In my experience the best time to transition to a bed is between ages 2 and 3, with some room at either end depending on other factors, such as your child’s size and whether she’s a “climber.” Still, your daughter will be on the early side, and adding a sibling is already a lot for a little one. If you can, keep her where she is, get another crib—even if it’s just a portable one—for the baby, and let your daughter deal with one change at a time. Mom to mom: You don’t want more work for yourself if it’s not necessary!
But if space or budget won’t permit a second crib, transition your daughter to a toddler bed about two months before your due date. This way, she can get used to her new bed—and won’t feel like the baby is sleeping in “her” crib.