Abstract: The existing fetal monitor is prone to false detection in the case of high maternal interference in the second stage of labor. With this background, the article designs and implements a combined maternal-fetal fetal heart monitoring system. The system obtains the Doppler signal of the abdominal fetal heart and the blood oxygen signal of the mother's finger, and estimates the maternal interference degree in the fetal heart rate Doppler signal according to the maximum correlation value between the maternal finger blood oxygen signal and the abdominal fetal heart Doppler signal, and switches the fetal heart rate extraction algorithm between the autocorrelation method suitable for lower interference and improved template method suitable for higher interference according to the maternal interference degree. The accuracy of our method is 9.2% which is higher than that of the improved template matching method and 6.1% higher than that of the autocorrelation method.