Screening for trisomy 21 in twin pregnancies in the first trimester: does chorionicity impact on maternal serum free β-hCG or PAPP-A levels?

In a study of 180 twin pregnancies I have examined the distribution of maternal serum free β-human chorionic gonadotrophin (β-hCG) and pregnancy-associated plasma protein-A (PAPP-A), in addition to fetal nuchal translucency thickness (NT), in twins classified as monochorionic or dichorionic, based on ultrasound appearance at 10–14 weeks of gestation. In 45 monochorionic and 135 dichorionic twin pregnancies the median MoM free β-hCG was not significantly different (1.00 vs 1.01), whilst that for PAPP-A was lower (0.89 vs 1.01) but again with no statistical significance. Previous reports of an increased fetal NT in monochorionic twins pregnancies could not be confirmed (1.03 vs 1.00). It is concluded that the existing pseudo risk twin correction algorithm is appropriate for both monochorionic and dichorionic twins in providing accurate first trimester risks for trisomy 21. Copyright © 2001 John Wiley & Sons, Ltd.     

Early prenatal direct gene diagnosis of cystic fibrosis in a twin pregnancy and subsequent selective termination

We present a case of prenatal diagnosis of cystic fibrosis (CF) in one twin at 11–12 weeks of gestation. The parents had previously had two children, one of whom is alive and healthy and one who died of CF at the age of 2½ months. The parents were both known to be carriers of the ΔF508 mutation. Chorionic villus sampling (CVS) was performed and direct gene analysis showed that one fetus was homozygous for the ΔF508 mutation, while the other fetus did not have the mutation at all. Both fetuses had normal karyotypes. Selective termination was subsequently performed. The pregnancy continued without complications except for mild pre-eclampsia at term. The woman had a Caesarean section. The genetic diagnosis was confirmed after birth.     

Down's syndrome screening in multiple pregnancies using alpha-fetoprotein and free beta hCG

Second-trimester distributions of the free beta human chorionic gonadotrophin (hCG) and alpha-fetoprotein (AFP) levels in 420 twin and 19 triplet pregnancies were measured and compared with the distributions in 6661 singleton pregnancies. On average, the levels of both analytes were twice as high in twins and over three times as high in triplets. Eight sets of twins discordant for Down's syndrome showed elevated levels of free beta hCG and reduced levels of AFP after correction of the multiple of the median for the presence of a twin pregnancy. Screening for Down's syndrome using the twin correction of the multiple of the median is expected to achieve a 51 per cent detection rate at a 5 per cent false-positive rate using these two markers.     

Selective intrauterine growth restriction in monochorionic diamniotic twin pregnancies

Selective intrauterine growth restriction (sIUGR) occurs in 10 to 15% of monochorionic (MC) twins, and it is associated with a substantial increase in perinatal mortality and morbidity. Clinical evolution is largely influenced by the existence of intertwin placental anastomoses: pregnancies with similar degrees of fetal weight discordance are associated with remarkable differences in clinical behavior and outcome. We have proposed a classification of sIUGR into three types according to umbilical artery (UA) Doppler findings (I-normal, II-absent/reverse end-diastolic flow, III-intermittent absent/reverse end-diastolic flow), which correlates with distinct clinical behavior, placental features and may assist in counseling and management. In terms of prognosis, sIUGR can roughly be divided in two groups: type I cases, with a fairly good outcome, and types II and III, with a substantial risk for a poor outcome. Management of types II and III may consist in expectant management until deterioration of the IUGR fetus is observed, with the option of cord occlusion if this occurs before viability. Alternatively, active management can be considered electively, including cord occlusion or laser coagulation. Both therapies seem to increase the chances of intact survival of the larger fetus, while they entail, or increase the chances of, intrauterine demise of the IUGR fetus. Copyright © 2010 John Wiley & Sons, Ltd.     

The sonographic diagnosis of chorionicity

The differentiation between mono- and dichorionic placentation in twin pregnancies is of clinical importance because of the significant difference in perinatal morbidity and mortality between the two, and the increased surveillance indicated in monochorionic gestations. Application of ultrasonography has enabled very precise prenatal determination of chorionicity. While this is best performed in the first trimester when accuracy approaches 100%, even in the third trimester, using a composite cascade of available sonographic features, accuracy has been reported to approach 97%. While two clearly separate placentae or discordant fetal gender conform to dichorionicity, in most twin pregnancies other features need to be assessed to determine chorionicity. The presence of the ‘lambda’ or the ‘T’ sign in the presence of a single placenta, best determined in the first trimester, is the most reliable indicator of chorionicity, with measurements of the inter-twin membrane thickness and counting of the membrane layers being less reliable. In this article, we review the sonographic features that help in the accurate depiction of chorionicity. Copyright © 2005 John Wiley & Sons, Ltd.     

Discordant growth in twins

Discordant growth in twins contributes significantly to rates of perinatal morbidity and mortality. These rates vary according to chorionicity, timing of onset and severity. We have reviewed English language literature in Medline since 1980. It is clear that diagnosis of discordant growth has improved due to the use of serial ultrasound examination. Following the detection of differences in fetal size, diagnosis is facilitated by umbilical artery and fetal Doppler studies. Management options vary according to chorionicity, timing of onset and umbilical-fetal Doppler studies. The mode of delivery in discordant twins remains controversial. We conclude that ultrasound surveillance of twin gestations, combining serial biometry and selective Doppler studies, is effective in the recognition of siginificant intrauterine growth restriction in co-twins. Differences in etiology and management underscore the imortance of establishing chorionicity routinely as soon as twin gestation is diagnosed. Copyright © 2005 John Wiley & Sons, Ltd.