n Obstetrics and Gynaecology Forum - Prenatal diagnosis in multiple pregnancies : review articles

Volume 12, Issue 4
  • ISSN : 1027-9148



Fetal abnormalities are more common in twin than in singleton pregnancies. Prenatal diagnosis is a major challenge for obstetricians. Not only are two or more babies involved, but also multiple pregnancies typically occur in older mothers who are at increased risk of genetic abnormalities. Recent advances in prenatal screening and diagnostic and therapeutic procedures have necessitated a review of current methods of screening for aneuploidy. This is particularly relevant to twin pregnancies where nuchal translucency measurement and selective termination of pregnancy offer options not previously available Twins account for about 1% of all pregnancies, with 2 / 3 being dizygotic and 1 / 3 monozygotic. A crucial prerequisite to prenatal diagnosis in a multiple pregnancy is the determination of chorionicity. Chorionicity helps to predict perinatal risk especially with regard to twin-to-twin transfusion syndrome. Chorionicity also determines the management of discordant fetal abnormality as well as being relevant to the obstetric management of singe intrauterine death or fetal compromise. Chorionicity can be determined by ultrasound, and relies on the assessment of the fetal gender, number of placentas and the characteristics of the membrane between the two amniotic sacs. Screening options for aneuploidy in multiple pregnancies include first and second trimester biochemical screening as well as screening by nuchal translucency measurement. The detection rate by biochemical screening is very poor when compared to the detection rate by maternal age alone. A "screen positive" test also provides no information as to which of the fetuses is affected. Other problems limiting the usefulness of serum biochemistry in multiple pregnancy include the fact that the information is available only after 15 weeks of gestation at which time selective termination in a pregnancy discordant for aneuploidy carries an increased risk compared to first trimester selective termination. With the evidence available on screening tests for aneuploidy in a multiple pregnancy, the measurement of nuchal translucency thickness, is superior to first or second trimester serum screening. It offers a better detection rate for a similar false positive rate, and in a dizygotic pregnancy will allow an individual screening test for each fetus. In pregnancies found to be at increased risk, it allows earlier diagnostic testing. Fetal karyotyping requires invasive testing by amniocentesis, or chorionic villus sampling (CVS). The choice of invasive technique in twins should be based on the use of individual risk calculated by maternal age and fetal nuchal translucency thickness. When the risk for chromosomal defect in at least one of the fetuses is high (more than 1 in 50), it may be preferable to perform chorionic villus sampling. For pregnancies with a lower risk, amniocentesis at 16 weeks would be the favoured option.

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