Is there a polite way to tell very kind generous family members to "butt out"?

I have wonderful in-laws. They have been very kind and VERY generous to my husband and me. However—my mother-in-law somehow keeps bringing up the topic that she is sure that my daughter will be an only child.

CONSTANTLY. Every time I see her, she mentions my health, my age, our finances, or several other reasons why it wouldn’t be smart for us to have another child. Suggesting a vasectomy to my husband while we were eating dinner was a last straw for me.

She has been very very kind to us, like I said. But I’m frankly TIRED of her talking about this when my first child isn’t even born yet. It’s not really her business anyway.

Chances are, we aren’t likely to have another child. But I REALLY don’t want her to keep mentioning it over and over. I find it insulting. I’m just worried that in my overly hormonal state, I’ll just spaz our on her rather than being grateful but firm.