These things scare me

So far on facebook, two girls who are around my age 18-20 have both lost their babies. One at 23 weeks and one at 37 weeks. This makes me so nervous and scared that something similar is going to happen to me. However, both girls before they got pregnant were hardcore into drugs and i've never done drugs in my life, maybe it has something to do with that? So far my pregnancy is okay, i have knly gained 7lbs and baby is head down and weighing 4lbs and 12oz at 33 weeks 4 days. Im trying not to think about it to much, i feel so bad for these other moms.. plus i have pcos and im overweight (200lbs)