AceShowbiz - Actress Vanessa Kirby was so in awe of the process of childbirth she held her breath as she observed a woman in labour as research for her new movie.

In "Pieces of a Woman", Vanessa portrays Martha, who is left grief stricken after her baby is stillborn, and she felt daunted by her first scenes, which depicted her character giving birth.

As "The Crown" star has never had children of her own, the actress contacted officials at London's Whittington Hospital and asked if she could sit in on a birthing experience - and was stunned when an expectant mother agreed.

"I walked in in my scrubs. We had asked permission and unbelievably she said yes. Not sure I would. Some random actress in there...," Vanessa remarked to The Sunday Times Culture magazine.

"Anyway, I sat next to her on the bed and vaguely waved. I was in awe. Her mother was there too. I hardly breathed for six hours."

Asked if she offered encouragement, Vanessa exclaimed, "No! Can you imagine? 'Go on, girl!' No, I was silent. It was a greater achievement than I've seen anybody do. There were forceps."

"At one point she looked at me. Halfway through a difficult contraction. I blew her a kiss. Why? So embarrassing."

Although she didn't know the family, Vanessa found the experience of watching the mum welcome her son very moving.

"When he was actually born, the nurses brought me round to see him come out. I was crying my eyes out," she recalled. "All colour came back to the mother. It was holy. And then I was introduced, and they went, 'Oh God, Princess Margaret!'"

Vanessa portrayed a young Princess Margaret in the first two seasons of Netflix's royal drama "The Crown".