I think you could miss it.
I'm assuming you mean a
very early miscarriage? As in, around the time AF is due or just a few days later? Maybe before you'd want to test?
If your temps go up ... and up ... and your chart is triphasic, then you'll think you might be pregnant.
But not all "triphasic" charts are pregnancy charts and not all pregnancy charts are triphasic. After the uterine lining starts shedding your temp drops and you get your period. Well, I bet it would look the same if an embryo detached, the lining started to shed, and then AF started. The same dip and all. It might just look like a long luteal phase (or if it's really early, maybe even a normal luteal phase).
So let's say your chart looks triphasic. You buy a test for the next day. Your temp drops that morning, the test is negative, and the next day AF starts. How do you know whether it's a normal period or an early miscarriage? There could have been an embryo there and maybe it only produced a very little hCG before it miscarried. It wouldn't show up positive on the test. Or maybe there wasn't an embryo and it's just AF. There's really no way to know.
It's possible that 50% of pregnancies miscarry before implantation is fully finished anyway, and most of those women have
no idea they've had a miscarriage or that they were ever pregnant. I will have to find it but I read somewhere that in fertile couples, fertilization actually occurs in 60% of their cycles. Now you know those women don't get pregnant 60% of all their unprotected cycles. Either the embryo doesn't implant, doesn't implant properly and is miscarried, or implants but miscarries a few days/weeks later before the woman is aware of the pregnancy.
So I would say with a chromosomal abnormality
and PCOS, you still aren't that much worse off than most of the rest of us. Any of us could miscarry without knowing it, during any cycle. Probably almost as often as you might and possibly even for the same or a similar reason (genetic abnormalities aren't always hereditary ... some are just random mutations).
Don't lose hope. You have one child (a beautiful one, btw!) so obviously it
can happen for you. You are not totally sterile by any means.