In the film, "The Crown" star plays Martha - a woman who loses her baby during a home birth. However, as she's not a mother herself, Vanessa was unsure how to tackle the 26-minute birth scene.
So she threw herself into preparing for the gig by shadowing a midwife and a paediatrician at London's Whittington Hospital. And, as part of her time at the hospital, Vanessa actually got to watch a little boy entering the world.
"Just by pure chance an amazing woman allowed me to watch her give birth," Vanessa told the U.K.'s Daily Mail newspaper. "I didn’t know her but she was so generous and allowed me to be there when her boy was born."
In addition, Vanessa spoke to several women about how they had coped after losing their babies, and was told by one friend who had had a miscarriage that she "received more support when her dog was put down by the vet."