A toddler has now died after a four-car crash that killed a 29-year-old woman and her 6-year-son boy on Friday morning.

Colorado State Patrol says a call first came in about the wreck at Highway 52 and Weld County Road 41 at 8:08 a.m.

According to their preliminary investigation, a vehicle heading north on County Road 41 ran a stop sign and was t-boned by a car on Highway 52, killing driver Sheila Sinclair of Lochbuie and her young son Jaxon Sinclair. Neither of them were wearing seatbelts.

Sunday, CSP told 9NEWS a 3-year-old girl in the car with Sheila passed away two days after the crash. The toddler's name has not been released.

A 1-year-old girl remains in serious condition after this wreck. She and the 3-year-old who died were both in child safety seats.

The people driving the third and fourth vehicles involved in the crash weren’t hurt, however, there was a child in one of those cars who was taken to the hospital as a precaution.

Troopers with CSP think Sheila ran a stop sign and another car ran into them. Fog may also have been a factor.

This stretch of highway is between Fort Lupton and Hudson.