I promised myself I would write something like this, having myself scoured the internet for "extremely low" stories while terrified and pregnant. So here goes, I hope this helps someone.
Tl;dr - It was a lab error. Baby is born and fine. I'm glad that I asked for a redraw of blood before proceeding to more invasive tests.
I got pregnant with my second child at age 42. I was pretty nervous due to previous losses (plus my age) so we went for the NIPT tests (with a scan) at 10 weeks. Everything seemed normal. Then had the NHS scan done at 12 weeks (at Wexham Park Hospital), again normal. I initially declined the NHS bloodwork screening, because of already having had the NIPT, but was persuaded to by the sonographer, because PAPP-A can "flag up concerns about the placenta". My bloods were taken by my midwife.
Some time afterwards I received a sombre phone call from another midwife saying that my blood tests had come back as abnormal. I told her not to worry, as the NIPT tests were normal, but she told me that my PAPP-A levels were very low, and that the fetal medicine team wanted to see me. She read me the results and I noted them down. It was only when I started googling that I realised that things were really worrying. PAPP-A levels below 0.1 are extremely rare, so there weren't actually many articles, but the research indicated a very high risk of either chromosomal abnormalities or severe placental issues. A level of 0.02 seemed basically unsurvivable and the literature mostly seemed to be about "preparing" parents for a bad outcome. I was absolutely devastated.
Feeling a bit desperate, I started looking for reasons why the test might be wrong. And it turns out that there are, in fact, a couple of possibilities. One is that you have some kind of rare antibodies that screw up the test. This can be tested by re-running the test on a different machine. The other is that a mistake was made during the blood draw. In particular, the right tubes must be used for different tests, and this is indicated by the colour of the lid. If you use the wrong colour tube it can produce an extremely (falsely) low result. This is why phlebotomists are trained to draw blood in a particular colour-order, to avoid contamination between tubes.
Both of these issues are uncommon, but then so is getting an extremely abnormal blood test result. I'd had quite a few other tests done at the same time as my screening draw, so this seemed like a possibility worth investigating. In fact, one of the other tests I'd had drawn at the same time was in the "wrong" colour tube, i.e. one that would affect PAPP-A results. I texted my midwife and asked whether she was certain that she'd done the tubes in the right colour-order. She texted back and told me she'd never even heard about order of tubes.
I relayed this to the fetal medicine team at my appointment, and although the consultant was sympathetic to my request for a further blood draw, this was denied by the management, who seemed completely indifferent. I was told that if I wanted to know more I would have to have an amniocentesis. There are significant risks associated with this, and I really didn't want to go ahead unless i was certain that there was a good reason.
As Wexham were being unhelpful, I decided to contact the Wolfson Institute Antenatal Screening service (at Queen Mary University of London). They were lovely, and thought my results were weird enough to be of interest from a research perspective. They invited me to come for a redraw (free of charge), and called me on the way home to tell me that according to their tests my PAPPA-A results were completely within the normal range. They also conducted a second-trimester screening blood test (quad test) and this was normal too.
I called Wexham the next day to tell them the good news, but they refused to accept the Wolfson results, saying that it was "impossible to tell" which hospital's results were correct. I also got a letter from Wexham's lab manager, saying that they were "satisfied" that everything was fine with their own procedures (although no justification was given for this assertion).
In spite of all the drama, happily, my baby was born at 39 weeks, apparently healthy and weighing on the 35th centile. No issues were detected with the placenta or chromosomal abnormalities, and he is now walking and talking, and is a total cutey.
I don't want to falsely raise anyone's hopes with this story: if there are other signs that something is amiss - other abnormal bloodwork or abnormal scans - then there may be a genuine problem. And I should also emphasise that lab errors etc are much more likely to apply if the test result is quite extreme. However, I'm basically certain that my first test result was wrong, and if I was ever in this situation again, I would definitely demand a redraw, ideally with a different lab.
I hope that my story has been useful to someone. If you are going through something similar, the Wolfson centre are a fantastically useful resource for anyone who is navigating the prenatal testing process. I don't want them to be bombarded with callers, and I don't wish to imply that they do free prenatal testing as a matter of course. But I can say that they were kind, knowledgeable and patient, and genuinely interested in helping. A big thank you to them (and a big fuck you to Wexham Park!)