...at least for first time moms of newborns who are less accustomed to caring for babies (those of us who haven't changed a nappy in a long time...or ever).
If the baby is calmly tolerating the nappy change instead of screaming her head off- just stop and do it later. Don't assume that the series of farts that caused her whole body to shake mean that the nappy is now full and she is done. Because she aint. And when you get those legs up to wipe that bum, well...you've got a perfect fully loaded poo cannon just waiting to go off. And it will. Several times over as you frantically try to minimize the disaster zone.
So...just keep the poo-y nappy on and make yourself a cup of coffee and wait it out. When the screaming starts, the poos have probably stopped. Probably.