The kooky star played deranged witch Bellatrix LeStrange in the hugely successful film series, and started work on the sixth movie in the franchise just 12 weeks after welcoming her daughter Nell.
But her madcap, high-energy scenes came too close to the birth, and Bonham Carter admits she had several accidents during filming - so she resorted to wearing a child's diaper to save herself from embarrassing drying-off sessions.
She tells Britain's Style magazine, "I really couldn't jump up and down and scream without losing most of my bodily liquid. They had these air dryers (on set), these huge things - that's where I would dry myself off.
"None of the men (knew I wore diapers). They don't know, you see. But all the girls were, like, 'How can you?' But, actually, I don't think I was too embarrassed. There's something quite liberating about having a baby... I loved having a baby. I would have a baby every day if (I could). The birth day is fantastic. It is magic. It is complete magic."