Low incidence, very low number of centers, striking change which reverses in two months: Operator (new staff) or recording error (new system for recording or identifying) would be the first two things you would need to rule out. Specific locality of residence of the mothers would be the next research. Cross-referencing of the OB/Gyn provider would be next (creepy, yes - but one doctor covering a population area this size would see far more than this number of patients over an 8 week period). Manmade environmental factors would probably be next (leak, spill?). Covid, while not a remote guess given the timing, is pretty far down the list of likely suspects imo.