Toddler advice

My son is 21 months (aka a year and 9 months) and he has learned a new thing that I need to put a stop to but dont know how. He will say no mama and smack me all the time if I tell him to stop doing something or anything pisses him off and he does it to dad too and says no mama and smacks dad. How do I stop it he is a very well behaved kid other wise eats his food and everything just when he gets mad he smacks and most of the time it's my belly (I'm 29 weeks preg) because that's what he can reach first. He will be a brother in October and turn two in October and I do not want him exhibiting this behavior then especially to the new baby. So you moms who have had or have a toddler how can I stop this behavior??

Here is a pic of him when he isn't being a turd :)