he might have asked for a headcount already

It is not completely unreasonable for a manager to plan around a pregnant employee's schedule by asking for additional headcount. While it may seem OK to you, your co-workers may not be able or willing to pick up your slack for three months or longer while you are away. after all, nobody knew about your maternity plan until now. The fact is that there are a fair number of new mothers who either (1) take the entire maternity leave/vacation/sick time -- therefore maximizing the benefits received-- but then quit the job; or (2) take all the vacation time/sick leave, and then basically dedicate themselves to the baby and leave the work as an afterthought. It might sound harsh, but it does happen a lot and managers have to plan around it without knowing whether the subject on hand belongs to one of these two categories.

If you do plan to take 3 months off plus additional sick days, and you want to fight vigorously against the new headcount without an alternative solution (e.g. are you willing to work from home?), it might backfire on you and you might be setting yourself up for a lot of animosity among your co-workers.