Screening for trisomy 21 in twin pregnancies in the first trimester using free ?-hCG and PAPP-A, combined with fetal nuchal translucency thickness

Abstract

In the first trimester of pregnancy the biochemical markers free beta-hCG and pregnancy associated plasma protein-A (PAPP-A) are used for the prenatal screening of trisomy 21, either alone or in combination with nuchal translucency (NT) thickness. In this study, I have analysed the distribution of these biochemical markers in 159 twin pregnancies and compared this with 3466 singleton pregnancies. On average free beta-hCG values are 2.099 times greater in twins than in singletons and PAPP-A some 1.86 times greater. The width of the analyte distribution in twins is very similar to that in singleton pregnancies. Using statistical modelling techniques I have predicted that at a 5% false positive rate the detection rate in twins discordant for trisomy 21 will be 52% and in twins concordant for trisomy 21 will be 55%, if correction for twin pregnancy is carried out using the 'pseudo risk' approach. The detection rate using biochemical parameters is less than that achievable for twins using NT (75%). However, the combination of NT and maternal serum biochemistry will give detection rates approaching 80%. These rates are some 10% less than in singleton pregnancies, but nevertheless combining NT and biochemistry will allow high rates of detection of affected twins with the benefit of ultrasound and NT being able to specifically locate the affected twin. Twin screening using both modalities should be considered when introducing first trimester screening.

Screening for trisomy 21 in twins using first trimester ultrasound and maternal serum biochemistry in a one-stop clinic: a review of three years experience

“…The median MoM PAPP-A after weight correction and before correction for twin pregnancy was 1.93 with a mean log 10 of 0.2628 and a log 10 standard deviation of 0.2178. These values are very similar to those published in a previous series 9 .…”

“…Using a 'pseudo risk' approach, combining fetal nuchal translucency thickness and maternal serum biochemistry was predicted to identify 80% of cases of twins discordant for trisomy 21 (at a 5% false positive rate) and 81.5% in concordant twins. Thus, it was argued that while maternal serum biochemistry alone could not specifically identify the fetus at risk in the presence of twins discordant for trisomy 21, it would be possible to enhance the detection rate in twins by some 5 -6% yet still retain the benefits of nuchal translucency thickness in identifying the specific affected twin 9 . Furthermore, it has been shown that chorionicity has no impact on the maternal serum biochemical marker levels in twin pregnancy 10 .…”

“…In the first trimester, both fetal nuchal translucency thickness 8 and maternal serum biochemistry can be combined together 9 to provide detection rates approaching those achieved in singleton pregnancies. Such algorithms do not seem to be dependent upon chorionicity 10 and an example case of twins discordant for trisomy 21 that had been identified by prospective screening has been described 11 .…”

Screening for trisomy 21 in twin pregnancies in the first trimester: an update of the impact of chorionicity on maternal serum markers

“…In twin pregnancies, both NT and maternal serum biochemistry can be combined (Spencer, 2000) to provide detection rates that would approach those achieved in singleton pregnancies. Whilst these early algorithms were based on relatively small numbers of cases of twins, retrospective use has shown them to be useful in identifying pregnancies discordant for trisomy 21 Nicolaides, 2000, 2003).…”

First‐trimester combined screening for trisomy 21 in a predominantly Chinese population

“…A higher screen-positive rate and a lower detection rate compared with that in singleton pregnancies was reported in a Caucasian population 15 . The higher screen-positive rate is secondary to a higher prevalence of increased NT in twin fetuses, especially among monochorionic twins 16 . The two first-trimester biochemical markers only add a further 5% to the detection rate compared with using NT alone.…”