Will she use other people's urine for examination? It should be accurate after 40 days of pregnancy!
In fact, I will not trust this maid anymore. She work hard because she afraid you will fire her before she notice you (pregnant), she scared because her plan was failed. No matter what, she is dishonest and never consider about your feeling.
In this circumstances, she afraid because her news was bring to public attention, she scare if you fired her, she will become a break contract maid and no one will hire her as DH, and she cannot stay with her boyfriend in HK!
Don't think she will work very hard after this case, look for replacement and terminate the maid before she got pregnant again or make any excuses to sue you money. Keep an eye on your property and statement (she might borrow money and dismiss some day then sue you later.)