Can I ask spouse to slow down?

On a basic level? No, it's not really fair.

But your feelings around wanting that are both fair and valid. You're afraid of what's going on and it's a lot of change on top of pregnancy and it's a huge deal. But fundamentally you have to be able to trust your partner.

I wrote an advice column on taking it "slow" and why that doesn't really work here: Rushing into polyamory

I also have a few columns on pregnancy and polyamory in general which might be helpful.

Pregnancy and polyamory

Episode 8: Polyamorous and Pregnant

/r/polyamory Thread