You have to make a distinction between a diagnostic test and the care of a skilled healthcare professional. The two are very much a different thing.
Scans are questionable because people make decisions which could seriously affect their and their baby's health often based upon the erroneous assumption that scans are extremely reliable.
It is easy to fall into the trap of thinking that medical tests are always objectively right because impersonal machines perform them. We must remember that medical tests like scans are interpreted by human beings who are fallable.
I personally used to work in hospital laboratory where I would perform breathing tests on patients and it scared me how much faith patients and doctors would put into these tests because I knew first hand how unreliable they actually were due to the inevitability of human error.
Midwives on the other hand have quite a different role. The only similarity I can think of is that they could be considered an authority on fetal health, but I think that is not actually a valid comparison as most midwives would refer any woman they suspected had problems with her baby to a consultant for investigations- they certainly wouldn't diagnose disease themselves.
I suspect the original poster actually meant that if we question one type of medical authority then shouldn't we question them all, and frankly my answer to that is a resounding YES. Medicine is a constantly evolving science not a religion, you needn't have blind faith!