I didn't know how to react

So I was at work talking to a co worker. We got on the subject of marriage and all. I was telling her how my SO and I want to get married this year and also want to start trying.

She then made a comment of not being able to get pregnant (I think her husband can't produce sperm or something). I had a shocked face and said "aww I'm so sorry".

Thinking about it .. what's the right way or how to respond to that. I don't know her well at all. She even replied with "oh that's ok" and brushed it off like it was nothing.

Now I feel bad cuz maybe I responded in the wrong way? We work at a preschool and this was the last day I'm going to see her.