A lot of the weight gain will be extra fluids and blood.
Weight gain IS a normal part of pregnancy, unfortunately!
I was an 8-10 pre-preg and went up to a 14-16, despite having no appetite whatsoever throughout pregnancy. Two years later, despite plenty of exercise and healthy eating, not back to pre-preg size and doubt ever will.
Don't rely on popular culture and celeb post-birth pics nonsense, they have rigorous exercise, private healthcare and nannies to help them, not to mention airbrushing and nothing like a contract with some company or other whose products you're promoting to get you back 'into shape' quick!!
Have intuitive feeling my metabolism DID change after birth though, was always 'fast metab', runs in family, we're all skinny minnies, although could be an age thing, over 40 now! I notice the younger you are, the quicker new mums seem to ping back into shape, unless they were already 'average' or plumper before getting pregnant.
Having been so skinny since childhood and suffering from beanpole verbal abuse, have to say now enjoy the extra pounds, feel more womanly, I think as long as you look fairly toned up and healthy, extra weight doesn't matter.
My stance has changed though, it really is mindset thing, in my 20s and 30s as a single and attractive skinny, my sexuality and attractiveness defined me, now in my 40s and a mum, that's what defines me, I'm a 'Mummy Portal', it's my job, my life, my total fulfilment spiritually, I don't care whether I'm sexually attractive anymore or not (but not in an 'I've let myself go' kind of way) because my children have made me feel so secure in my self-image - nothing like the unconditional love they give to make you feel wonderful inside and out!
Worry about weight later, if it goes on it can come off again, just depends how rigorously structured and determninedly you want to shift that weight post-birthm - my guess like most mums you'll be too blissed up with your new bubba to care, though that might seem ridiculous right now!!