Whilst you are pregnant hormones soften the collagen in your ligaments, however the collagen in your skin also suffers.

Diet is a major factor - you can do a lot to help your skin deal with pregnancy by ensuring that your body is getting enough antioxidants especially Vit C as it plays a major role in the synthesis of collagen.
Eat foods that are rich in zinc, vitamins A, C and D and a good mix of proteins.
Dark Green Veg (Spinach, silverbeet) and oily fish (salmon, mackerel, etc...) are good inclusions in your diet.
Drink lots of water....lots of water! - this does wonders for the elasticity of your skin.