Are tests for genetic disorders accurate in Multiple Pregnancy?

"We screen all pregnancies at around 11 to 13 plus 6 weeks of gestation for what we call as the chromosomal abnormalities and the factors what we use for the screening is the maternal age and the scan what is called as the NT scan, which is done around this time, along with the biochemical test, what we call as a biochemical screening test. In twin pregnancy, the most accepted test for Downs is a combination of the maternal age and the NT scan, because the biochemical scan cannot alone be used because of the variation of the level of these markers in the twin pregnancy. However this can be included with the maternal age and the NT scan to increase the accuracy and one thing is under normal circumstances, that is the singleton pregnancies, the age limit or the risk of Down’s is about 35, whereas in twin pregnancy, it is a little early, about 32 to 33 years"