This open neural tube defect occurs very early in pregnancy, like all deformities and malformations of the neural tube.
I won’t get into the (actually very interesting, if complicated) specifics of the notochord and neural plates and the gene signaling going on, but around the third week of development a groove forms down the back of the embryo. This is called the neural groove, and it will form the canal that the spinal cord and spine will develop within. One of the most critical phases of development is the closure of that grove to form a tube, and much can go wrong.
Though an embryo may have such a malformation that they’re unable to sustain life independently, the genes and cells in other parts of the body don’t know that, and since they continue to receive blood and nutriment from the mother, they continue doing their thing and forming other structures. That’s how such a catastrophic defect from so early on can end up at this stage - fully formed except for the critical neural regions.
Manual of Pathology. W. M. Late Coplin, 1905.
I sourced this image from wikipedia. It shows strong signs of a terminal lung disease in a patient who was taking medicine for a really screwed up heart rhythm. The medicine has a side effect of causing the disease, but apparently the heart deal is so deadly that it’s worth the risk. This CT was the first evidence of this person, who has had this terrible heart affliction so intensely and for so long that they are required to wear fucking dogtags to signify it, has finally succumbed to the very thing they were dreading their whole, sickly life. Also I’m pretty sure this is from a kid.
That. Is fucked up.