Birthdate is required when you have Age Based lists. If we do not have an age, we would not know which list to put that person on.
If the parents have a due date, put that as the birthdate. You can update it when the child is born.
If they do not yet have a due date, you can check the “Trying” checkbox. This will put them on the youngest list on your account. If you do not have a Trying checkbox, go to Settings > Site Settings > Site Details and check the “Show Trying Checkbox” setting.
On the list, these children will have a birthdate of “Expecting”. You’ll also notice they have a red exclamation mark next to their name indicating they are too young for the list. This icon will also appear if a child’s due date in is the future, or your lists start at a later age then the child is.