A major theme is the struggle to adapt. They suddenly have to change their lifestyles, whether it's their diet, their sleeping habits, or their daily activities. Also, relationship dynamics play a big part. If they're in a relationship, it can either strengthen or strain the relationship. In some fanfictions, the couple becomes closer as they face this new situation together, while in others, it causes a rift. And in stories where the character is single, it's about her finding support and dealing with the stigma or judgment from society. There are also themes of hope and new beginnings as they look forward to the arrival of the baby.
One common theme is self - discovery. The characters often have to re - evaluate who they are when they find out they're pregnant. Another theme is the element of surprise. It's a shock to them and to those around them. For instance, in many stories, the friends and family are as stunned as the pregnant character.