John Gill Commentary Luke 2:6

"And it came to pass, while they were there, the days were fulfilled that she should be delivered." — Luke 2:6 (ASV)

And so it was, that while they were there
At Bethlehem, waiting to be called and enrolled in their turn;

the days were accomplished that she should be delivered;
her reckoning was up, the nine months of her going with child were ended, and her full time to bring forth was come.