When do you book into the hospital?

Tara • Im 23 yrs old have a baby girl 20mths old i love her with all my heart. I have an amazing partner bein together 2yrs.
Hey everyone :) im 7weeks & 2 days with my 2nd i have gone to the doctor and got my antenatal letter to book into the hospital but at what week do you book in i know most hospitals wait till your at 12 weeks??