Fashion Designer Says Dress Shrinks Waistlines

Shani Grosz's line of dresses features shapewear sewn right into the garment.
5:07 | 02/05/13

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Transcript for Fashion Designer Says Dress Shrinks Waistlines
We know that ladies like to look fit, too. What if you could look like you lost ten pounds just by getting dressed? That's the beauty, apparently, of the miracle dress. Just by putting it on, they claim women go down one dress size. We'll put it to the test, live, in just a moment. First, here's abc's paula faris with more. Reporter: When christy looks in the mirror, she still sees the weight she put on when she was pregnant with her infant son, wes. This would be the area that I'm not sure about now. Reporter: Priscilla says she sees a tummy that's not flat enough. And julie says she wishes she had a smaller bottom half. I'm pear-shaped. I'm always trying to hide that area. Reporter: The solution to all these women's body image worries isn't this. Or even this. Believe it or not, it's this. What she calls a miracle dress. Our dresses are designed to make a woman look about ten pounds thinner, one size smaller. Shani is a fashion designer, whose new line of dresses features shapewear sewn right in the dresses. No lumps. No bumps. Reporter: The dresses are selling like hotcakes. That doesn't surprise this stylist. As long as women want to look flawless, this trend is not going anywhere. Reporter: But can a dress really make a woman look and feel ten pounds lighter and one size smaller? We decided to find out. With the help of six women, sizes 2 to 16. Frock number one, a royal blue dress from shani. The women agree, it eliminated muffin top. I feel in it. It smooths things out. I have no back roll. No back fat. Reporter: The dress actually saved inches. Before donning the frock, her hips -- how about that? She lost two inches in both areas. You look amazing. Frock number two, a versatile number by sassybax, the figure-forming dress that retails for $76, can be worn by a tube dress or a skirt. We're not sure if the women were scared or impressed by the little black dress. My husband would like this. Reporter: But the proof of the power was in the measuring tape. This time, lauren was just 38 inches in the hips, three inches slimmer than without the dress on. I feel like I'm probably back IN MY 20s. Reporter: As for the opinions of our ladies whether the effects of the so-called weight loss dresses are for real, we'll let them tell you. It's a miracle. Reporter: For "good morning america," paula faris, abc news, new york. Joining us now is lori bergamotto, contributing style editor of "lucky" magazine. We know this started with celebrities. Kate winslet, perhaps, is credited with starting this dress. She kick-started the trend of THE stella McCartney miracle dress. You can get this look. And you can, like everything, a few inches. It kind of looks like it acts like a suction cup. It does. That's accurate. You're looking for materials that really keep you nice and stay put. Almost like a girdle effect. And look for creative color blocking. Black panels on the side, no matter what the color is down the middle, creates an optical illusion. You found two lucky audience members to see if they wanted to lose a dress size and a few inches. We have marlene from iowa. She originally had a 44-inch waist and 43-inch hips. Let's do the work here. Marlene, if you don't mind. We're going to do her waist. 34 we started with, right? She's at 31. Wow. At her waist. Three inches. Let's do her hips. She started at 43 at her hips, correct? Now, she's down to 41. So, three inches. That's nothing to sneeze at. She looks fantastic. Marlene, thank you very much. Thank you, marlene. Next, we have briana. She's from rochester. Both of these women look beautiful in these dresses. I like how youd it with the jacket. Throw a little blazer. She had a 36-inch waist. And 46-inch hips. She lost three inches on her waist and three inches on her waist. She is down to 33 and 43. If this really has that big of an effect, are we going to see shapewear like this? Many my closet, yes. Designers are hip to this idea. And consumers want it. Why not lose a few inches and look great while you're doing it. Lori bergamotto, thank you so much. We have more information on our website, goodmorningamerica.Com on yahoo! We turn, now, to prince

This transcript has been automatically generated and may not be 100% accurate.

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