The growth of your baby relies on a steady stream of nutrients. Most nutrients are transferred unaltered across the placenta, but some substances are made by the placenta itself and a few are produced from scratch by your baby. These include the hormone thyroxine, the production of which depends on iodine from the mother crossing the placenta. Thyroxine has several functions and its level needs to be controlled very precisely. The placenta forms a nearly perfect barrier to thyroxine, enabling you and your baby to adjust thyroxine levels independently of each other. In the early stages of pregnancy, genetic factors largely determined the size of your baby, but by now environmental factors are becoming more important. Overall, your baby's final birthweight is determined about 40 percent by genetic factors and 60 percent by environmental factors. Your baby grows at a steady rate from 24 weeks until the last 2-3 weeks, when growth continues but more slowly. (If you're expecting twins, your babies grow as if alone in the uterus up until 28 weeks but from this point there is a reduction in their growth rate.) Your baby's internal organs account for much of his current growth. The liver and brain, in particular, continue to enlarge and muscle mass increases. Later, fat will be deposited under the skin, rounding out your baby's contours.