There is no real science to support the blood type diet information, but I've found it to be helpful in terms of eating foods that help me feel good, and (which is the opposite) don't encourage weight gain.
For people with blood type O your basic diet according to Dr. D'adamo is "lean organic meats, vegetables and fruits and the minimization of wheat and dairy..." Here is a bit more http://www.dadamo.com/bloodtype_O.htm
But that means being clever and finding calcium and all the other things your baby needs-- dark green leafy vegetables that have calcium too, etc. are good in general.
I'm sure you have discussed your negative RH factor with your doctor, based on the bloodtype of your baby's father.
At one point in my first pregnancy my midwife suggested a I gain more weight. I started eating an extra apple and peanut butter for an afternoon snack and some cheese mid-morning. It worked.
I would recommend adding the appropriate digestive enzymes to your diet to assist with proper assimilation of food and absorption of nutrients.
Founder/Organic Mommy & Baby Healthcare Solutions