The former `Lost` actress was spotted in Hawaii sporting a sizeable baby bump which she has managed to keep secret from the world.
It will be the first child for Lilly and her long-term boyfriend Norman Kali, who she met when he worked as a production assistant on mysterious show `Lost` in which the 31-year-old star played plane crash survivor Kate Austin who was stranded on an island.
The brunette beauty has made no secret of her desire to start a family.
She previously said: "I have always wanted to have a child of my own and I will always want to go through the gift that women have been given to experience that, so I hope to be a mother at some point."
Lilly has also claimed she only wants to conceive one baby naturally and then she intends to adopt so she can give a home to a disadvantaged child.
She has stated in the past: "For a very long time, I have had intentions to adopt. We have a huge population problem on Earth. We`re not going to protect the Earth the way we need to protect it if we don`t stop making so many babies. I think I can allow myself one child - and from then on, I think I would have to adopt. It makes sense not to add to the population problem."