Maybe it's the influence of skinny celebrities like Nicole Ritchie or Nicole Kidman, who seem to gain only the perfect "bump" while pregnant.

Or maybe it's the fear of having to shed post-pregnancy weight, but some mothers-to-be are embarking on dangerous diet and exercise programs to prevent too much weight gain during pregnancy.

FOXNews.com managing health editor Dr. Manny Alvarez said Tuesday he's "absolutely" had to tell some of his pregnant patients to eat more.

"We still see many more overweight patients than underweight, but it's a challenge," said Alvarez, chairman of the Department of Obstetrics, Gynecology and Reproductive Science at Hackensack University Medical Center in New Jersey. "We have to convince overweight people not to eat as much and underweight people to increase their caloric intake."

Women considered to be "normal and healthy" are recommended to gain between 20 and 25 pounds while pregnant, Alvarez said. The recommendation, however, may be modified depending on whether the woman is under- or overweight before becoming pregnant.

The trend also has been seen in Europe. In England, for example, some mothers-to-be are dubbed pregorexics as they diet and exercise to the extreme so they can wear tight-fitting maternity clothing made and sold by everyone from the Gap to high-end designers such as Diane von Furstenberg, the Daily Mail reported.

"It's vital women know that pregnancy is no time to be starving yourself," Pat O'Brien, a consultant obstetrician at University College Hospital and the Portland Hospital in London, told the Daily Mail. "During the nine months it is in the womb, the baby is growing faster than it ever will in later life."

Gaining too little weight during pregnancy is associated with poor fetal growth, lower birth weight and the chance of a baby's being born prematurely, according to a study released in May from RTI International-University of North Carolina at Chapel Hill Evidence-based Practice Center.

Alvarez said expectant mothers need to consume certain vitamins such as folic acid and vitamin B, as well as calcium to ensure the health of their unborn babies.

"What we expect is that, whenever possible, women should have a physical before they get pregnant — that way they can be evaluated for risk factors such as being overweight or underweight," he said. "Being overweight can lead to complications such as diabetes and being underweight can lead to problems such as low birth weight."

Alvarez said low folic acid levels can lead to birth defects and neural deficits such as spina bifida, a condition in which the newborn’s spinal cord isn’t completely formed. Babies who don't receive enough calcium in the womb may steal it from the mother, damaging the mother’s bones and teeth.

"We recommend to women that rather than going on a diet while pregnant, they should consult a registered dietician and have a plan tailored to fit their needs," Alvarez added.

Click here to read more on this trend from the Daily Mail.