I have heard many different things over the years. When someone is getting married spur of the moment without major notice, and finally announces it to lots of people there is sometimes the thought that you are pregnant. I would assume it is because people think alike. Usually I would think the same thing.
The woman is usually the one that takes the bad end of the stick. She probably gets a bit concerned when many people ask if she is pregnant, and then she goes out and takes a pregnancy test. There are a few times in life that a bride does have a child, but not by the married father. Those times call for paternity tests to answer the real question.