On the whole, most women are still getting married when they have kids. But that majority isn't anywhere near what it used to be; 59% of all mothers are married by the time they have their first born, meaning that two out of five of all mothers are unmarried. Seems like that sacred institution necessary for a child's well-being isn't as highly regarded as it used to be.
Some analysts cite financial reasons for the shift in attitude regarding marriage. It's easier to receive welfare as a single mother than as a married one, so many poorer couples may opt not to tie the knot. But while the welfare-chasing single mother is a common fantasy among the conservative right, government benefits do not appear to be the only reason so many women are choosing to remain single. The stigma regarding single motherhood has lessened dramatically over the years. It's no longer so uncommon to see a woman raising a child alone by choice. She often won't be shunned by her family and friends for her decision, especially if the father of the child is still in the picture. After all, a woman living with her boyfriend is still a single woman on paper, regardless of the kind of paternal support offered by her partner off the record.
The more educated a woman is, though, the more likely she is to get married before she has children. It would seem that marriage and the whole cohabitation process that accompanies it is more palatable to the upper classes these days. The more assets a couple has, the more benefits they reap from the legal perks of marriage. And couples that seek to own property together would certainly be aided by a legal union. But for those couples with no such financial goals? Marriage seems more like an afterthought than an immediate necessity. It's certainly becoming less and less of a social cornerstone as the years go on as the individual becomes the more important discrete unit of society in lieu of the nuclear family. You no longer need to have a traditional household to be respected, so fewer people seek to build one.