Sacagawea was pregnant with her first child when the Corps of Discovery arrived near the Hidatsa villages to spend the winter of 1804–1805. Captains Meriwether Lewis and William Clark built Fort Mandan. They interviewed several trappers who might be able to interpret or guide the expedition up the Missouri River in the springtime. They agreed to hire Charbonneau as an interpreter when they discovered his wife spoke Shoshone, as they knew they would need the help of Shoshone tribes at the headwaters of the Missouri.