I found when I rang (at 5 weeks) that you need to book in early, otherwise you'll have to go and get the scans done yourself (via your doctor) rather than the hospital arranging them (that is if you're a public patient like I am, no idea what happens in the world of the private patient).

Most places, unless there are problems, don't have you in for your first appointment til around 10 weeks anyway, but their waiting lists are long enough to need to book early.

Last time I was 14 weeks pregnant when I booked in and had to arrange (and pay for) my 20 week ultrasound, even though I was a public patient.