The 36-year-old woman, from Dordogne, was travelling on the D934 from the si station at Artouste in the Pyrenees Mountains when the rock, believed to weigh around 400kg plunged from the cliff above and smashed into her car.
The home carer, named in local newspapers as Karine Vocant, is believed to have been killed instantly.
Her 46-year-old husband, Patrick Marchand, who was driving the car, escaped uninjured, but was taken to hospital in the nearby town of Pau suffering from shock. He was receiving specialist counselling.
The couple were expecting their first baby, French newspaper Sud Ouest reported.
"We're all in shock," the husband's mother Marie-Thérèse Marchand told Sud Ouest. "Karine and Patrick had known each other for six years. They were having their first child together. Patrick was overjoyed."
In this image, taken by photographer from the local TV station France 3 Acquitane, you can see the giant boulder wedged into the wreckage of the couples car.
— France3 Aquitaine (@F3Aquitaine) January 3, 2014
Other images taken by a witnesses showed emergency crews desperately battling to get the couple out of the car.
Local police in the region are investigating how the boulder broke free from the steep mountainside.
“For some unexplained reason the rock detached from the mountainside,” a police source told Sdu Ouest. “The local prefecture has asked an expert to visit the site to find out why and to see if there is a risk of others falling,” the source added.
SOme local people reacting to the news spoke of they felt unsafe when driving on the same road.
"The last time I took this road I don't think there were any safety nets. The road is right under the mountain and rocks do fall off from to time," said one commenter on Sud Ouest's website.
Another said: "I know this route and I never feel safe."
(This image from google maps shows where the accident happened)
The boulder and the remains of the car were taken to the police station in the town of Arudy to be examined.