Reliability of Temporal Thermometer

How reliable are the temporal scanning thermometers? We have the iProven brand. My baby will be 4 weeks on Thursday and I'm not sure if this is considered accurate or not. We do not have a thermometer to use for rectal temp. We have a really upset tummy and have been spitting up more than usual - but my MIL also babysat last night (never again) and overfed her.

The head scanner is ranging between 99 and 99.3 But I also have a baby that tends to be warm in general and she's been crying on and off - so idk if that can effect the temp.

FTM if that isn't obvious.