Why is one twin smaller than the other? Answer could lie in the placenta MRI study finds differences in prenatal oxygen transport from mother to baby  When a baby is born small, it&#x27;s often attributed to genetic factors or maternal risk factors like poor nutrition or smoking. But a twin study now finds that slower transport of oxygen from mother to baby across the placenta predicts slower fetal growth, as well as a smaller brain and liver.  This is the first study to make a direct connection…