It's not that unusual and typically goes like this: The parents are unmarried and sometime before or around the birth of the baby have an argument. Maybe dad has taken up with another woman, or maybe it's something small, but being unmarried the mother has the right to control who visits the baby, and in her anger towards the father uses one of the only weapons at her disposal - visiting privileges for their child. Sometimes the parents fight and make up and visiting privileges change daily for the father, depending on the couple's status. If the parents are married or the father officially claims paternity, then the mother cannot keep the father from visiting, but those circumstances are fairly uncommon in our NICU.
Our social worker and I really don't like this, and even though it's legal for mother to do this we generally tell them they cannot do it. Let's face it; there are too many kids in urban America without a strong father or male influence in their life, and we hate to allow anything that might contribute to the father's absence from his child's life. Just because the parents don't get along doesn't mean the kid should be deprived of his or her father. Sometimes when we put it to the mother in those terms, she will let the father visit (although not necessarily the father's girlfriend, and who can blame her for that?)
It's not that we don't sympathize with the mother. Any father who impregnates a woman and then leaves her before the baby is born doesn't rate real high in my book. He's especially a rat if he shows up in the nursery with another woman as his girlfriend. But parents splitting up are a fact of life, and the children shouldn't be pawns in the disagreement between them, whether it's at birth or later in life.
Sure, the father should claim paternity, and we encourage them to do so if they want to visit, but that is not always done immediately. Also, the flip side of the father's bad behavior in showing up with another woman is the mother having multiple affairs so the father isn't sure if he's really the father. There's crap all around, but let's let the kids have two parents, even if they're not together.