One of the best ways to deal with a man touching pregnant woman’s belly would be to touch it back. You can confer the fact that this is a strong verbal message which means that they should move their hands. The hands are going to take up space which they had for their own hands. They are going to move away without saying a word. This would be one of the easiest ways in approaching the whole situation.
Just in an affirmative voice you can say don’t touch my pregnant belly. After all it works out to be your body. No one has the permission to touch your body if you do not want it to be. This point is compounded by the fact that we do not walk up to a stranger and start touching them everywhere. It is on the expected lines that these things should not be done with a pregnant woman. Yes this might fascinate you, but in case if you really want to touch a pregnant belly then seek permission from the lady in question.
Another trick would be to take a step back with a gentle smile on your face. In this way they will get the message that you are not being unkind to them. This works out to be a nonverbal method and you can say that this works out to be an easy method to deal with things. Here the concept of pregnant bellies and strangers would be opposite poles apart. In fact you can go on to give it to the stranger without having to utter a word in the first place.
Then you can go on to pass a comment that you really value your space to be honest. You can convey to them that their hands are taking up the space. Personal space is important in the long run. A lot of people do not like when someone is too close to them or touches them. This would not be the case until and unless they happen to a close relative of theirs. If they rub your belly button for sure that they have crossed the line. Just tell them that you really like your personal space and there is no harm in doing so as well.
You can go on to adjust your body. Trust me the other person will be forced to make a change as well. This works out to be another nonverbal clue that you can give as well. It does work out to be pretty natural and there would be no need for you to scream at this point of time as well. Just tell them in a casual tone and they will not feel awkward as well. There are thousands of reasons which you can give on why you intend to shift in the first place.
Then you could make a joke of the whole situation. This would depend on the general mood at that point of time.