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Why Do Some Women Have Enormous Babies?

As birth weight approaches 10 lbs, vaginal delivery becomes tricky, putting the the mother and baby at risk.

Babies with very large birth weight are not uncommon. In mid-May, a Florida woman raised eyebrows when she gave birth to a 13-pound baby girl by C-section. “It looked like they pulled a toddler out of my belly,” the mother told ABC News. “When the doctor was pulling her out of me I just start hearing them all laughing and excited.”

In the past few years, we’ve seen gargantuan, 13-plus-pound babies born in Pennsylvania, California, Germany, and Spain. And in March 2013, a British mom gave birth — vaginally, no less — to a whopping 15-lbs., 7-oz. son. It’s a phenomenon known as macrosomia and, at least by some estimates, it’s on the rise.

Macrosomia, and the related condition LGA (Large for Gestational Age), typically refer to babies that weigh more than 8 lbs. 13 oz. at birth. That’s big, but also fairly routine — about seven percent of infants born in the U.S. (and nine percent worldwide) qualify. Studies suggest that mothers who are older than 35, obese, or diagnosed with diabetes (either before or during pregnancy) are more likely to give birth to large children. Male babies are more likely than female babies to be born enormous, and overdue babies are also at higher risk.

But only one percent of babies weigh more than 9 lbs. 15 oz. at birth, and that’s when doctors get nervous. As birth weight approaches 10 lbs., vaginal delivery becomes tricky, putting the mother and baby at risk. Delivering giant babies vaginally can injure the mother’s birth canal, tearing vaginal tissue and potentially rupturing the uterus, which can lead to severe bleeding and, if left untreated, death. Meanwhile, even if the enormous kid somehow makes it out of his or her mother, studies suggest the child is more likely to develop diabetes, obesity, high blood pressure, and abnormal cholesterol levels, opening the door to a life of long-term cardiovascular problems.

We’re seeing more large babies now than ever before, and that might be because the factors that cause macrosomia—maternal diabetes and obesity—are also on the rise. One 2004 study suggested that, as mothers gained more weight and smoked less frequently, babies were starting to grow to unhealthy sizes in utero.

Unfortunately, high birth weight is difficult to predict. There are a few warning signs — an abnormally long distance between the mother’s uterus and her pubic bone (known as fundal height), for instance, or excessive amniotic fluid. But even ultrasounds are notoriously unreliable when it comes to estimating birth weight. In fact, suspicions of macrosomia are among the most common reasons for doctors scheduling unnecessary C-sections and inductions—only to deliver babies with normal birth weights. One 2008 study found that interventions to protect women from high birthweight babies carry more risks than delivering a giant baby vaginally.

“Our ability to predict macrosomia is poor,” the authors of the 2008 study wrote. “Our management policy of suspected macrosomic pregnancies raises induction of labor and cesarean delivery rates without improving maternal or fetal outcome.”

In other words, it isn’t easy delivering a giant baby. But even worse? Getting worked up about the relatively remote chance of having one.