The stress of fearing a pandemic is perhaps quite different from the everyday stresses of work. We may find them both severe - even the fear much worse; but it's still conceivable the everyday stresses are worse specifically to the extent they influence birth weight. Perhaps everyday stress is more immediate; perhaps it's more physical; or perhaps it's more social - or perhaps we're grasping at straws and it's something else entirely.

In any case; just because a pandemic is stressful doesn't necessarily mean that stress that matters here isn't lower. All those various strains don't affect us in the same way.