How to convince hubby to threesome

Husband and I have been married over 4 years. We’ve always played with the idea of inviting another girl. I’m really wanting to try it but he’s on the fence because he doesn’t want to hurt my feelings. We had one set up but the girl backed out the day before. I’m currently 29 weeks pregnant so we’d obviously wait awhile before pursuing anything again. How do I gently convince him and then how do we find a chick to join us?