Booking the 3D/4D ultrasound

Lindsey • Our daughter will be 3 when her sister is born!
So my husband and I are looking forward to finding out the gender of our baby at 16 weeks at a separate establishment from our doctor. My doc doesn't determine the gender until 23 weeks.... Crazy! Anyway, how far in advance have you ladies booked it? I can't wait to see those awesome images and get that teddy bear with my baby's heartbeat in it!