Does anyone know why this happens?

I was playing this family when I went into edit town and split up the household.
Both of the sims g/fs were preggo.
The first sim g/f had the baby and it showed the male sim as the father.
Then his official g/f had a baby and on the family tree it showed him without a father.
I moved both the g/f and the male sim together into the same house does this matter?
Has this happened to anyone else?