The horrible condition in which a twin is relegated to "top" status in the mother's womb. Due to being on top, the fetus is relegated to only receiving "leftowver's" in terms of nutrients. In addition to lack of nutrients, the top baby receives all the negative substances the mother brings in (smoke, alcohol, drugs, etc.). As a result, the "top baby" will be physically smaller, and weaker, as well as substantially less intelligent, and more prone to learning disorders.
Danny DeVito suffered from Top Baby Syndrome in the movie Twins.