Remember where you learned this? I'd be interested to learn more.
http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5127a1.htm#tab1
Here is another quirk of the measurement system I just discovered while googling. Low birthweight rates increase with income (at least in Brazil):
http://www3.interscience.wiley.com/journal/118696841/abstrac...
Remember where you learned this? I'd be interested to learn more.