Because an ultrasound is very common, MRI’s are used to scan for brain abnormalities (which is far most the important development issue in children), and from my understanding a maternal blood analysis is when the mother gets her blood drawn and tested, which also seems very important and very safe.
I chose these because they seem to cause less harm to the fetus, and they do not have risk of miscarrying a baby.
One thing that bothers me in the book is that authors do not communicate well when describing the methods, what genetic defects are they looking