If you are a lady, and you think you might be pregnant, you can take an at-home test to find out. You simply pee on a stick. Whether the results are measured in pink lines, blue lines, plus and minus symbols, or a "pregnant"/"not pregnant" digital readout, all the home pregnancy tests on the market are really looking for the same thing — Human chorionic gonadotropin (HCG).

HCG is a pregnancy hormone. It's produced by the placenta, a temporary organ that only forms in female bodies when an embryo has attached to the uterine lining. And so it was kind of weird when a male friend of a Reddit user known as CappnPoopDeck peed on a home pregnancy test and it came back positive.

Turns out, HCG can show up in men, too. And when it does, bad things are happening. You might have seen this story on Gawker earlier this week, but the science behind it is so crazy that I wanted to discuss it in a little more depth.

First, let's talk about HGC in ladies. The specific cells that produce HCG are called trophoblasts. At the point where a human is quite literally nothing more than a ball of 70-100 cells, trophoblasts form the outer layer. The mochi to our ice cream center, if you will. These cells eventually grow into the placenta, but at the earliest stages of pregnancy, their primary function is to rip through flesh and secure the embryo to uterine wall. Seriously.

I'm going to quote from Jon Cohen's excellent book on miscarriage, Coming to Term, because it has heavily influenced the mental pictures I see when I think "trophoblast".

Successful implantation occurs only if the embryo properly "invades" the lining of the uterus, which requires that the placenta's trophoblast cells penetrate deeply, boring into maternal arteries to shunt blood to the growing baby. If the mother did not stop the trophoblasts' drilling, they might burrow so deeply into her uterus that she could hemorrhage and die. [He goes on to explain how the female body prevents this. Although he notes that, sometimes, it doesn't work.]

Fun!

The outermost cells in a layer of trophoblasts are called syncytiotrophoblasts. These are the cells that actually produce HCG, which plays a huge role in making sure that you actually miss that first missed period. The chain of command works like this: When you ovulate, you release an egg from a follicle on your ovary. But that follicle doesn't just instantly disappear afterwards. Instead, it hangs out, producing progesterone — a chemical that is instrumental in preventing you from shedding your uterine lining and having a period.

On a normal, non-pregnant cycle, progesterone levels will go up for a bit, but then the spent follicle — the corpus luteum — will wither away. Your progesterone levels will drop. And you'll ride the crimson wave.

But, if you're pregnant, the corpus luteum doesn't go away. It keeps on producing progesterone, and no period happens. What keeps the corpus luteum alive and kicking? Human chorionic gonadotropin, that's what.

So why on Earth would HCG end up in a man?

When that pregnancy test came back positive, CappnPoopDeck made a rage comic about it, and posted it to Reddit. The very first response, from a user named goxilo, was this: "If this is true, you should check yourself for testicular cancer. Seriously. Google it."

Yes, HCG in men can be a sign of a rare (and dangerous) form of testicular cancer — choriocarcinoma. This is a cancer made up of syncytiotroblastic cells, said Katherine McGlynn, a senior investigator with the National Cancer Institutes. The tumor secretes HCG because that's what syncytiotroblasts do. They secrete HCG. And they don't particularly care whether they're secreting it into a man or a woman.

But how do they get into a guy, to begin with? That's where things get really weird. The truth is that nobody is entirely certain, McGlynn told me. But there are a couple of theories. One possibility is that these syncytiotroblasts that turn cancerous were leftovers — remnants of the time when that guy was just a ball of 70-100 cells. One way or another, they persisted in his body and then started to grow out of control.

The other theory: Somehow, normal cells in the man's testes just start regressing, reverting to one of the earliest forms of cells in a human's life cycle. Either way, one thing is certain, "It's exactly the same cell as in the placenta," McGlynn said.

The bad news: Choriocarcinomas move really fast. They're more common in men under 30 and the prognosis is usually bad, because most of the time nobody catches them until they've already spread to other parts of the body, especially the lungs. In that, CappnPoopDeck's friend is incredibly lucky. Both that he decided to pee on a stick for LULZ and that his friend posted the news to Reddit. In a follow-up, CappnPoopDeck reported that doctors found a very small tumor in his friend's right testicle. But they found it early enough that it's going to make treatment much easier.

The good news: Men should know that their chances of developing a choriocarcinoma are extremely rare. A 2002 paper in the Canadian Journal of Emergency Medicine reported that only about 2 men in 100,000 will get any kind of testicular cancer. Pure choriocarcinomas — the dangerous kind that I'm talking about here — make up less than 1% of those diagnoses. It's not clear whether CappnPoopDeck's friend has a pure choriocarcinoma, or a much-less-deadly form of testicular cancer that happens to incorporate some syncytiotrophoblasts.

Either way, McGlynn wanted to make it clear that you all shouldn't feel like you need to run out and stock up on home pregnancy tests. In fact, at least twice during our interview, she marveled at how amazing it was that this story even happened. "The actual odds of the man having this particular cancer, and then using a pregnancy test, are sky high. It's kind of amazing that this happened," she said.

Read More:



• Read the Gawker story about this case



• Read the original Reddit thread



• Read a history of home pregnancy tests, which includes a lot of information about HCG



• Read a full research paper about a different case of testicular choriocarcinoma

Photo: Shutterstock