Herein, we are trying to use the novel analytical technique -Dual Polarisation Interferometry (DPI),to achieve detection the minimal amount of the human chorionic gonadotropin for early detection and strict monitor of the GTD. Under this circumstance, maintenance chemotherapy is continued until hCG values is totally undetectable, in the hopes of eradicating all viable tumors. Besides this method could be more precise in sensitivity and specificity to avoid the false positive result which could led to unnecessary chemotherapy or surgery.