Exactly the same thing happened to us, but luckily I was not pregnant at the time. My hubby and I decided to keep our maid, on the following reasoning: 1) She was pleasant and we felt sorry for her, 2) There is no guarantee that your next maid would not get pregnant during employment unless she is past menopause, and it is much less likely that your maid would get pregnant again knowing the trouble she is in, 3) She would need money to look after her baby, so would need to have to work even harder, and 4) It'd probably take just as long choosing and hiring a new maid as waiting for her to come back after she has given birth.
Anyway, she was also afraid to tell her parents but we convinced her that it is not really plausible for her to have the baby in Hong Kong and she would need her family support in looking after the baby in Phillippines. She was very grateful and reassured when she knew that she still had a job to come back to, she managed to sort it out with her family very quickly after that.
Since she was pregnant when she started her employment with us, we did not have to pay for her maternity leave. Although she had to leave for 3-4 months and it was a real nuisance, we have not regretted our decision. She has been with us for over 6 years now and looks after our baby almost like her own, even volunteered to stay in to help on Sundays (paid of course) during our baby's first 8 months.
May not be the solution for everyone, but since we found our maid to be hardworking and pleasant, we decided not to ask/bribe her to leave. As anticipated, she has not gotten pregnant again and works hard to send money home for her daughter. Hope that gives you an alternative view.