To date, the earliest reported birth where a child survived (and survived relatively unimpaired) is that of Kane Benoit who was born after 16 weeks gestation. The baby, whose twin sister died within hours of birth, weighed less than 500 grams. A reporter noted the despair that descended on his parents because of the enormous medical costs incurred in saving Kane.
I googled it to no avail. As the earliest birth I'd heard of a live birth is 21 weeks, I wondered about this.
I would have liked to have investigated the matter further, but I only had a limited amount of time to read this book at the library.