The maternal AFP, or triple screen test, that is done between 15 to 20 weeks, is known to have a high false positive rate of 10 to 15 percent. However, since the risk looks elevated in your case, you will need to make the decision about whether or not you want to have an amniocentesis done to determine with certainty if Down's syndrome is present in your twins. I am assuming, since you say your twins share one sac, that they are monozygotic, or identical twins. If that is the case, only once amniocentesis procedure will need to be done to collect fluid since the twins would share the same chromosomes.

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