A police spokesman said officers were called at around 1.25pm following a concern for safety near to junction 25 (Brighouse).
He said: "Initial enquiries are ongoing, but it is believed a woman has fallen from a bridge.
"She has sustained serious injuries and road closures are in place."
The eastbound carriageway was shut between junction 25 and junction 26 (Chain Bar) while emergency services worked at the scene.
A diversion was put in place until the carriageway reopened shortly before 4pm.
Highways England said there had been severe delays stretching back as far as junction 22 (Outlane) but these were now easing.