28 weeks onwards is the most likely time that a growth scan will be done. Every baby is different and at this scan you will find out If you baby is likely to be big or small. This can be very important if you have known gestational diabetes etc. There are many things that influence the imaging we get at this scan. Placental site, amount of fluid around baby position of baby etc. If things go right you will get amazing pictures however this may not always be the case and you may even find it hard to understand the ultrasound at all.
NOTE: No need to have any fluid in your bladder for this scan.