Bump measurements are woefully inaccurate. My midwife said they mainly look for consistency. This pregnancy I have continually measured 4/5cm bigger than my dates at every appointment and although baby has shown to be on the larger end of the scale, it's definitely not that much bigger!
If the growth scan shows a bigger baby there are many different meanings/options etc. Firstly, they'll probably get you in to do a glucose tolerance test just to rule out gestational diabetes. If the comes back fine then they'll probably give you a couple of extra scans to check baby's size. If baby is measuring off the charts, and it would to be seriously huge to do that, then a cesarean might be offered as an option. If it's in the higher end of normal they can offer a variety of thing- it all depends on your local trust and their policies.
Caesarean might be offered as an option, and induction too possibly. With my dd they were happy for me to reach term but I had induction booked for 6 days post due date instead of the usual 12-14 because they didn't want her to get too big. As it turned out she came before then.
Her abdomen was measuring very big, and her other measurements were a bit above average. She ended up weighing just shy of 9lb, so big but certainly within the range of normal. Unfortunately she did have shoulder dystocia, but I've since read that it's mainly down to position of baby and mothers bone shape etc. I personally know people who've given birth to 9lb 8-9oz babies and had no trouble. Because of the previous shoulder dystocia I'm having a cesarean this time (which is probably why I'm unconcerned about my measurements being so over!!)
Just remember, they won't make a plan for you based on one measurement alone, as they can be so unreliable (even the scan sometimes) so they'll probably just want to monitor you a little more at first and take it from there.