The room was lined with baby after baby, and it was almost too warm for Keri Dykes to tolerate.

She remembers squeezing between the incubators to see her son Jesse, who had been born only 25 weeks into her pregnancy.

A neonatal intensive care unit in Tupelo, Mississippi, is where Jesse spent his brief life in 2008 -- a life cut short by an infection that got into his bloodstream and lungs.

The first time Dykes, 29, got to hold him and look at him face to face, she remembers that his pretty blue eyes were open as he was dying.

"It's just something you never, ever think will happen to you," she said.

Jesse happened to be born in the state where babies are less likely to reach their first birthday than in any other state, according to the most recent data (PDF) from the Centers for Disease Control and Prevention.

For every 1,000 Mississippi babies born in 2011, 9.4 died before their first birthday, according to the state's health office. That makes Mississippi's infant mortality rate more comparable to countries such as Costa Rica (9.2), Sri Lanka (9.5) and Botswana (10.5) than the United States (6.0), according to latest estimates in the CIA World Factbook.

The bottom of the list

Mississippi is 50th out of 50 states when it comes to infant mortality, and it's been that way for a long time, says state health officer Mary Currier.

There's not one clear explanation. Experts cite a multitude of factors that are also seen in other parts of the country and around the world.

For example, Mississippi leads the nation in obesity, which can carry with it a host of complications that might affect a baby, such as hypertensive disorders, says Dr. Michelle Owens, an obstetrician-gynecologist at the University of Mississippi Medical Center. Other Southern states with a high prevalence of obesity -- Alabama and Louisiana, for instance -- also have some of the nation's highest rates for infant mortality. A 2010 study also found that overweight and obese women are at higher risk for preterm birth.

In general, Owens said, many pregnant women don't realize the importance of getting their own medical problems treated.

"We know that the way we get the best pregnancy is to have a healthy mom before mom and baby are together," she said.

Birth defects are the leading cause of infant death in the United States. Not all are preventable, but a mother can reduce risk by keeping her own health under control by eliminating smoking, drinking and illegal drugs.

The same is true for premature births, another leading cause of infant deaths in the United States. The March of Dimes recently released its Premature Birth Report Card and gave failing grades to Mississippi, Alabama and Louisiana. Mississippi has the largest premature birth rate based on the 2011 national statistics used in the report.

Poverty, low socioeconomic background and low education contribute to high preterm birth rates, said Dr. Eve Lackritz, senior program officer for the Global Alliance to Prevent Prematurity and Stillbirth, an initiative of Seattle Children's. Lackritz was once the branch chief for the CDC's maternal and infant health division.

Southern states tend to have some of the biggest poverty problems in the country. Mississippi led the country with the highest percentage of people whose income was below the poverty line in 2011, according to the American Community Survey (PDF).

"Infant mortality goes along with poverty, and we have a pretty low average income in the state," says Currier.

Mississippi also ranks in the top 10 states for the percent of the population that is uninsured (19%), according to the Kaiser Family Foundation, and in the proportion of residents receiving Medicaid (20%).

Teenage births are also more common in the South than in most other parts of the country, according to the latest CDC data. Mississippi had the highest teen birth rate in the country in 2010, at 55 births per 1,000 women compared with a national average of 34.2. Alabama (43.6) and Louisiana (47.7) are also on the higher side in this regard.

Babies born to teens are more likely to be born prematurely and have a low birth weight, according to the Mayo Clinic. Not receiving proper prenatal care contributes to the complications that can arise in teen pregnancies.

An international reach

The interplay between teenage births, poverty and infant mortality isn't unique to the United States.

Catalina Escobar, one of the top 10 CNN Heroes of 2012, knows all too well how much young mothers need medical support and education to give birth to healthy children. In Escobar's country, Colombia, the infant mortality rate is 15.92, much higher than any state in the U.S.

Colombia's colorful city of Cartagena is an attractive tourist destination, with its historical grandeur and beach resorts. Acclaimed author Gabriel Garcia Marquez set one of his famous novels, "Love in the Time of Cholera," there. But Cartagena has a darker side that visitors may not realize: One-third of residents live at or below the poverty line.