The collision between a pale blue Citroen and a dark blue Mazda happened around 12 noon on Wednesday (December 3) on the westbound carriageway near the Fulbridge Road junction.
The woman, who is in her 60s, and her one-year-old grand-daughter, who were travelling in the Citroen, suffered life threatening injuries.
A man who was driving the Mazda suffered serious but not life threatening injuries.
Police are investigating and would like to speak to anyone who saw the collision or the vehicles involved just prior to the incident.
Contact Sgt Paul Tanner at the roads policing unit on 101
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