Prenatal diagnosis
Screening for common trisomies is more complex compared to singletons, and the detection rates vary depending on the chorionicity. It is important that women are appropriately counselled about the implications of detection of a discordant chromosomal abnormality, the risks associated with an invasive or a diagnostic test, and the practicalities and implications of selective fetal reduction, where appropriate.
Zygosity and not chorionicity mediates the risk of chromosomal abnormalities in a multiple pregnancy. In monozygotic twins, the risk of aneuploidy for each fetus is similar to that in singleton pregnancies. Very rarely, heterokaryotic monozygotic twins with chromosomal discordance can be noted due to postzygotic mitotic errors (21). Dizygotic twins are genetically distinct and thus aneuploidy, when present, is usually discordant. Each twin has an independent risk. As a result, the chance of at least one fetus in a dizygotic twin pregnancy being affected by chromosomal defect is broadly twice as high as that for women of the same age with a singleton fetus (22).
First-trimester screening for chromosomal abnormalities
It is recommended that in a twin pregnancy, screening for trisomy 21 should be performed in the first trimester using the combined test, which includes maternal age, measurement of nuchal translucency (NT) and serum beta-human chorionic gonadotrophin (β- hCG), and pregnancy associated plasma protein-A (PAPP-A) levels (3, 11). An alternative is the combination of maternal age and the NT recorded between 11+0 and 13+6 weeks of gestation (11, 23). In case of a vanished twin, if there is still a measurable fetal pole, β- hCG and PAPP-A measurements are biased and in such cases, NT alone should be used for risk estimation (24). The risk of trisomy 21 in monochorionic twin pregnancy is calculated per pregnancy based on the average risk of both fetuses, whereas in dichorionic twin pregnancy, the risk is calculated per fetus.
The detection rate for Down syndrome is thought to be lower in twin compared with singleton pregnancy (11). However, a recent meta-analysis reported similar performance (89% for singletons, 86% for dichorionic twins, and 87% for monochorionic twins, at a false-positive rate of 5%) (25). For triplets or higher-order multiple pregnancies, NT and maternal age is the only available screening method.Second-trimester screening for chromosomal abnormalities
In twin pregnancies, conventional second-trimester serum screening tests are not as reliable as in singleton pregnancies and do not provide a fetus specific risk. This may not be a problem for monochorionic twins, but for dichorionic twin pregnancies, which are more likely to be discordant for aneuploidy, second-trimester biochemical screening is not an effective or an accurate option. The NICE guidance on multiple pregnancy does, however, recommend offering second-trimester serum screening for twins (11). However, where first-trimester screening is missed or not possible in monochorionic twin pregnancies, the 2017 Royal College of Obstetricians and Gynaecologists (RCOG) guidance does recommend offering the quadruple test (3).
Investigators have suggested calculating a ‘pseudo-risk' using the biochemical markers (alpha-fetoprotein, hCG, and oestriol) for twins similar to singletons, albeit with a lower sensitivity of 51-73% for a 5% false-positive rate (26). Some studies have suggested some improvement in performance of screening in the second-trimester twin pregnancy with the quadruple test) (27). As such, the norm is to offer first- trimester combined screening to all pregnancies including twin pregnancies. For triplet or higher- order pregnancy, there are no second-trimester screening options.
Non-invasive prenatal testing in multiple pregnancy
Non- invasive prenatal testing (NIPT) of maternal blood for common fetal trisomies can be offered as early as 10 weeks of pregnancy. It has a better detection rate and lower false-positive rate compared to the conventional combined screening, especially for trisomy 21 and less so for trisomies 18 and 13 (28).
NIPT should never replace diagnostic testing in both singletons and twins. In monochorionic twin pregnancies, due to the same genotype (except for a very small incidence of heterokaryotypia), this may be an effective option for screening. For dichorionic twins, with current techniques it is difficult to distinguish individual contribution of the fetuses. The largest series on NIPT in twin pregnancy was a recent prospective study in 438 twin and 10,698 singleton pregnancies undergoing screening for fetal trisomies by cell-free fetal DNA testing at 10+o to 13+6 weeks' gestation, showed that in twin pregnancies, the median fetal fraction was lower (8.0% vs 11.0%; P risk of miscarriage can be minimized by delaying amniocentesis until the third trimester, but this is only relevant in those countries in which late termination of pregnancy is permitted. Although there is a small risk of preterm delivery, the risks to the fetus from preterm birth are minimal when invasive testing is performed at or after 32 weeks. In twins, late fetal karyotyping carries an additional advantage of avoiding loss of an unaffected co-twin from earlier selective reduction if twins are discordant for anomaly. Due to the higher risk of fetal cell contamination and failure to sample each sac with CVS, amniocentesis is the preferred option for prenatal diagnosis in triplets and higher-order multiples.