8 mm long and 10 mm long embryos with spina bifida
To get an idea how long that is, a pinhead is appx 1 mm across. One of those ubiquitous tiny red ants is 5 mm long, on average.
You can see here that the neural tube is created VERY early on pregnancy, which is why folic acid (critical to the cell signaling which closes the neural groove) is recommended for all women of childbearing age - this stage of pregnancy is often before the woman even knows she’s pregnant, and if the pregnancy is carried to term, the lack of a single vitamin can lead to lifelong ramifications for the offspring.
For those who are into embryology/developmental biology: You can see the dorsal differences between Carnegie stages 15 and 16 if you look at these two embryos. The caudal neural pore has not yet attempted to close at all in the smaller of the two, and there are also only 3 pharyngeal arches clearly visible. In the larger of the embryos, a telling factor of which stage it’s in is that you can clearly differentiate the thigh, leg, and foot parts of the hind limb buds.
A Study of the Causes Underlying the Origin of Human Monsters. Franklin P. Mall, 1908.