I agree that the tone of your post is rather unnecessary, so perhaps I'll be a little more forthright than I might otherwise be in correcting your misconceptions;
Firstly, the basic maths involve far more components than just baby, placenta, blood and breasts.
maternal stores of nutrients and muscle development 7.0 lbs
Increased body fluid 4.0 lbs
Increased blood 3.0 - 4.0 lbs
Breast growth 1.0 - 2.0 lbs
Enlarged uterus 2.0 lbs
Amniotic fluid 2.0 lbs
Placenta 1.5 lbs
Baby 6.0 - 8.0 lbs
Total 26.5 - 30.5 lbs
Secondly, You were told that 18lb is the maximum you can gain without stretchmarks? You were told wrong, I'm afraid. I can personally attest to gaining 4.5 stones (the horror) with my first pregnancy without a single stretchmark to show for it.
Do you understand that a pregnant woman's metabolism can change fundamentally from its non-pregnant state? The same number of daily calories intake can still result in weight gain because the body is deliberately conserving fat stores to ensure healthy continuation of pregnancy and the successful establishment and continuation of breastfeeding.
Haven't you ever questioned that our bodies have been designed to alter in a way that best ensures the survival of our babies?
Oh, and for the record, my face (and thighs and arms) quite obviously reveal my pregnancy. I change from being rather sculpted and 'cheekbony' to much more round faced. This effect disappeared last time within a couple of weeks of the birth, and I'm certainly hoping it will this time too.
So, please do "question and maybe reappraise" your understanding and even, dare I say it, attitude?