Vanishing twin and NT/AFP tests
We had a nuchal translucency test done (with associated blood work) at 13+ weeks. The width was 1.5mm, everything looked normal, and when combined with age and blood work, the results came back at 1:1650 for T21 (I have mentioned this in another post). The issue is - we had had an ultrasound just 2 weeks prior to this, and had 2 fetuses at that point - so one had passed away within (at most) 2 weeks of this screen.
2.5 weeks later (16 weeks), we had a quad screen (or maybe triple screen) done, and there the age -adjusted result said 1:800 for T21. The level II u/s at 18+ weeks showed no abnormalities, and the perinatologist suggested we could take the 1:800 and add roughly 1/3 to it (just over 1:1000).
I have thought about this a lot (too much), and see that a VT would cause HCG to be abnormally high AND PAPP-A to be abnormally high. Of course, these act in opposite directions - high HCG raises risk of t21, but high PAPP-A lowers risk, so I cannot even make a wild guess as to what effect that would have.
I can only find one paper specifically addressing this, and they simply show that where there is a VT, there tends to be significantly higher levels of PAPP-A and HCG, and so results are untrustworthy.
My question (finally!) is - what are your thoughts on trusting these tests? Frankly, most cases I hear about are where VT causes false positives - I rarely hear people talk about false negatives. Our perinatologist seemed unconcerned when we mentioned it. Do you have any light to shed here?
Thanks!
