We had ours booked into be installed at 39 weeks (they were fully booked!).

Unfortunately DD had big ideas of arriving early at 38 weeks! LOL! so DH had to run around like a madman trying to convince them that bubs HAD arrived and we needed to have the seat put in...in the end he refused to leave the office and someone took pity on him and helped out! LOL!

so yeah, can recommend getting it in early!