Rosamund, 35, is seven months pregnant with her second child, and Charlotte, 39, is expecting her first child in February.
The pair met on the show this morning (September 30) to discuss Gone Girl, a film based on the best-selling novel by Gillian Flynn - in which Rosamund plays Amy Elliott Dunne, a woman who disappears on her fifth wedding anniversary to husband Nick (played by Ben Affleck).
But unsurprisingly talk soon turned to babies:
"Having a baby is a great project to have after playing a character as un-maternal as Amy Dunne," Rosamund joked.
"After playing a character like that, it's good to be bringing a new life into the world!"
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Gone Girl hits cinemas on October 2, so luckily Rosamund will be able to get through all her promotion work before giving birth in November.
This child will be her second baby with partner Robie Uniacke, 53. They already have a two-year-old son called Solo.
Charlotte has a little longer to go.
She announced that she's expecting the 'first Good Morning Britain baby' live on the show on Tuesday, August 12, and revealed that her due date is next February, which means she is currently only four to five months pregnant.
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