Hi Op
MN disclaimer ? please be away I am a first time mum to be and this note is based on my constant research as paranoid of putting too much weight on etc. 
I thought ?read via several sources?that by your due date, just over a third of your extra weight will come from the baby, the placenta and amniotic fluid. Here is a breakdown I got from one of my sources:
?At birth, an average baby weighs about 3.3kg (7.3lb)
?The placenta, which weighs 0.7kg (1.5lb)
?The amniotic fluid weighs 0.8kg (1.8lb)
The other two-thirds of extra weight is due to the changes that happen to your body while you?re pregnant. On average:
?The muscle layer of your uterus grows dramatically, and weighs an extra 0.9kg (2lb)
?Your blood volume increases and weighs an extra 1.2 (2.6lb)
?You have extra fluid in your body weighing about 1.2kg (2.6lb)
?Your breasts weigh an extra 0.4kg (0.9lb)
?You store fat, about 4kg (8.8lb) to provide you energy for breastfeeding
If we consider the NHS says that on average most pregnant women gain between 8kg and 14kg (17.5lb to 30lb), most of which is gained after the 20 week mark then the baby, placenta, extra blood and fluid etc. (not incl. fat stores for breast feeding) contributes on average to 19.3kg (18.7lb). Any addition weight outside c. 27.5lb is additional weight gain by mum and not really to do with baby. Clearly this is all internet / NHS based research as I am paranoid about putting too much weight on.
It?s probably all wrong and there is different advise out there.
Ha ha ha
But I have found that most advice I have read is consistent with this. So hopefully when the baby is born I am hoping/praying to drop at least 1st 4lb straight away ?she says naively hoping this will really happen-- 