It's a serious comic about miscarriage that everyone thought was hilarious due to its contrast with the usual funny series.
As someone who's been through the loss comic in real life, multiple times, it's pretty fucked everyone uses it as a meme. It's a miserable, soul crushing experience I wouldn't wish on anyone. And that's from the perspective of the father. I can't begin to imagine the emotional pain my wife was in.
The first part of the joke is the type that people like me will never understand without someone explaining. I can tell it's mitosis but I can't tell that's what fills the gap. It just never comes to me on my own.