Lucia Sebastian Diego left northern Guatemala with her 10-year-old son in early July hoping to reunite with her husband, who lives illegally in Indiana.

She never made it — her journey and life coming to an abrupt end on July 19 on the desert floor on the Tohono O'odham Reservation.

The 41-year-old mother of four became one of the latest victims in a grim trend among illegal border crossers attempting to traverse the Arizona desert: Twice as many women, often traveling with children, die in July as in any other month of the year.

They are more susceptible to the brutal heat and often held back by the burden of caring for their children, creating a fatal formula that explains the spike, said Melissa McCormick, senior research specialist at the Binational Migration Institute at the University of Arizona, which released a study on border deaths in February.

With more women and children crossing illegally into the United States each year to reunite with husbands and fathers, the death toll is likely to climb.

This July, 16 of the 46 bodies found in Arizona, or 35 percent, were women, numbers that are in line with the past four years. From 2004-2007, women have accounted for 36 percent of all bodies found in July, more than double the 17 percent they account for the rest of the year, numbers from the Arizona Daily Star border-death database show.

In the absence of evidence suggesting more women are crossing in the middle of summer than at other times, experts and analysts who study border deaths theorize that three factors likely contribute to the phenomenon:

● Women aren't as used to extreme heat.

Women may not be as accustomed to the heat or as well-conditioned as men who have worked outdoors in construction and farm jobs, said Dr. Bruce Parks, chief medical examiner at the Pima County Office of the Medical Examiner, and the Rev. Robin Hoover, the founder of Tucson-based Humane Borders.

"If you are not used to being in the heat, somebody that is is going to respond better," Parks said.

A 2006 study from the UA's Binational Migration Institute found women are 2.87 times as likely to die of exposure to elements than men. The study was based on a review of more than 900 autopsy reports from the Pima County Office of the Medical Examiner from 1990 to 2005.

The findings reinforce what Hoover had anecdotally noticed.

"They are not as tough as the men, nothing disrespectful intended by that," said Hoover, whose organization places water tanks throughout the desert and charts border deaths.

Desert survival is correlated with social support, said Nestor Rodriguez, one of the principal researchers on the University of Houston's groundbreaking studies on border death in the mid-1990s.

Women sometimes get left behind by fast moving guides, or "coyotes," especially if they are pregnant or having menstrual pains, he said. That's a death sentence in the desert in the summer.

Illegal entrants who have survived after getting sick or lagging behind received help from someone else in the group who ran to a highway or found somebody for help, he said.

"The critical variable is: 'Is someone with you?' " said Rodriguez, director of the Center for Immigration Research at the University of Houston. "If you are by yourself, you are less likely to survive."

The only person with Lucia Sebastian Diego when she died was her 10-year-old son. The "coyote" left her behind when she couldn't keep up after falling in a small ravine, injuring her back, said her husband, Valtisar Mateo Felipe.

● Women traveling with children are at greater risk.

Traveling with children often forces them to walk slower or sometimes carry the child. They also carry extra food, water, clothes and supplies.

"That's like trying to walk across the desert with an anchor," Rodriguez said. "It creates an additional disadvantage."

On successive days in July, Border Patrol agents found women dead with their children alive nearby. On July 18 on the Tohono O'odham Nation, agents found the body of Maria Resendiz Perez, 33, of the central Mexican state of Queretaro, along with four survivors, including her 10-year-old son. The next day, about 15 miles south of that recovery, they found Sebastian Diego.

In both cases, their sons survived and were in relatively good condition.

The caregiving instinct usually kicks in, which sometimes prompts women to give their own food and water to their children, said McCormick, of the Binational Migration Institute.

"She's going to do everything to make sure her children come out alive, including giving up water," she said.

● More women are crossing to reunite with husbands.

Women began crossing at greater rates in the mid-1990s following increased enforcement efforts that made it more difficult and expensive for their husbands to sneak back and forth across the border, said Douglas Massey, a Princeton University sociologist who has studied international migration for nearly three decades.

"Rather than face the gauntlet again, people who had been circulating back and forth, hunkered down and decided to stay," Massey said.

Men began paying coyotes to smuggle their wives and children across the desert or through ports of entry with false documents, he said. The share of women coming has been gradually increasing and will continue to increase, he said. Between 20 percent and 25 percent of all illegal border crossers are women, he estimates.

Women account for 15 percent to 16 percent of all apprehensions made by the Border Patrol since 2003, up from 11 percent to 14 percent from 1996-2002, according to agency statistics.

An Arizona Daily Star database — which includes deaths from Pima, Cochise, Santa Cruz and Pinal counties — shows that women account for 22 percent of illegal-border-crosser bodies found from 2004 through July 2007.

Sebastian Diego was headed for a reunion with her husband, Mateo Felipe, 43, who has been coming to the United States to work since 1993. He used to stay for a couple of years at a time, then return to Guatemala, he said in a phone interview. He's been living in Logansport, Ind., and working at a meatpacking plant for the past two years and five months.

This year, he decided to send for his wife of 23 years and youngest child, Valtisar Mateo Sebastian. He planned for the family to stay for another four years before returning to Guatemala to buy land, he said.

Economic opportunities are scarce in their Guatemalan hometown of San Sebastian Coatán, Huehuetenango.

Mateo Felipe last spoke with his wife when she was in Chiapas, Mexico, in the second week of July. Sebastian Diego and her son traveled aboard buses through Mexico and crossed with a coyote and about 20 people into Arizona, he said. They had been walking for about 18 hours when she fell in a small ravine and injured her back, his son told him.

On the morning of July 19, a Border Patrol agent found her son wandering alone one-half mile north of the border on the Tohono O'odham Reservation. He told the agent his mother had died. They backtracked his footprints and found her body.

It was her first attempt to cross and they didn't know about the extreme heat, which the medical examiner determined as the cause of death.

She died on the final day of a near-record heat wave during which temperatures hit at least 100 degrees for 37 straight days in the Tucson area. It reached 110 degrees on July 18 when she began walking and and 104 on July 19 when she died.

Mateo Felipe's son is with him in Indiana but he grieves for his wife and doesn't know what he'll do next, he said. Her body is scheduled to be returned to Guatemala this week for a funeral, said Patricia Meigham, Guatemalan vice consul in Phoenix.

"Thank God, my son still has his life ahead of him and nothing happened to him and he's with me," Mateo Felipe said.

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