No matter how big your bump gets, the law says you still need to wear a seatbelt when you're pregnant, unless your doctor has given you a Certificate of Exemption from Compulsory Seat Belt Wearing. You'll need to keep the certificate in your car at all times so you can show it to the police if you're stopped.
The NCT has some useful tips on wearing a seatbelt when pregnant:
Always wear a three-point seatbelt.
Place the shoulder belt over your shoulder, collarbone and down across your chest, between your breasts.
Keep the lap belt as low as possible under your bump.
Adjust the seatbelt so it fits as comfortably as possible, and the seat too if necessary.