I got mine at 3 month because I was sleeping in my stomach and I enjoyed it till 6 months But now as 7 month pregnant I wish i bought the small one
I got mine at 16 weeks, but you can pick one up whenever you desire! Good quality sleep is important whether one is pregnant or not. A great tip if you're unsure on what shape of pillow to get: if you have extra pillows around the house, form those into the shape you're interested in buying (J-shape, U-shape, wedge, ect.) It may save you some time and money by doing this ahead of ordering :)
Can have a pregnancy pillow when not pregnant if it means you sleep comfortably! Get that pillow and sleep well Mumma!