Follicle size/ovulation?

I had my hcg trigger today with my biggest follicle measuring 18 and three measuring 15. I'm not too concerned about the 15's but it's my 18 I have a question about. I'm looking for an answer from a woman who had an 18 follicle and had a successful pregnancy. Is 18 big enough? My doc said "unlikely to conceive" but didn't want to wait a few days to grow just in case the 15's got bigger too.