yes it is really, its a deep wound in an area without much flesh. lacerations of 5cm are more likely to become infected plus He will have given you a 5 day course of a first line antibiotic- not a major problem. I presume he knows you are pregnant?
If you don't take it you are at increased risk of infection and near a bone these can be fast developing.Its a risk you can take but if you develop any infection that's nasty you might need a stronger or more specific antibiotic and many of these aren't safe in pregnancy.
Drs don't prescribe antibiotics wily-nilly to pregnant women particularly if your Dr isn't an over-prescriber of them. Id say trust his judgement on this one. You might get lucky and be infection free or you might not.