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Slower diffusion of oxygen from mother to baby across the placenta may dictates slower fetal growth.
Identical twins shared a placenta that is divided into two sections, oxygen may diffuse easily to one of the two and the fetus will be healthier and bigger than the other.
MRI showed that dysfunctional placentas have large regions with delayed diffusion of oxygen to the fetus.
Reserchers followed different sets of identical twins from conception to birth, monitoring pregnancies in which one twin was smaller than the other.
Reserchers measured how long it took oxygen to diffuse into the placenta, and duration it takes to pass through the umbilical cord into the fetus and pass through the brain and liver.
They discovered that the longer time it takes for the oxygen to reach the placenta; the smaller the liver, brain and newborn birth weights.
If the oxygen gets to the placenta sections at the same time, the twins will have the same birth weight.