If the placement was by a state agency, or another foster care or adoption agency that is approved by the state (or the court) then yes, a legally placed foster child is treated the same as a biological child and you can claim them as a dependent. Make sure in Turbotax you indicate they lived with you "all year" which is the correct answer for a newborn. If you only indicate 5 months in turbotax, the program won't think the child is eligible.
However, if this was a private placement, and the state or courts were not involved, then the answer is no.
