It's been my experience that the great majority of expectant employees manage perfectly well with the 'standard' office chair provided. I suggest you go for accessories (lumbar and other cushions, footrests etc.) on an 'as required' basis.

I have had to deal with this a few times and replacing the chair has not been the answer one single time. I wholeheartedly agree with Hilary and Ron but another important thing is to ensure regular breaks. One lady we had suffered pretty badly with back pain but found she was ok if she was able to stand while working so we arranged a bench for her to work at, problem solved. In any case, I would start with the basics of workstation posture, ergonomics, position of monitor, keyboard and mouse, height of screen, seat adjustment yada yada and go from there.