A mom used this extreme diet to shed 30 pounds and look like a model 5 months after giving birth

Gabrielle Rein, 31, gave birth to her first child late last year. Then, she looked like most very-pregnant people -happy, with a rather large belly. She's the woman on the left in the photo below:

Five months later, Rein's body was completely transformed and she shed 30 pounds. Her motivator: a nude photoshoot she was asked to do for work.

Rein is the creative director of Viceroy Creative, an advertising agency that decided to rebrand itself in a powerful way last March. As part of the rebranding, it asked a few of its executives, including Rein, to pose naked for a marketing campaign. The photoshoot was meant to resemble a buzzy nude partnership announcement the firm Sagmeister Walsh made in 2012.

This is one of the photos from the Viceroy's nude rebranding shoot (Rein is the woman lying on her back):

Here's how she slimmed down from baby body to bikini ready in just a few months.

Start getting back in shape while you're still pregnant

Rein's transformative journey began while she was pregnant.

"What really starts to set the tone is actually exercising during the pregnancy," Rein tells Business Insider. "That's what sets you in the right direction. So, for me, I was able to maintain a very regular exercise regiment all the way up until three or four days before I delivered."

Rein's routine involved working out Monday through Saturday every week up until the last week before delivery. As her pregnancy progressed, she began to feel more tired and had to alter the types of exercises she did to accommodate for the baby bump.

Here was her workout schedule for each of her three trimesters:

• 1st trimester: For the first few months of pregnancy, Rein says her workout routine didn't change. Monday through Friday she would complete one hour of combined cardio (StairMaster, treadmill, stationary bike, and elliptical) and weight lifting (the basics, as she puts it, for legs and arms using the machines at her gym). Then, on Saturdays she completed a one-hour-long, total-body workout through a Bar Method class (read more about these types of classes here.)
• 2nd trimester: The next few months, Rein says she felt more tired and her belly grew noticably. So she exercised for half an hour to an hour - depending on how she felt - Monday through Friday. And during the Bar Method class, she did modified versions of the exercises that the instructors provided her.
• 3rd trimester: Things got a lot tougher during the final months because Rein's larger belly hindered her balance, making exercise more difficult. Her exercises, which lasted for about 30 minutes, involved limited weight lifting and mostly low-impact cardio like walking on the treadmill and stretching.

She even managed to avoid the swollen-foot syndrome notorious among pregnant women. What actually happens, Rein explained, is that the arch of pregnant women's feet flattens under their weight, which then makes it impossible for them to fit in their shoes and also harder to exercise. To avoid this:

"I exercised my arches daily by ... basically just going from flat foot position to tippy toe and back to flat, 40 times in a row," she told Business Insider. "That will keep your arches high and keep you from getting that flat-foot expansion."

From a baby bump to a nude photoshoot

Before she was pregnant, Rein weighed 120 pounds. (She's in her early 30s and is five feet, five inches tall, so 120 pounds is considered a healthy weight according to the National Heart, Lung, and Blood Institute.)

The pregnancy got her up to 155 pounds, but right after giving birth, she lost a lot of water weight along with the weight from the baby and was back down to 135 pounds.

To prepare for the photoshoot, she trained, along with the other two naked executives in the photo: account manager Raegan Gillette and president David Moritz, for five months - a healthy amount of time to shed those post-baby pounds. Each of them finished by following an extreme diet the last 30 days. By the day of the shoot, Rein weighed 125 pounds.

More impressive, when Rein agreed to the photoshoot, she had 28% body fat - a normal amount for recent moms. By the time she was posing for the cameras, she had just 18% and she gained about six pounds of muscle.

Juggling work, training, and a new born was incredibly difficult for Rein, but she said that on the days she felt extremely tired, her trainer gave her lighter exercises.

To look like Rein, you have to exercise constantly and watch what you eat

While the intense exercise plan she followed with Gillette and Moritz - helped her regain the shape of her body, especially her stretched stomach, she says the 30-day diet is what really made a difference.

Here's her completing one of the exercises of the training plan:

"What exercise will do is change your body shape, but a diet will change the chemistry of your body. A diet will actually make you healthier from the inside which is very important," she says.

Diet is really the number one thing that's going to change you.

For the last 30 days, Rein and her colleagues worked out every day for an hour and a half, seven days a week with the help of professional trainers at their local Equinox gym. Rein's diet was constricted to 1,300 calories over the course of six meals, which consisted of the following:

• Meal 1: 1/2 cup of a rice-based cereal with 1/2 cup almond milk and 1/2 cup blueberries and one scoop carb-free protein shake
• Meal 2: 3 egg whites
• Meal 3: 3 oz. ground turkey, low-carb wrap with a cup of romaine lettuce
• Meal 4: 3 oz. grilled chicken and 1/4 of an avocado
• Meal 5: 6 oz. fish with a 1/4 cup steamed jasmine rice and six pieces of asparagus
• Meal 6: 6 oz. of 99% lean ground beef with 1 cup romaine lettuce
• No alcohol was allowed and most condiments were banned (with the exception of hot sauce, since it added a negligible amount of extra sugar or fat)

Rein also took iron supplements (the pregnancy significantly depleted her body's supply and made her border-line anemic) and other vitamin supplements for extra nutrients because she was nursing.

Rein's one piece of advice for women during their first few months as a mom: don't binge eat.

"A lot of women when they're nursing think that means that they can just eat very high-calorie things," she tells Business Insider. "Some people really go for it and indulge, but indulging all the time is what gets you into trouble. It's ok to indulge every once and a while."

How to totally transform your body on the inside and out

After the training, diet, and photoshoot were done, Rein didn't stop exercising and continued to watch what she ate. In fact, the diet completely changed the way she eats today. For example, she used to eat high-protein foods, like meat, for one meal a day - usually dinner - but now she eats protein for every meal.

"I still eat healthy, and I'm always aware of how much protein I'm eating and try to make sure I'm having enough," she says. "I try to avoid dairy and sugar."

By following her new diet plan - she says she consumes about 2,000 calories each day, although she doesn't track them anymore - she lost another 10 pounds. Rein now weighs 115 and her body fat is even less than it was during the photoshoot. She's down to 12%.

During a recent blood test, Rein's cholesterol was also down and her iron levels were back to normal. "I had the best blood test I've had since I was 25," she says.

Doctors know best

Although Rein is in even better shape now than before she was pregnant, she stresses that each woman is different and should not expect the exact same results as her over the same amount of time.

"Everybody's body and pregnancy is different and just because one person can do doesn't mean that if you can't do it you shouldn't' feel bad about yourself," Rein says and then added encouragingly:

"But anyone can do something like this if they put their mind to it and they want to be diligent and disciplined, and if their doctor says it's ok."