I have my own experience of giving GA for all kinds of obsteterics elective and emergency cases up to 2500 cases.From my experience I think the following reasons 1)Not enough pre-oxygenation for 3 mts.Low FRC in pregnant women would have caused low Spo2. 2)Endobronchial intubation.3)Undiagnosed PPH(Primary Pulmonary Hypertension)Intubation would have caused increased Pul Art Pressure/Increased shunt fraction/desaturation.4)Increased intra abdominal pressure(polyhydromnios)-markedly reduced FRC/Inadequate pre oxygenation-all combined together-relieved once intra-abdominable pressure is relieved(after delivring the baby) and lungs are fully ventilated. 
                                                 Dr.J.Edward Johnson.