Thank you yeaher and kaplinghat. I've asked around in China but have not been given definite and positive answers yet. I know that in China, people who are born in hospitals usually are issued birth certificates from the hospital immediately; but those born at home (like me) do not have any authority to testify the birth. The only similar official documents are house registry, but I'm not sure if immigration officers know what a house registry is and would consider it similar to a birth certificate, plus our house registry is changing with the change of one's marriage status.