Because its come up recently in discussions and I’ve ranted a little about it in a comment too.
Of course you can fly in a pregnancy. It wasn’t recommended in the first and the last trimester, and it still isn’t, but unless you’re in a high risk pregnancy, no ob/gyn will likely strongly discourage it.
This change in attitude is mainly due to a change in diagnostics. We know today, that most pregnancies that self-terminate do so because there is a structural or numerical chromosomal aberration in the embryo, as only very few of them are viable.
However, what we know about flying holds true for everyone, pregnant or not:
Clotting is a major issue and you can clot even in the First Class or Business Class. Pregnant women are predisposed to clotting anyway and the cramped room of a plane doesn’t make it better (mind, not all pregnant women, but the older you get, the more concerned doctors become about it and that goes even for fit women, it’s just such a strong strain on the body).
Radiation, which the plane hull is supposed to shield you from but especially the older models do it insufficiently.
Toxic fumes, only recently discovered, you obviously breathe in, depending on the model of the plane. And no one can tell you if those toxins will be able to cross over to the baby.
So if I was pregnant, would I fly, independent of what my ob/gyn said?
If I absolutely had to, yes. But I’d avoid long hauls if I anyhow could, because after 24 hour on a plane… I think after the way to and fro, I’d always think you could find that baby in my belly with a Geiger counter. Generally though, I’d try my best to avoid it. I’m a strong advocate of the “I’m pregnant, not sick” way to go about it, but there’s risks worth taking, and avoidable risks.