It's widely believed that unwed parents are most receptive to marriage immediately after their baby's birth, a period that some refer to as the "magic moment."
"It turns out the 'magic moment' lasts longer than conventional wisdom has held. And for some subgroups, that moment lasts even longer," study author Christina Gibson-Davis, of Duke University, said in a university news release.
She analyzed data from more than 5,200 children in the United States who were born out of wedlock, and found that 64 percent of the children's mothers got married. Black mothers tended to get married later than mothers of other races or ethnicities, with most of them tying the knot when their child was older than 3.
Overall, nearly half of marriages where parents already had children end in divorce and the rate is even higher for black women, according to the study published online July 2 in the journal Demography.
"These marriages are fragile," Gibson-Davis said. "If you think that stable marriage is beneficial for kids, very few kids born out of wedlock are experiencing that."
Marriages are somewhat more likely to succeed if mothers marry their child's biological father. After 10 years, 38 percent of marriages involving biological fathers had failed, compared with 54 percent of marriages involving stepfathers.
The American Academy of Pediatrics discusses different types of families.