HantsPants. I am sorry to be blunt but it isn't actually your decision about replacing her, you know.
If I were that nanny and had seen your post from last night, I could take you to an ET and win a case for sex discrimination.
You have to keep her job open for her unless she tells you she will not be coming back after maternity, and any action you do now to replace her before she has give you that decision is just piling up the evidence against you.
For all you know she may be putting into place plans to have someone look after her child so that she can return to work for those 3 days.
Assuming she will become a less flexible and more problematic employee simply by dint of having a baby makes my blood absolutely boil. How would you feel if your own employers were saying the same about you and your post-maternity return to employment?
Get your head around employment law, and start acting like a good employer.
If you do not want the baby coming along then fine, you can say no to her bringing the baby to work, but you cannot throw out the nanny.
Hesaysshesays - am also disgusted with you. Where does it say that her DP will not be taking time off to care for the baby (if ill), therefore enabling the mother to go to work? Sharing the time off is what happens in our house, in Athene's house and many others - your lazy sexism is the kind of low-level stuff that annoys the hell out of me. If you want to be sexist, then fine, fuck off and be sexist. But don't ever pretend that you are being anything other than that.
Jeez, I thought the posters on the childminder boards at least might have some inkling of appropriate behaviour towards working mothers.