Please, please, please do not terrify a pregnant lady (particularly a first-time pregnant woman) with labor and birth horror stories. Even if it does work, even if she has a complete mental breakdown from fear, it's still inappropriate and dangerous behavior to try and scare someone into having a mental event or losing their mind from fear. There are many other ways that you can disturb or annoy a person without jumping out of nowhere. For example, you could run down the street with your hair flailing in the wind, play loud music, or throw rocks at their window. These things will definitely get their attention, but they would not be considered "scares" as we know them today. Here is how one website describes proper "jump scares": "Jumping scares are extremely popular in video games and movie special effects. They usually involve an actor being placed in a situation where they believe they are safe, then suddenly confronted by something horrifying (or funny). The key ingredient is surprise." So basically, what you want to do is make someone feel scared for no reason other than to cause a reaction. This might sound fun, but it can also be very dangerous if you do not know what you are doing.