Some of them may be, but I think this is honestly a reflection of the marriage trend. People are waiting longer to marry, sometimes until after the big 3-0, because they are focused on their education and becoming established in their chosen careers before taking that leap.
Not every mother chooses to be married, of course, but most are concerned with being financially stable, and ready to tackle the responsibilities of raising a child. Happily, that’s not happening so much at age 15 anymore. = )