Wah, I totally understand how stressful this must be, I was in a similar situation and honestly felt quite lost when my options became so limited. From what I've heard, the structural ultrasound at week 12/13 (the nuchal translucency scan) can actually be combined with other markers, so it's worth asking your gynae specifically whether they're doing a full combined first trimester screen, some hospitals bundle in more detailed measurements that push the accuracy a bit higher than the standard 80% they quoted you. My neighbour actually went through something similar with a vanishing twin and her specialist recommended waiting until the empty sac had fully resolved, then revisiting whether NIPT became viable again at a later gestational week, which I hadn't heard of before. I'll be honest though, not all doctors are willing to do that, and some mums I know were just told to proceed with the structural scan alone and accept the limitation. If you really want a more definitive answer, invasive testing like CVS (chorionic villus sampling, around week 11-13) or amniocentesis (around week 15-20) are still on the table regardless of the vanishing twin situation, since those test the baby's actual cells rather than a blood marker, though of course they carry a small procedural risk. Definitely worth having a very frank conversation with your gynae or asking for a referral to a maternal-fetal medicine specialist to go through all the options properly.
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