I'm 35 weeks pregnant and the manager of my office. Planning to go on maternity leave next week. I've realized over the last couple of months that my staff has been getting increasingly nervous about my presence here now that I'm in the third trimester; they are concerned about putting any bit of stress on me and causing problems, or that I might go into labor early and force one of them to deliver my baby.
I think my staff would LOVE it if I were working shorter hours (so would I). I've had a low-risk but not...pleasant... pregnancy and they have seen the toll it's taken on me without me having to say anything.
I'd second/third/fourth the suggestion about just giving a heads up about when and how you will be available with your new schedule, and don't make an attempt to explain, because I'm betting they will all understand the reasons without being told.