Why are girls with babies typically single?

I was wandering about this girl. Here's a little background: (it's not complete but has been simplified)One day I was at work and I met this girl. She was very nice and friendly. It seemed she was interested in me to some extent because she was looking at me when I wasnt looking, she was also kind of flirty she would laugh and talked to me often. So we bacame friends on facebook. I got another job and so did she. One day I noticed she was single and some time has passed since then but I didnt talk to her her for a while (our own ways). One day I'm on facebook and notice that she had a kid and was posting some picrure of her preganancy, the baby and everything. But one thing that was throwing me off was that there was no picture of the husband or even a hint of a husband through a marriage or something. its all photos of just her and her family. Adn when I checked her relationship status she is single. I know it can be a variety of reasons (he doesnt use facebook, etc). But what I'm trying to find out without putting any weird strain on our relationshp(as friends) that she has baby but yet has no picture of a marriage or husband but she looks happy but reamins single. What ususally causes these types of situations? (is it that the husband left her, dvicorce, etc) Usually I see photos of coupes with this significant other, etc. just curious of what you all thought of this.