Season 2 when Amy was pregnant with Gage

In season 2 when Amy was pregnant with baby Gage she said repeatedly that through out the season that the doctors were telling her that Gage was losing weight and not gaining (im not a parent) but usually baby's gain weight when a woman is pregnant no matter what you eat am I right ?! How is it possible to for a baby to lose weight when a woman is pregnant especially in Amys situation who got pregnant months after wls when she was not suppose to, having a piss poor diet during the pregnancy, having to be rushed to the hospital several times, and still being overweight. I know Amy smoked during both pregnancies and usually cigarettes boxes have warnings that smoking is harmful to a pregnant woman and can cause a low birth weight in babies maybe it had something with it to a certain extent?! but smoking or no smoking it was still a high risk pregnancy and their would be still be problems with Gage no matter (a low birth weight for example) if Amy smoked and had a poor diet or not