Possible explanation here:

New Suite of Chemicals May Cause Disease Generations Later: Plastics and Jet Fuel Raise Incidence of 'Epigenetic Transgenerational Inheritance

Epigenetics in a nutshell:

Genes are a blueprint of how the body operates that are passed down from your parents to you. It isn't destiny that if you have a particular gene you'll have a disease, however if you have two copies of a certain gene you'll be much more likely to have that disease (or trait) and sometimes you are guaranteed to have it, it just depends. Genes are potential traits.

There's an environmental component to it too. On top of the genetic structure is another layer that's relatively newly discovered called the "epigenome". This influences the genome and activates or disables genes.

Over a lifetime, identical twins become less and less identical. At the gene level, they're still identical, but at the epigenetic level they begin to diverge at birth.

Nova has a wonderful program on epigetics

Anyway, apparently they are starting to see how epigenetics works and how the epigenome can be passed down to offspring from the parents.

So, the title I posted this as gives us a possible explanation for the obesity epidemic. Things which began becoming commonplace in the 1950's and after start to enter the environment. Mom's begin acquiring epigenetic changes. Those changes might not affect the Mom herself, but her offspring.