Your landlord can not force you to move simply because you are having another child.
Familial Status is a protected status and discrimination on that basis is prohibited.
However the local zoning laws relative to habitation may not allow siblings of different genders to occuoy the same bedroom or their could be other limitations on the number of people allowed per number of bedrooms.
These would be local ordinances that you can verify at the township.
Good Luck and congratulations
Fran and Mark Redding
Prudential Fox Roach
1010 Stony Hill Road
Yardley PA 19067