Development of the external form of the embryo
In the early development of the embryo, a dorso-ventral differentiation first ensues in that the umbilical vesicle is positioned ventral and the amniotic cavity dorsal to the embryonic disk (stage 5a). Soon afterwards rostro-caudal differentiation takes place through the germinal streak (stage 6). The embryo is still bilaterally symmetric. The processes that lead to right-left determination are known, but not all of them in detail
With the inception of neurulation (stage 8), due to the enormous increase in volume of the brain, the head bends over forwards until the telencephalon (endbrain) lies on the umbilical cord (stage 14). With the formation of the cartilaginous spine, the embryo straightens itself out again towards the end of the embryonic period and is then bent forwards only slightly.
Development of the form and position of the fetus
In the fetal period large changes of the form no longer take place. It is a period of growth. With the increase in size, especially of the inner organs, and with the overall growth of the fetus it stretches itself out again and takes on its typical shape.
Normally, it positions itself so it is aligned with the longitudinal axis of the mother and, 96 % of the time, with its head downwards (head presentation). Probably, the pear-shaped form of the uterus is responsible for this, in that the head fits better into the narrower lower part than the lower extremities do.