Unknown reasons for pregnancy loss?

Shanta
Hi all! I'm 26 and am currently pregnant with baby #3. Back in Jan of last year my husband and I became pregnant with our first child and I ended up having a miscarriage at 4 weeks along. (Jan. 25th) Shortly after (May 22) we became pregnant with out baby girl Nylah but I ended up going into preterm labor at 21 weeks (Nov. 13)but we had to bury our little girl. With each loss, our OB never gave an explanation as to what happened. I know as far as the misscariage goes that it could be hard to determine because I wasn't far along but to just go into preterm with no explanation seems odd to me. Has this happened to anyone else?