Why is "quedarse embarazada" instead of "ponerse embarazada"

Question

I recently read "se queda embarazada", and I also found it on 20Minutos. However, apparently, being pregnant is not a permanent change. Is this considered a special case?

Answer

Hola Wenli
The cases that we state in the lesson are the most common context where each of these verbs are used. On top of those there are other examples that don't quite fit exactly in those categories. There are also set phrases that are used with that specific verb and not others. Quedarse embarazada is a set phrase that, despite not referring to a permanent thing, it always uses quedarse. Quedarse sometimes also implies an action that is seen as a result, which is this case: there was an action taken and we see this as a result. We all know what that action refers to : ) 
If you think of those examples that we included in the block for permanent things with quedarse, you have the example of "quedarse calvo" - this indeeds shows something that is going to be permanent, but it also talks about something as a result of a person losing their hair gradually (we can see this action as the action that takes place before the result).
I hope this clarifies it.
Saludos cordiales