The effect of a 'vanishing twin' on biochemical and ultrasound first trimester screening markers for Down's syndrome in pregnancies conceived by assisted reproductive technology.

BACKGROUND: Previous studies have found that 1 in 10 in vitro fertilization (IVF) singletons originates from a twin gestation. First trimester Down's syndrome screening markers are altered in assisted reproductive techniques (ART) pregnancies compared with spontaneously conceived pregnancies. The presence of a perished embryo may further complicate prenatal screening among women pregnant after ART. The aim of this study was to assess the impact of a 'vanishing twin' on first trimester combined biochemical and ultrasound screening in pregnancies conceived after IVF and intracytoplasmatic sperm injection.

METHODS: From a national prospective cohort study concerning first trimester combined screening among women pregnant after ART, 56 cases of pregnancies with a vanishing twin were identified. As control group 897 cases of ART singleton pregnancies were used. All women completed a first trimester combined ultrasound and biochemical screening programme comprising serum PAPP-A and free beta-hCG together with nuchal translucency (NT) measurement.

RESULTS: There were no significant differences in geometric mean MoM free beta-hCG and PAPP-A between pregnancies with an early (gestational week <9, EVT) or late vanishing twin (gestational week 9-13, LVT) or singleton pregnancies (0.98, 1.13 and 0.95 for free beta-hCG and 0.84, 0.80 and 0.74 for PAPP-A, respectively). Likewise, no difference was seen for NT measurements. The gestational age at the time of blood sampling and NT scan was similar for the three groups. The proportion of EVT pregnancies with a PAPP-A and free beta-hCG log(10)MoM value below the 5th%iles and above the 95th%iles of the value in the singleton pregnancies were 4.3%, 4.3%, 6.4% and 8.5%, respectively, which did not constitute a significant difference from singletons. The corresponding values for LVT pregnancies were 0%, 22.2%, 0% and 11.1%, respectively; however, these numbers were too small to allow for statistical calculations.

CONCLUSIONS: First trimester biochemical screening markers in women pregnant after ART, and with a vanished twin diagnosed at early ultrasound, do not differ from those of other ART singleton pregnancies. In cases where the fetal demise was first diagnosed at the time of the NT scan, it is doubtful whether the serum risk assessment is as precise as it is in singleton ART pregnancies. No difference was seen for NT measurements.