Yes, I'd like to stick up for TMPOB too. DH is a doctor and I have Pure Science degree, and yes, the methidology is possible and valid. Sadly I think the big problem with this test is the fact that it's done at home and if the instructions are not followed properly, the sample can be contamimated with y chromosomes. They test the white part of the blotting paper as well as the blood, to check for contamination.
The test works by detecting the Y chromosome from the baby, some of it leaks into the mothers blood. If there is no Y chromosme, then it's a girl.
If you have a m/c or birth then male dna can stay in your blood for months (rarely, for years), and about 10pc of preg start as twins, and usually one dies before you know it was there, and you can get a "wrong" test result because of that.
Even given all of the above, this test is still more acurate than ultrasound at 12 - 16 weeks. The later an u/s is done, the more accurate it gets.
People who do get a wrong result tend to shout louder than people who get the right result, and that makes it appear that the test may be a con. It's not, it's real science.
I took the test with my last preg, not long before my m/c, and it was a comfort to us to get the result through a week later saying it was a girl. we are happy to go with that result, 100%. And I have been in touch with the lab and I have been told that since they confirmed the m/c was a girl, I can do TMPOB if I get preg again soon (normally you can't do the test for a few months incase it was a boy).
To prove this is real science, not a con, tests using this same technology (but more complex) are being developed right now, which detect chromosomal abnormalities in the baby, using a sample of the mothers blood. They take a much bigger blood sample and look at many chromosomes, (TMPOB only looks for the presence or absence of the Y chromosome) There has been a small set of test done in the US, and a bigger trial underway in Hong Kong. These new tests do not cause m/c or abnormatities in the baby, unlike amnio and CVS, but they are not yet available.