I have lots of pregnant Moms over the years. I ask outright what the plan is. Do they intend staying full time? Do they intend leaving ? And how soon after baby due are they going? I then advertise.

If parent not know, I ask them to let me know a couple months at least before hand so I'm not stuck losing income due to empty place. Then I fill based on their information. Sometime they want few months to adjust. Sometimes children stay FT. Sometime they wish for PT but I don't offer that so it's all or nothing, just need to know when.