What happens if 2 o negative blood types have a baby?

When both parents have O- blood, all of their offspring will also have O- blood. This is because the O blood group is recessive, meaning that two copies of the O allele are needed to express the O blood type. If both parents have only O alleles to pass on, then all of their children will have O blood.

The O blood type is the most common blood type in the world, accounting for about 45% of the population. It is also the universal blood donor, meaning that people with O- blood can donate blood to anyone, regardless of their blood type.