You can't have an accurate nuchal scan at 20 weeks......

The thickening on the back of the neck dissipates over time, thus giving an inaccurate/false scan result. Pretty sure 16 weeks is too late for a NT test. The time frame is somewhere near 13-14 weeks.

The blood test test doesn't have the same time constraints. So you could just have that done. But the highest accuracy is when both tests (combined with your age) are done and risks are calculated.