Is It Safe To Get A Paternity Test In The First Trimester?

How safe is it to get a paternity test in the first trimester? I thought I ovulated Oct. 1st, had sex with one guy a few days before and a few days after. I had sex with another guy on Oct 9th, just once though. My question is, when I had my ultrasound, they measured the baby a week behind what I though I was.. Is it possible that I coud really be 9 weeks pregnant, but the baby is just smaller?