MIL is denying the gender of our soon to be first born. We have showed her ultrasounds clearly showing its a girl but she insists it's a boy and he is just hiding himself because that is what her son (my husband) did?

This is such a blessed event. Please WHY pick this as a battle? Just love your MIL for her heart means well but when your beautiful daughter is born, none of this is gonna matter.

My opinion---- NO BIG DEAL. Life is too short to sweat the small stuff. When the baby is born, the answer will be revealed anyway.

I guess that's her choice to believe.  But the truth will be found out when your baby is born ... Don't let it get to you. 

Well, I hope she defers buying anything blue for a while.

A response like "Well, that would be an interesting outcome" would probably be a good way to leave it as a possibility without having to think on your part that it is in any way likely.

Obviously, this issue will be resolved in a reasonably short period of time.

Can I give you some advice? Do not go there with her. Accept that things will be difficult with this one. Accept that you will most likely ALWAYS be the bigger person. Know that she will get under your skin. YOU picked this man, and ALL that he entailed. He AND his family are a package deal. Take my advice: You really LOVE your man? Then never ever make him have to pick between you and his mother. Never ever complain about his mother. Even though she's a pill, you must leave her be. Complaining about his mother, to him, hurts him. You cannot have a tug of war if you are not holding one end of the rope. For the sake of your husband, leave his mother alone.

If she's as much of a PITA as it sounds from your responses here ... Talk to your husband.  You may need to consider cutting her out of your life, until she changes her ways.