Hi their maidenoz,
An elevated white count is actually a very normal thing in pregnancy. When one performs a white count, it's done from blood which is collected from a vein. You are only counting the white blood cells which are floating around in the bloodstream. The truth is, though that there are tons of white cells which are stuck to the sides of the blood vessels and aren't sampled when you draw blood. There are certain conditions where larger numbers of these blood cells are released from the wall and it makes the number of cells floating around more numerous. This process is called demargination and it doesn't have any clinical relavance.
In your case, unless you are having worrisome symptoms of an infection or physical stress, the white count doesn't mean much. Yes, 18 is starting to get high, but in isolation it doesn't mean anything and in and of itself is no harm to you or your baby. It is very important they have checked you for urinary tract infection as that can result in a fever and white count and any amount of bacteria in a pregnant mom's urine should be treated (unlike in an otherwise healthy non-pregnant adult).
I think it's a good idea to keep an eye on that result. Your obbie isn't "shrugging it off"--it just seems that way because you're probably not familiar with normal, clinically irrelavant variations in bloodwork which your obbie is. Sometimes doctors take for granted that certain things which are seemingly so simple (because they've been inundated with it for years during their training) may not be so simple to someone who has no medical training. Again, your obbie knows that an elevated white count in isolation is of no consequence, but rest assured both the OB and the midwife are keeping a close eye on things--that's what they've gone through multiple years of training for!