"They're saying the series of co-incidences are too high for them to believe this genuinely, medically happens often "
No, they aren't. Several posters have said that they flat-out don't believe that it's possible at all.
I don't know anyone in RL this has happened to. But I know two women who didn't suspect that they were pregnant until they were six months, two who had no identifiable bump and one who didn't feel any significant movement. Extrapolating from that I think genuine cases must happen - not very frequently, but probably there are a few cases each year given how many babies are being born.
DD2 has a genetic condition that only arises spontaneously (i.e. not inherited from either parent) in one in 400,000 births. It wasn't at all likely she'd have it. In the year she was born there would statistically probably only be her and maybe one other child born in England and Wales who had this genetic condition but no family history of it. And yet here she is.
Weird shit does happen. Babies get unlikely genetic mutations. Women get unlikely combinations of absence of pregnancy symptoms.
The most obvious explanation of an "I didn't know I was pregnant" story is that the woman was just in denial. But then the most obvious explanation of DD2's condition is that DH isn't her father. "Most obvious" explanations aren't always true. And it's incredibly offensive to insinuate that they are just because that's what seems most likely to you.
If a woman was in denial about pregnancy and you just nod and accept her story that she genuinely didn't know, what harm is done? If she genuinely didn't know and you start in with chinny reckon comments to her face and gossiping behind her back then a lot of harm is done.