On Feb. 3, a 13-pound boy was born to Amanda Byron and fiancé Eric Rozzi in a Massachusetts hospital. At almost twice the weight of an average newborn, Jonathan Rozzi was the largest baby his doctor had ever delivered naturally.

As big as he was, Jonathan was barely more than half the weight of the heaviest newborn on record. That title goes to a short-lived 23.12-pound boy who was born in 1879 to two circus performers who suffered from gigantism, Anna Bates, who was 7-foot-6, and Martin Van Buren Bates, who was 7-foot-9. Sadly, the baby, whose feet were fully 6 inches long, died of respiratory failure 11 hours after birth.

Though the world record has gone untouched for 132 years, big babies like Jonathan Rozzi are growing more common. According to Thomas Jansson, M.D., Ph.D., a researcher at the University of Texas School of Medicine, the ever-worsening obesity epidemic is causing an upward trend in fetal overgrowth, and though a plump, happy baby is usually a cause for joy, an overweight one is a cause for alarm.

"Half of American women are considered overweight or obese, conditions which greatly increase the risk of having an overweight baby," Jansson told Life's Little Mysteries.

Though diabetes (including gestational diabetes, a type that sets in during pregnancy), extreme height, and abnormal hormone regulation can also produce larger than average babies, obesity is responsible for the present upsurge.

"Obesity is on the rise," said David McCormick, a clinical professor of pediatrics at the University of Texas Medical Branch, Galveston. "For example in Texas, 30 percent of the population is obese. As a result a large proportion of our mothers who are delivering are obese. Obesity in and of itself during pregnancy, mothers who are obese, have a higher number of large-for-gestational-age (LGA) babies."

The size of a fetus depends on the quantity of nutrients it receives from its mother, and fetal growth mechanisms have only evolved to cope with shortages of nutrients, but not with surpluses.

"From an evolutionary standpoint, the fetus takes a lot of nutrients and resources from its mother, and it could therefore jeopardize her survival if the mother is undernourished," Jansson said. To avoid that, two hormones, insulin and insulin-like growth factor 1, react by decreasing the fetus's nutrient intake and keeping it from growing very large when the mother's food supply is scarce.

But when that growth mechanism is faced with an overabundance of nutrients coming from the mother, the opposite happens. "We think the mechanism is now being up-regulated in many fetuses in reaction to an excess of nutrients," Jansson said.

McCormick explained that sugar crosses over the placental barrier from mother to fetus very easily. "The fetus responds to an influx of sugar by increasing its insulin levels, and just like it does in the rest of us, the insulin stores that extra sugar in fatty tissue." In other words, the fetus gets fat.

The condition can have serious health implications. "In the short term, there are obvious difficulties at delivery. If a mother delivers an overweight baby vaginally, there's much higher risk of having traumatic birth injuries. If a doctor plans for a C-section, knowing that a baby is going to be big, this is also associated with certain risks," Jansson said.

Health problems can set in almost immediately after birth, too. "If a baby experienced high sugar intake in the womb that caused it to put out excess insulin, then as soon as it is born and doesn't have that sugar coming in anymore, the high insulin level drives its blood-sugar way down," explained McCormick. "This can be very damaging."

Fetal overgrowth can also cause continued weight problems and obesity later on. "Very good tracking by a number of studies shows that infants who are overweight at six months are highly likely to be overweight or obese at age three," said McCormick.

"Long term effects have been emerging more recently," Jansson added. "Babies born too big are at risk of developing obesity, diabetes, cardiovascular disease, or hypertension by age nine or 10."

Studies have been done that have dissociated the effects of learned bad eating habits from the effects of simply being large at birth. "Simply being an overweight baby carries many health risks," Jansson said. "It's a vicious cycle: a woman can effectively transmit her condition to her baby."

Though no treatment for obesity-induced fetal overgrowth has yet been developed, Jansson and his colleagues are looking into ways of modifying pregnant mothers' diets, and in particular, changing the types of fatty acids they eat.

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