Amazingly, a baby born to an obese surrogate mother with a skinny biological mom may harbor a greater risk of becoming obese than a baby from a big biological mom born to a slim surrogate. Identical twins don’t just share DNA; they also shared a uterus. Might that also help account for some of their metabolic similarities? Fetal overnutrition, evidenced by an abnormally large birth weight, seems to be a strong predictor of obesity in childhood and later in life. Could it be you are what your mom ate?
A dramatic illustration from the animal world is the cross breeding of Shetland ponies with massive draft horses. Either way, the offspring are half pony/half horse, but in the pony uterus they come out much smaller (thank heavens for the poor pony). This is presumably the same reason why the mule (donkey dad and mare) is larger than the hinny (stallion and donkey mom). The way you test this in people is to study the size of babies from surrogate mothers after in vitro fertilization….