While I’m of the opinion that trans-generational epigenetic inheritance is not typically important in human biology, the most convincing argument I’ve heard is

While I’m of the opinion that trans-generational epigenetic inheritance is not typically important in human biology, the most convincing argument I’ve heard is that the egg cells that will become a grandchild are present and post-meiotic in the fetus of the mother, so exposures to the grandmother could have effects 2 generations down without requiring some relatively exotic pattern of inheritance.