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hi Stefanthat's a good thought, but flawed for the following reason. Human foetal mean arterial blood pressure is about 60mmHg. The hydrostatic difference between a head up and head down position - in other words how much extra pressure is added by lowering the head will be about 60-100 mm of WATER (because blood is effectively water); therefore arterial perfusion pressure is not significantly affected by the position of the head. Compare this with a giraffe, however, where the extremely long neck adds thousands of millimetres of water pressure and you're talking a different story entirely.The other point to bear in mind with a human baby is that the mother is often recumbent (lying down) and hence a developing foetus could not always rely on this gravity assist effect. If it were therefore this critical to neurological perfusion then women who are on their feet more should have brainier babies, whilst women who give birth to breech (feet-first) babies should have less cerebral children. This doesn't seem to be the case and although breech delivery is associated with more problems around the time of delivery, in the long term there is no developmental or health impact; at least as far as I am aware.Therefore, I stick to my original conclusion that the foetus takes up the most space-efficient position, which later in pregnancy in a normal uterus means head first downwards.Chris