The 39-year-old actress is currently expecting her second child with husband Nnamdi Asomugha.
In an interview to People magazine, Washington explained why she chose to make the big reveal at the prestigious fashion event.
"I hadn't been public about it (before then). I still haven't made a statement about it because it's not really how we roll in my family.
"I knew by the Met Gala that I wouldn't be able to hide (my pregnancy). The idea that my bump made its debut at the Metropolitan Museum of Art is pretty fun," Washington said.
Washington and Nnamdi also have two-year-old daughter Isabelle together.