I too had spotting with my first pregnancy. And a big bleed at 16 weeks, which i thought i was miscarrying, but he held on! I think it was caused from a combination of having a weakened cervix (from having a biopsy due to pre-cancerous cervical cancer cells-which is a common operation) and also my partner and I were only together for 3 months then i fell pregnant so we were still getting hot and heavy in bed, i think the pressure from sex caused the massive bleed, cos it happened not long after we had a hot session!lol
Luckily i can laugh about it now!

Anyway, i've since read alot of other women bleed during pregnancy, i think its from hormones softening our body tissues and the pressures of certain things can rupture the veins or capillaries of the cervix...

Im now 16 weeks into my second pregnancy and i havent had any bleeding at all!

All you can do is try not to stress too much and take it easy! .. Goodluck!