U.K ladies help.

Megan • Our little rainbow baby girl due September 17"
So I received my appointment letters via post today and have been booked for my 12 weeks scan, which is normal. I also have a 28 weeks ultrasound and obstetrics antenatal appointment. What about 24 weeks and why 28 weeks? Help? Will I still be given a 24 week scan? Why have I got a 28 week scan?