Of course you can request one. The doctor of course can refuse it, but they might just agree to it. Be aware though that a scan could bring more worries (as my growth scan at 32 weeks did, for no reason at all as it turns out).

If it's the size of your bump that's concerning you (and I'm guessing that it is) try not to stress about it. I never got that big with my pregnancy - late in the 2nd tri some work colleagues didn't even know I was pregnant and in the third tri I had a number of people comment on how small my bump was. My baby was 7lb 13 ounces at birth so was not small.

It could be that you just aren't putting on any fat weight and be like me and within 1 week of the birth only have 2.5 kgs to lose to get back to your pre pregnancy weight.