In the premiere Yes, Then Zero, Eleanor finds a book on pregnancy hidden and it's assumed it's hers. However, Dorota comes out and announces that she is expecting her second child. Later, Blair's stylist asks how far along she is after receiving her measurements. While she denies it, it's revealed that she definitely is. Soon after, she accompanies Dorota to her obstetrician and after she causes a scene asking bizarre questions, Dorota figures out Blair is pregnant. She begins to experience morning sickness, and eventually admits to Dan that she is pregnant (Beauty and the Feast).
After sleeping with Chuck (The Wrong Goodbye) and Louis (multiple times over the summer), Blair isn't sure who the father of the baby is. After encouragement from Dan and Dorota, she gets a paternity test done and finds out it is Louis' baby (The Jewel of Denial). Once she and Louis share the news with Eleanor, Cyrus, and Sophie, problems arise when no one can agree on how to raise the baby. Sophie has a contract drawn up threatening to take custody if Blair misbehaves, but neither her or Louis sign it (The Fasting and the Furious).
After different events cause Blair to question whether Louis is trustworthy, she realizes she is still in love with Chuck. After they reveal their true feelings to each other, she intends to end her engagement to Louis and raise her baby with Chuck. However on the way to end it, she and Chuck end up in a serious car accident. As a result of the accident, Blair loses the baby, thus ending the unborn child's arc (Riding in Town Cars With Boys).
- Had she be born, this character would have carried the surname Grimaldi and, as a child of Prince Louis, would have been styled Her Serene Highness and a princess of Monaco.
- In an interview with People Magazine, Gossip Girl producers Josh Safran and Stephanie Savage reveal that the unborn child of Blair and Louis was, in fact, a girl.