And it's changing quickly. Here's the same statistic from the last 30 years:

1988–1994 54.9

1999–2000 64.1

2001–2002 65.6

2003–2004 66.5

2005–2006 67.3

2007–2008 68.3

2009–2010 69.2

2011–2012 69.0

2013–2014 70.7

https://www.cdc.gov/nchs/data/hus/2016/053.pdf