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.

It could also be that they are sleeping more, which people will try and shoehorn into: "that's a form of stress!" Stress is a buzzword, I'm wary of manhandling the theory too much as counterpoints come up, especially if it involves playing with semantics.

Exactly; stress isn't one clearly defined thing, and the forms do matter: therefore the focus on overall stress isn't that useful.

It's worse than that: "it's stress" is a lexical tautology. By definition anything that causes a miscarriage was a stressor.