How to shadow/black out photos?

I don't even know the correct terminology for what I'm wanting to do. My best friend is wanting me to take some maternity photos of her tomorrow and this is an idea I'm going with… (see links).
I don't know how to get this effect for the pictures, where everything else is blacked out, and her belly and body are shadowed.

I'm shooting with a Nikon D5100. Thanks!

http://www.coltonphotography.com/blog/2011/12/rob-danielle-dc-maternity-portraits/

http://www.louisedieselphotography.co.za/photography/black-white-photography-2

Not going to happen without some sort of lighting away from the camera. You get a strong light source from one side and you meter on the light side of the subject. That's a very basic explanation of something that can easily take a day's workshop to get the hang of

I wouldn't say not going to happen without strobes, but difficult without strobes. The irony is you need the experience of doing with them to be able to do it without.

What you can try is find a sunny window.

This one was done with just a sunny window in the morning.

This one was done with all the fancy lights.

Go get yourself a strong light work light at HomeDepot. If you are shooting in color make sure you balance for tungsten light. If you will end up in B&W it really doesn't matter. You'll need some dark fabric or dark paper as a background and a bit of distance from the person to the background. If you have a big enough room just put the model far away from the wall. Put the light just behind the person and look at it. Slowly move your light around until you like the shot. The light will be super contrasty and as long as no light hits the background it will go black. The lighting on those shots is very simple and easy to do.

Easy to get with manual flash and your camera in Manual Mode, it's difficult with window light or any continuous light as some light will fall onto your background, the best, deepest black is the absence of light.

You'll need a manual flash, off camera so some means of syncing it to the camera, I use cheap radio triggers (YN-602), a light stand (cheap), preferably a soft-box to make the shadows softer edged, keep the flash close to the subject for the same reason.

Set your camera to Manual mode and set to your cameras X speed, the fastest speed you can use and sync flash, usually in the 1/160th to 1/250th of a second. Set your aperture to a small value (high f number around f11 or even f16), this setting will kill most of the ambient light, if you were to take a picture at these settings it would be totally black.

Now turn on your flash and take a test shot, what you're looking for is the highlights properly, exposed with no burn out. If it's too bright either turn the flash power down and/or move the flash further away, if it's too dark the opposite.

Now as long as the flash to subject distance doesn't alter too much you can just concentrate on getting the 'moment' and the right relaxed pose. Note that the distance from the camera to the subject doesn't matter and you can move around and shoot away.

Make sure none of the light from the flash doesn't falls onto the background, a piece of card will do it. This is how I got this shot.

In a room that was lit by daylight, no need for a black backdrop.