The preparations are similar to any other photo shoot: just be prepared to look your best. It's a good idea to avoid any new or radical changes to your normal hygiene regimen just before the shoot, as new skin cleansing and hair care products could bring about unexpected results. Being well rested will help, though it's sometimes difficult during pregnancy. Also eating light and healthy the day of the shoot is a good idea so you don't feel bloated or hungry. This will help you look your best.