It used to be assumed that protein needs during pregnancy stayed the same throughout, but a recent study shows that as the pregnancy progresses, the amount of protein needed also increases. A 2011 study has shown that outcomes are improved when mothers supplement with higher levels of Vitamin D than the 600 IU/day that is currently recommended. The mothers body is amazing, if the diet doesn't provide everything baby needs, then the body will start pulling nutrients from the mother's body. This can impact the mother years after pregnancy, so eating to support both Mom and baby in pregnancy is important not just for pregnancy.
If you are curious to learn about prenatal nutrition more in depth, and learn what the latest evidence shows, I highly recommend reading Lily's book. It's easy to read, the information is easy to understand and her recommendations are easy to implement. She even has a small section with recipes to try. Knowledge is power, and in this case knowledge is vital. We are what we eat, and babies during pregnancy get the building blocks they need to grow organ systems, and be ready for life outside the womb from what Mom eats. Eating in a way that supports the pregnancy appropriately will make a world of difference for baby, and for how Mom feels during pregnancy, as well as Mom's health in the years after pregnancy.