"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."

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Pregnant pre-humans appeared to have stood the same way. And it may save women from even more back pain than they already have, the researchers report in this week's issue of the journal Nature.

The bodies of women do two things when they are pregnant -- they adjust their stance to move the center of gravity to accommodate the growing fetus, and the lower vertebrae have evolved a distinct shape to allow this shifting to take place without damaging the spine, Katherine Whitcome of Harvard University and colleagues found.

"It was one of these things like, 'Oh my god, no one's ever thought of this,' and it looks so obvious," Liza Shapiro of the University of Texas at Austin, who helped supervise the work, said in a telephone interview.

Whitcome and Shapiro followed 19 women through their pregnancy, using digital cameras and motional analysis equipment to map the changes in stance and movement as the months passed.

"What women do when their pregnancy reaches about half of the expected mass ... they shift backwards," Shapiro said.