Since January, the US government has ordered 16,000 migrants under 18, including nearly 500 infants, to wait with their families in Mexico for US immigration court hearings, a Reuters analysis of government data found.
Along the US-Mexico border, babies and toddlers are living in high-crime cities - often in crowded shelters and tents or on the streets - for the weeks or months it takes to get a US asylum hearing.
The risk of violence and illness runs high and is of particular concern for families with young children or those with chronic health conditions, according to interviews with health professionals, migrants, aid workers and advocates.
The children, whose numbers have not been previously reported, are among tens of thousands of migrants returned to Mexico under a Trump administration policy known as the Migrant Protection Protocols (MPP). Most are from Guatemala, Honduras or El Salvador.
US immigration officials did not respond to requests for comment on Reuters' data findings.
According to the Department of Homeland Security, decisions about whether a person is placed in MPP are made by border agents on a case-by-case basis and include consultation with medical professionals. Unaccompanied minors should not be sent back to Mexico, according to the program guidelines, but children can be sent back with their parents.
Trump administration officials have said they are doing everything possible to discourage migrant families from making dangerous journeys to the United States, often in the hands of human smugglers, which they say needlessly put children at risk.
Numbers Growing As Flu Season Looms
About one third of the nearly 50,000 migrants in the MPP program as of Oct. 3 were children under 18, according to the latest data available from the Executive Office for Immigration Review (EOIR), which oversees US immigration courts. Of those, Reuters found around 4,300 under 5 years old and 481 under 1 year old.
Blanca Aguilar, a 27-year-old mother from Guatemala, is living in a makeshift encampment of around 40 small tents cramped together in the back rooms of a church outside Tijuana, across the border from San Diego. Children can be heard coughing and crying throughout the night, she and other mothers told Reuters during a recent visit.
When one gets sick, they all do, Aguilar said. Her two-year-old son Adrian has had a recurrent cough with wheezing, as well as bouts of diarrhea, since they arrived in August.
"He's been sick a lot," she said, adding that she suspects he may be developing asthma.
Another mother at the same shelter, 34-year-old Marla Suniga from Honduras, said her 1-year-old daughter Montserrat recently had a convulsion due to a high fever and had to be taken to a hospital. "She couldn't breathe," she said.
Suniga said she fled violence in her home country but plans to return there because she fears for her daughter's life in Tijuana.
DHS said it could not comment on individual cases. Mexican officials did not respond to requests for comment on the conditions in migrant shelters.
Reuters was unable to corroborate the diagnoses of the Suniga and Aguilar children. Doctors and nurses visiting shelters and camps in Mexican border towns, however, told Reuters they have seen cases of chicken pox, scabies, respiratory infections, skin rashes, eye infections and gastrointestinal issues among children and adults.
Children under 5, and especially under the age of 2, are at high risk of serious flu complications, according to the US Centres for Disease Control, and the flu season is about to start.
American doctors and nurses volunteering in Tijuana with the Refugee Health Alliance hope to be able to provide flu shots in a few of the shelters there, but the effort is hard to organise, said coordinator Phil Canete. The vaccines need to be stored in cold, regulated conditions, and the Mexican government requires a physician licensed in Mexico to supervise the effort, as well as signed consent for every patient, he said.
The US government has said in guidance documents that migrants with known physical or mental health issues are not candidates for the MPP program. But advocates say it's not clear what qualifies as a medical exemption from MPP.
Jennifer Jimenez, a 30-year-old Salvadoran, said she arrived at the border in July with 11-year-old twins and her eight-month-old son Jacob, who was born with lungs that had not fully developed.
Although she explained Jacob's condition to border agents, she said, the agents sent her and her children back to Ciudad Juarez, where the family ended up sleeping on the floor of a crowded shelter.
Recently she managed to find a doctor who noted in Jacob's medical records - seen by Reuters - that living in the shelter had complicated his health care. US officials recently admitted the family to stay with relatives in the United States, a rare occurrence.
Reuters found that 1% of migrants assigned to MPP have so far been transferred out of the program.
The US government has signed a series of bilateral deals with the governments of Honduras, Guatemala and El Salvador to push more people to seek asylum closer to home. In May, nearly 85,000 family units - parents with kids - were arrested at the US-Mexico border, a monthly record. In August, the number of families arrested dropped by 70% after the administration ramped up MPP and other measures to deter migration.
US President Donald Trump has said that lax US asylum laws encourage people to show up at the border with their children. Before MPP, it was common practice to release arriving families into the United States to wait out their US court hearings – something Trump and others said allowed many migrants to disappear into the country to live illegally.
Immigrant advocacy groups say most released immigrants show up for their court hearings. Some are suing to halt the MPP policy, and a federal appeals court - the 9th US Circuit - is due to rule on the case soon.
'Nothing For Them Here'
According to the Reuters analysis, thousands of children are awaiting court hearings in border towns where the risk of kidnappings, rapes and assaults is high.
About 5,600 children in the MPP program had their cases assigned to San Diego immigration court, north of Tijuana. Most of the others were assigned to Texas courts: 6,800 to San Antonio and El Paso and 3,400 to Brownsville.
Jimenez, the Salvadoran mother, said that when she heard her family was being sent from El Paso to Ciudad Juarez, known for its high murder rate, "it was like they threw a bucket of cold water on me. ..Mexico isn't a place I trust to go out alone with my three children."
Brownsville is just north of Matamoros, in Mexico's Tamaulipas state, a violent battleground for drug cartels.
Florida resident Helen Perry, a nurse in the US Army Reserve who joined a volunteer aid group headed to Matamoros on Labor Day weekend, told Reuters she saw families camped out in donated tents - each with between 5 to 10 people sleeping inside - a few dozen feet from the border.
One family, with four small children, was curled up under a tree, she said. People lacked access to fresh water and proper restrooms and were bathing in the Rio Grande river, she said.
One baby Perry examined had a chronic eye infection and was beginning to develop scarring, threatening his vision. Another had a fungal rash under his arm so severe it limited his movement.
Perry said she saw breastfeeding mothers so dehydrated that they could not nurse their babies and parents chewing up donated pizza into mush to feed their infants. Some children were showing early signs of malnutrition, she said.
"There is really nothing for them there," Perry said.