Interesting ....

[gregmonsta]

Think of hormones acting like steroids or something to that effect, amplifying the creation of the necessary goods ... some tissue weighs more than other tissue even if it is made of the same key ingredients .... therefore the direct conversion of tissue x to y, even with minimal input can = something with higher density/weight (depending on various factors, of course).

re-read ^

All I'm saying is the Law of Conversation of Mass. You CANNOT have weight gain without some input. Retaining a bunch of water and using that for some "reconstruction" and gaining weight I'll buy.

The steroid note didn't say there was no food input ..... but the hormonal change includes the incoming food to make something that is denser that the usual fat-stores.
Females build up fat stores for the latter part of pregnancy before the process of egg-production and then concentrate on producing the egg masses. Even females kept on a normal diet (ie owner unaware of pregnancy) and weight gain will still exceed the input in the initial stages.
My radix last year went through vitellogenesis and produced some slugs (unfertilised egg masses). I wasn't expecting it but even then a small increase in growth rate was evident even with the regular / 'normal' food input.
Even the 'Law of conservation of mass' allows for flexibillity and is a general 'rule' and a constant point of debate (several parts of this theory stand contended in science), cases like this have been argued over. This (a living organism) is a flexible system and allows for increase of mass at a molecular level. A garter snake is not exclusively an open or a closed system. There are other factors involved, in my opinion. A pregant female will retain as much energy as possible (so it is possible to think that this would also be stored from internal processes and not just food). She will also, I imagine, combine existing fat stores with the food input when necessary, or for certain stages of embryo development.
At any rate it's an interesting thing to consider.
The role of water in these processes may indeed be quite significant and difficult to account for if you don't see an increase in drinking activity or drier stools. Again, this could be water retained from the food source too.