The findings of studies like this can be skewed, because it's limited to a biased population. For example, they are not surveying all mothers who gave birth, but rather only mothers who participated in specific government programs or those who chose to participate. Also, some counties and states choose not report data, and where you live matters for a numbers of reasons. So, even though there may be broad participation across income, age, ethnic, and geographic groups; it still can be biased.
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho