Don't feel bad about posting, OP - it is entirely reasonable for you to be upset that you have found out by accident when you have gone all this way through pregnancy without knowing.
However, I strongly disagree with those who think that you should write/complain to the hospital.
The doctor's only real role in antenatal care is to ensure that the mother and baby are healthy, and to identify any problems. Anything else is, in the greater scheme of things, trivial. It is not trivial to you and your family and you are quite entitled to feel that it is a big deal, and to post about it here looking for support. However, the second you take that personal disappointment and bring it to the attention of the hospital then you are taking something trivial, irrelevant, and time-wasting into an environment where they deal with major, heart-breaking, life or death decisions and scenarios.
Hope that makes sense - important to you, but not really something that will be, or should be, important to the hospital. And I know you weren't thinking of complaining, OP, but I was surprised to see that others thought you should.