Hurt and shocked

So my fiancé and I have been quietly trying to get pregnant for 2 years,no baby for either of us yet. For a weekend we kept his niece and went to his moms house for a visit. While there she says “y’all always have the baby,don’t be jealous you don’t have your own. When y’all have kids they’ll probably come out retardant”. I was so hurt and shocked I ran out the house without saying anything. He didn’t think much of it but I feel in my heart if we have a child with disabilities or not I would t feel comfortable with sending them to they’re grandmothers house. She won’t apologize and says she was joking. What should I do?