Re: Negative blood types
I became rH positive with my first pregnancy; I think my blood somehow mixed with the baby's. I only found this out when I went in for the shot of rhogam! That sent me on the track for high-risk pregnancies with weekly ultrasounds to monitor the baby; I was devastated and very scared about what it would mean.
I'm currently pregnant with #3 and have had no issues, though with the first two there were seemingly unrelated issues that weren't serious (baby #1 was small for gestational age and with baby #2 I had excess amniotic fluid). I think I'm an exception, though, as my midwives often say this is a first for them. From what I hear, each pregnancy could potentially get worse for the baby, healthwise.
Before continuing with testing during the second pregnancy, my husband was able to get a blood test to determine whether his blood type was +/+ or +/-. If the former, we would always make a baby with a positive blood type; if the latter, it was possible that it might be negative. In any case, we discovered that he's +/+ so we don't need to get the baby tested in utero or anything like that.