Blake, who recently welcomed her second child with husband Ryan Reynolds, posted a picture of herself with her head resting on Tamblyn's shoulder and holding her The Sisterhood of the Traveling Pants castmate's stomach on her Instagram page.

She added the caption: "I am asking you to rethink women, period."

"Whatever your beliefs, one thing is certain, the world must be a better, safer place for women and girls. The time is now," she added.

Tamblyn announced she was expecting her first child - a girl - on 26 October (16), in a published essay for Glamour.com, in which she addressed the challenges women face in today's society.

Amber wrote: "Motherhood has been heavily on my mind because I am going to be a mother soon. I'm pregnant, with a daughter on the way. I think constantly about the world I am bringing her into."

"I've been thinking about motherhood a lot lately," she continued. "What it means to be one, what it means to have one, what it means to know one, what it means to make decisions as one and have conversations as one.

"I am very lucky to be surrounded by strong mothers, from my own mom to some of my best friends - those who are raising young women to accept themselves and those who are raising young men to accept women."

Amber and Blake aren't the only Sisterhood stars with motherhood on their minds - their castmate Alexis Bledel became a first-time mum last year (15).