I think that you are right to be nervous, but perhaps you're worrying too much. Both responses I've seen address different but valid issues.
On the one hand, you have to think of your needs as an employer. On the other hand, you have to keep in mind the rights of your employee.
So, in an attempt to address both your needs/rights/responibilities, you should talk with her honestly and let her know that you are concerned for what she will no longer be able to do; beit now or in the near future, or once her child is born. I would definitely talk about her time off. Will she be taking the usual 6 weeks? Will she want to take more time (or less). I don't know if you pay SS on her, etc. If so, she can collect disability during her time off, etc. During that time, you WILL need a replacement. A good idea would be to go on CARE.com or similar sites and start looking for someone that can come in ONCE a week until October, and then do a maternity leave cover, with the possibility of it turining into permanent full time or part time after that. This way, you have your current housekeeper train her replacement while she is still working and you have someone trained by the time your current housekeeper needs the time off. This would also address your housekeepers ability to stay on for as long as possible, AND plan "heavier" duty for the one day a week when she has help.
I absolutely would not consider letting her go only because she is pregnant. Even illegals can sue for wrongful termination in California. It would be incredibly unfair, since, as one of your responders said, women are usually able to do most of their own housekeepin AND work an outside job, right until they give birth, and then right after. Just be sensitive to the heavy lifting, climbing on step ladders and doing big jobs like "all the windows" or "spring cleaning". You should both be ok!
