The birth in a field hospital is thought to be the first time a serving member of Britain's military has gone into labor in a combat zone.
The solder, a Fijian national serving as a gunner with the Royal Artillery, delivered the child Tuesday at Camp Bastion. The sprawling British base in southern Afghanistan's Helmand province last week suffered a major attack in which two U.S. marines were killed and six American fighter jets destroyed.
Britain's defense ministry said Thursday it had not been aware the soldier was pregnant, and stressed that it does not allow female soldiers to deploy on operation if they are pregnant. It declined to say whether the soldier, who has not been named, was aware of her pregnancy.
"Mother and baby are both in a stable condition in the hospital and are receiving the best possible care," the ministry said in a statement. It said a team of doctors would fly out to Afghanistan in the coming days to help the solider and her son return safely to Britain.
The woman had deployed to Afghanistan in March, meaning her child was conceived before her tour of duty began. She is one of about 2,000 Fijians who serve in the British military, even though the country became independent from Britain in 1970.
Camp Bastion, which hosts the U.S. Camp Leatherneck, is home to most of Britain's 9,500 soldiers in Afghanistan, including Prince Harry - who arrived there earlier this month to serve as an attack helicopter gunner.
"This sort of thing makes life difficult for everyone else, but the important thing is the welfare of the female soldier. This could have gone wrong and we don't know if the attack on Camp Bastion might have forced the birth," said Maj. Charles Heyman, a retired officer and author of "'The British Army Guide."
Heyman said it may have been "that the excitement of the tour masked the symptoms of the pregnancy."
Belinda Phipps, chief executive of the National Childbirth Trust, a British parenting charity, also suggested the soldier's demanding duties could explain why she either didn't know she was pregnant or attempted to ignore the signs.
"It could be that she was so very focused on other things, and because she was in a life-or-death scenario, that she simply didn't recognize that she was pregnant," Phipps said.
Phipps said the pregnancy may not have been obvious to the soldier's colleagues. "Not everyone has a very big baby bump, some women carry their baby far inside," she said.
Patrick O'Brien, a consultant obstetrician at University College London Hospital, said cases of unnoticed pregnancies were unusual but he encountered at least one each year.
"There are some women who have very irregular periods, often women who are very fit and exercise a lot. There are women who don't have sickness during pregnancy. Some women - particularly those who are overweight - don't recognize they have put on weight, or feel the baby moving," O'Brien said.
Many cases involved women who refused to accept that they were pregnant and attempted to disguise it, particularly young women living at home.
"It's not just that they hide the pregnancy from their parents, they often become in denial of the pregnancy," he said.
"If you have a combination of any or all of those things, a pregnancy can go undetected, or the woman can be in denial of it if the implications to their life are so great," said O'Brien, a spokesman for the Royal College of Obstetricians and Gynecologist.
A study published in 2011 by Glasgow's Victoria Infirmary said that denial of pregnancy was more common than expected, suggesting it occurred in around 1 in 2,500 births.
In a 2002 German survey of Berlin obstetric hospitals, researchers found that 40 percent of women who didn't realize they were pregnant had seen doctors who also failed to spot the signs.