It's natural and healthy to gain weight while you're pregnant, but not everyone loses the weight as fast as celebrities with personal trainers. The fear of keeping pregnancy weight on is not baseless - research shows that 75 percent of new moms are heavier the year after giving birth, according to NewsMax.

Researchers followed 800 low-income women and found that one-third of women whose weight was in the normal range prior to pregnancy became overweight or obese the following year. Twenty-five percent kept on more than 20 pounds.

"This unfortunately showed that pregnancy itself is leading to obesity or [being] overweight for a substantial number of women," said study author and assistant clinical professor at the University of Chicago Dr. Loraine Endres, according to NewsMax.

"It's a very important issue," Endres said. "We all see the rising number of obese people in our country and the health consequences that come from that, such as diabetes and high blood pressure. I really wanted to see where this is starting for women and to see if there is any way to turn it around."

Thirty-five percent of women over the age of 20 are obese and another 34 percent are considered overweight, according to the U.S. Centers for Disease Control and Prevention (CDC).

"The biggest problem is that a large number of women gain too much during pregnancy," Endres said, according to NewsMax. "The more you gain, the harder it is to ever lose that weight. From the moment women conceive, as health care providers we need to start talking with them about appropriate weight gain and remaining active."

Pregnant women don't need to "eat for two" - 300 to 400 extra calories a day is sufficient when there is one bun in the oven, Endres said.

Endres noted that the low-income of study participants may also be a factor in weight retention. They may not have money to join a gym or a safe place in the neighborhood to walk, according to NewsMax.

Dr. Timothy Hickman, director of reproductive endocrinology and infertility at Houston Methodist Hospital in Texas, Endres' study finding are "exactly what I expected," but he also mentioned that exercise and breastfeeding can aid in post-partum weight loss.

"You're not predestined to have this happen," he said, according to NewsMax. "Anyone is at risk for [excessive] pregnancy weight gain if you don't have a specific plan in place. This [research] speaks to getting in the best possible shape before conceiving and developing a plan to lose the weight afterward."

Endres' study was published Dec. 8 in the online version of the Obstetrics & Gynecology journal and will be available in print in January 2015.