At this stage of pregnancy, your baby's muscle mass and fat are continuing to increase. Her skin is now thicker and less translucent, and begins to look pink rather than red as the underlying blood vessels are overlaid with more flesh. Growth hormone is being produced by your baby's pituitary gland but before she's actually born this does not influence her growth. Instead insulin and insulin-like growth factors are key. As your baby's skeleton is now close to its final size, her overall length is established. Your baby is still, however, very skinny.
A sonographer can make a good estimate of your baby's weight from an ultrasound scan but her final birthweight will very much depend on when she is born. She'll continue to grow throughout the pregnancy, although in the last few weeks growth is mainly due to fat deposits rather than to muscle mass.
If you're trying to guess how heavy your baby will be at birth, the latest research indicates that the size of your baby has a lot to do with "imprinted" genes. These are genes that are marked as having come from the father, which promote the baby's growth, or the mother, which are growth limiting and attempt to preserve her resources.