If Quinn was even a month behind, it'd imply the baby would be massively overdue. Eventually she hits upon the concept of adopting Quinn's infant, when it truly is possible that everyone in the Glee club would be close to the start, and it'd be damn around impossible to disguise the point she now has Quinn's infant. An even smarter answer would be to fake a miscarriage with her medical doctor, then provide to undertake Quinn's toddler adequately.