A similar analysis was done for the prediction of TTTS as a separate complication. Significant predictors were difference in crown-rump length and discordant amniotic fluid in the first trimester and at 16 weeks, respectively. Although a 2-step assessment picked up 78% of TTTS cases, the positive predictive value was only 22%.
The low positive predictive value can be explained by the fact that the same factors predict TTTS, discordant
growth as well as intrauterine demise. As such, the combined outcome of TTTS, discordant growth and intrauterine fetal death is predicted more accurately than
TTTS as a separate entity. This is a drawback, since in our series, 60% of the mortality was attributable to TTTS,
in contrast to 12% caused by discordant growth. Consequently, we can identify a high-risk group, but as yet we
cannot distinguish the worst-outcome cases that will progress to TTTS or intrauterine demise from those with
discordant growth with a much more benign course.