Oksana Grigorieva welcomed the couple's first child, Lucia, on 30 October (09) in Los Angeles, even though she wasn't due to give birth until December (09).
The singer/songwriter admits the timing took them both by surprise - because Gibson was filming in New York and she was home alone when she went into labour.
Grigorieva tells Britain's Hello magazine, "She was almost a month early. I was at home and then I got rushed to hospital, and then eight hours later, I could look at her beautiful face. It was fairly straightforward, as births go. It happened very quickly.
"He (Gibson) came a few hours later because he had to fly from New York... He is ecstatic. He is in love with her already. He's very hands-on. He has been very doting and nurturing."
Lucia is Gibson's eighth child - he has six sons and a daughter with estranged wife Robyn.
Grigorieva, 39, also has a son, Alexander, with her former boyfriend, actor Timothy Dalton.