Births to unmarried mothers have risen sharply over the past generation, from about 21 percent of all births in the early 1980s to 43 percent during the 2009-2013 period. But as this issue brief from Wendy D. Manning, Susan L. Brown, and Bart Stykes at Bowling Green State University observes, the dynamics of this trend aren't quite what you think.
It's almost all about non-married people who are nonetheless part of cohabiting couples:
The large and growing share of children born into these families that are neither married nor single means you have to be careful with your demographic analysis. Marriage has traditionally been used as a proxy for things that plausibly matter for child well-being, but it's pretty clear that the growing absence of rings on parents fingers isn't actually the key issue. Research from Sweden, where raising children in non-married cohabiting relationships has been widespread for a long-time, shows that the level of actual family instability experienced by children of cohabitators varies enormously by education level of the adults involved.