According to this article it was an NCMEC study but I'm still looking for that.
http://www.slate.com/id/2063086/
Quote from the article (bbm):
"But a study by the National Center for Missing and Exploited Children shows that fathers are far more violent. And mothers frequently dispose of the corpses in what researchers call a "womblike" fashion. Bodies are swaddled, submerged in water, or wrapped in plastic.
Moreover, the NCMEC study showed that while the victims of maternal killings are almost always found either in or close to the home, fathers will, on average, dispose of the bodies hundreds of miles away."
The question of whether stepmothers who kill their stepchildren do it in the same fashion as mothers is an interesting question. I would say for the most part, no they don't have the same bond. There's exceptions to everything though.