More U.S. Couples Living Together Vs. Marrying
And more women getting pregnant while cohabiting
It's not clear what effect these trends may have on the health of families, women and children. Previous research has shown that people who are living together -- married or not -- tend to be healthier both physically and mentally, Copen said.
Children tend to be happier and healthier the more stable their parents' union is, regardless of whether the "union" has been formalized or not, she added.
This study did not look at how long couples stayed together beyond three years.
The Nemours Foundation has tips for a healthy relationship.