It's a choice, so technically it depends on their decision. It happens, but yet, sometimes they end up splitting, or staying a couple, but not yet marrying.
I have several different friends who are all different examples of what can happen.
1. Lets call this friend Jillian. A few years ago, she and her boyfriend ended up having an accidental child. They decided to keep the child, and stayed together a few months afterwards. The guy ended up leaving her, and she is now a single mother.
2. Lets call this friend Hannah. Her and her boyfriend have been together for several years now, and just recently I found out that she is pregnant. They are keeping the child, and getting married soon.
Hope this helps! :)
