder in the recipient fetus, which resulted in either miscarriage or fetal death or required intrauterine treatment or post-mortem evidence that the cause of death was TTS. The significance of differences in antenatal features between cases with and without severe TTS was carried out using the MannWhitney U-test and comparison of proportions.
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Results |
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Chromosomal defects were identified in two of the 303 monochorionic pregnancies. In one case the NT was just above the 95th centile in both fetuses and in the second case the NT of one fetus was greatly increased (8.2 mm) but it was below the 95th centile for the second fetus.
Intertwin membrane folding
In 153 ongoing structurally and chromosomally normal monochorionic twin pregnancies an ultrasound examination was carried out at 1517 weeks gestation with specific regard to the presence or absence of folding of the intertwin membrane as an ultrasonographic feature of early discordancy in amniotic fluid volumes (Sebire et al., 1998). Folding of the intertwin membrane was seen in 49 (32%) of the cases and 21 (43%) of these subsequently developed severe TTS. In contrast, only two (1.9%) of the 104 pregnancies in which membrane folding was not present developed this complication (Z = 6.6, P < 0.001). The likelihood ratio of membrane folding at 1517 weeks for the subsequent development of severe TTS was 4.2 (95% CI 3.06.0).
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Discussion |
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Increased fetal NT at 1014 weeks gestation is associated with chromosomal defects, cardiac abnormalities and a wide range of genetic syndromes (Snijders et al., 1998; Souka et al., 1998
; Hyett et al., 1999
). The heterogeneity of conditions suggests that there may not be a single underlying mechanism for the collection of fluid in the skin of the fetal neck. Possible mechanisms include: (i) cardiac failure in association with abnormalities of the heart and great arteries; (ii) venous congestion in the head and neck, due to constriction of the fetal body in amnion rupture sequence or superior mediastinal compression found in diaphragmatic hernia or the narrow chest in skeletal dysplasia; (iii) altered composition of the extracellular matrix; (iv) abnormal or delayed development of the lymphatic system; (v) failure of lymphatic drainage due to impaired fetal movements in various neuromuscular disorders; (vi) fetal anaemia or hypoproteinaemia; and (vii) congenital infection, acting through anaemia or cardiac dysfunction (VonKaisenberg and Hyett, 1999
).
In monochorionic twins the mechanism for increased NT, as an early manifestation of TTS, is likely to be cardiac dysfunction due to hypervolaemic congestion in the recipient twin. The immature ventricles of the first trimester fetus are disadvantaged from the point of view of filling because they have a less organized myocardial arrangement, fewer sarcomeres per unit mass, smaller diameter and operate at a significantly higher heart rate, allowing less time for inactivation of contraction (Nassar et al., 1987; Fisher, 1994
). Furthermore, there is impaired ventricular filling of normal first trimester hearts, compared with those of later gestations, based on the greater predominance of atrial contraction wave over early ventricular filling wave in both sides of the heart and cardiac afterload is significantly greater than that of later gestation because of higher placental resistance (Van Splunder et al., 1996
). The majority of pregnancies with increased fetal NT at 1014 weeks did not develop features of severe TTS in the second trimester, and it is possible that in the first trimester the placental vascular pattern is dynamic and therefore some pregnancies may become haemodynamically stable with advancing gestation.
During the second trimester the normal development of intrinsic renal function may lead to fetal diuresis with consequent resolution of the increased NT but this diuresis causes an increase in amniotic fluid volume and leads to intertwin membrane folding. This sonographic feature at 1517 weeks was associated with a four-fold increase in risk of subsequent development of severe TTS.
In the monochorionic twin pregnancies that did not develop severe TTS, the prevalence of increased fetal NT was similar to that in singleton pregnancies. In three of the four fetuses with trisomy 21, NT thickness was above the 95th centile. In one pregnancy the NT of both fetuses was similar, which is not surprising since they are genetically identical. In the second pregnancy, the NT of one fetus was within the normal range but the NT of the co-twin was very much increased, raising the possibility of discordance for a structural cardiac defect or cardiovascular dysfunction due to early TTS. The findings in these two cases clearly cannot constitute the basis for formulating a policy of fetal karyotyping in monochorionic twins. However, they highlight the dilemma as to whether invasive testing in monochorionic twin pregnancies should be reserved for those cases with concordance for increased NT or whether it should also be considered in cases with discordance for this marker despite the fact that in the great majority of cases the underlying mechanism is TTS rather than a chromosomal defect.
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Acknowledgments |
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Notes |
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References |
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Submitted on January 10, 2000; accepted on June 1, 2000.