Low motility :-(

Cheri • I`m a 39 year old mother of a 15 yr old son and 4 year old daughter. My husband and I are ttc our 3rd child. We suffered a heartbreaking MC Nov. 2015.
I just received my DH's semen analysis and everything was normal except for his swimmers motility. It was 31%, grade 2. I'm not sure what that means. I'll be 39 yrs old in 2 wks and he's 41. I'm worried about this number. Should I be? Have any of you conceived naturally with low motility of 31%? Is 31% considered a terrible number?