We consider that a low-frequency, sinusoidal rhythm detected by Electronic Fetal Monitoring (EFM) is a typical manifestation of a placental abruption. We present the case of a 26-year-old woman who was gestational 36 weeks. She had irregular contractions. The EFM showed a low-frequency, sinusoidal rhythm. The cycle of the sine curve increased in time from 90 seconds to 160 seconds gradually. A severely asphyxiated newborn was successfully resuscitated after an emergency Cesarean Section. The mother and child were discharged from our hospital in a good general condition seven days later. Upon examination it was proven that this was a case of a placental abruption. This is an original case study report about how to diagnose a placental abruption according to an EFM reading. We propose a definition of a low-frequency, sinusoidal rhythm having: 1) a stable baseline FHR (fetal heart rate) of 120 - 160 bpm; 2) a possible variation of 20 - 30 bpm; 3) a frequency of 90 - 160 seconds per cycle; 4) fixed or flat short-term variability; 5) oscillation of the sinusoidal wave from above and below a baseline; 6) no areas of normal FHR variability or reactivity. What is more, the lengths of the fluctuation cycle and the greater the amplitude has a close relationship with the prognosis of both mother and child. This pre-partum, low-frequency, sinusoidal rhythm is an ominous sign of fetal jeopardy needing immediate intervention. The medical literature has never reported such an association. Therefore, our case study report is possibly the first to mention this observation.