Generally first trimester abortions or abortions occurring in first three months are due to fetal abnormality, genetic causes, Rh blood
(this is ruled out in your case) and low progesterone hormone. Infection of the uterus, endometriosis, fibroids, etc are some other causes for early abortion. You need to consult your gynecologist and run some tests to rule out these causes. Eliminating these causes will increase the chances of full term pregnancy. Take care!