The Super actress announced in October that she and boyfriend Steven Rodriguez are expecting their first child together.
While Cardellini insists that she is feeling "fantastic" since getting through a difficult first trimester, she revealed that she has her concerns about motherhood.
"We're getting there - it's a wonderful, thrilling event," she told People about her preparations for the new addition.
"I think you can't help but be slightly terrified because somebody's whole life will be in your hands. [I'm] excited about this chapter in my life."
Cardellini, who appeared as nurse Samantha Taggart on ER from 2003 until its series finale in 2009, recently described her pregnancy as a "crazy miracle", saying: "I feel great, and it's amazing and I can feel [the baby] kicking and it's a totally unbelievable experience.
"It's a little bit like Alien in science fiction."