Really want a child!!!!

I have been married for 3 years but we have been together for 5 years. My husband has 3 kids from a previous marriage. I have been on an emotional roller coaster for the last couple of years because as much as I treat my 3 step kids as my own...I really want to experience raising a child from uterus. My husband is up and down about having another baby since the kids are teenagers.