Domestic helpers are like everyone else. Barring major medical problems, they are expected to work full-time until their maternity leave starts. Then they are allowed 10 weeks. If they choose to take much longer, then they risk losing their jobs.
I'm sorry if I sound hard-nosed about it, but the same rules apply to everyone.
So, no. You should not feel backed in a corner to go helper-less for "six or seven months."
The only concession is that you shouldn't ask her to do anything seriously damaging to a pregnant woman, like moving heavy furniture. But cooking and cleaning? I'm 6 months along and I'm doing my own cooking and cleaning!
I've had a hard pregnancy, but only took 3 days off work when I had a genuine medical scare -- a threatened miscarriage. Besides that, I worked full-day shifts through morning sickness, nausea, fatigue and leg cramps. It sucks, but that's what being a pregnant working woman is, whether you work as a maid or in an office.
Do sit down and talk seriously to this couple, and try to be firm but reasonable. Then ask immigration what recourse you have. The law is there to protect people from abusive employers who will fire people simply bc they don't want to give maternity pay. But it sounds like you have a legitimate problem on your hands.
Good luck!