if you are offering a home to a child you are related to, that would be considered. If you are starting a career as a general foster carer, that will have to wait.
it is perfectly possibly to start being assessed as a foster carer with a baby, and to work as a foster carer with your own little one, but no reputable agency is going to place unrelated foster children with someone about to give birth, or with a new born.
You cannot assess what a family is going to be like at those stages. You will have to wait, so if you start being assessed when the baby is 6 months or so old, depending on the timetable for assessment ( quite often they only start at certain times of the year) then you will be able to start when the child is one.
Or, if you are an established foster carer, there is likely to be a maternity leave, same as in every other job, although in some cases, an established relationship may continue, especially if it is a child you are related to.