In their impoverished communities in Uganda, the answers hinged on the fact that one girl had a birth certificate and the other didn't. Police foiled the planned marriage after locating paperwork that proved the first girl was not 18 as her parents claimed.
The other girl could not prove she was under the age of consent; her aunt, who's also her guardian, has struggled to press charges against the builder who seduced and impregnated her.
As Namirembe spoke, her niece sat beside her, her belly swollen and a vacant stare on her face.
In the developed world, birth certificates are often a bureaucratic certainty. However, across vast swaths of Africa and South Asia, tens of millions of children never get them, with potentially dire consequences in regard to education, health care, job prospects and legal rights.
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"They could end up invisible," said Joanne Dunn, a child protection specialist with UNICEF.
With support from UNICEF and various non-governmental organizations, many of the worst-affected countries have worked to improve their birth registration rates. In Uganda, volunteers go house to house in targeted villages, looking for unregistered children. Many babies are born at home, missing out on registration procedures that are being modernized at hospitals and health centers.
India is the biggest success story. It accounted for 71 million of the unregistered children in UNICEF's 2013 report, more than half of all the Indian children in that age range. Thanks to concerted nationwide efforts, UNICEF says the number of unregistered children has dropped to 23 million, about 20 percent of all children under age 5.
While obtaining a birth certificate is routine for most parents in the West, it may not be a priority for African parents who worry about keeping a newborn alive and fed.
Lack of registration hampers Uganda's efforts to enforce laws setting 18 as the minimum age for marriage. Child marriage remains widespread, due largely to parents hoping to get a dowry from their daughters' suitors. Even when the police are alerted, investigators face an uphill task pressing charges if they cannot prove, with a birth certificate or other official document, that the girl is a minor.
"When we reached there, I heard one man say, 'Ah, but the police have come. Let me hope the girl is not young,'" Atwebembeire recalled.
The girls' parents claimed she was born in March 1999, which would have made her old enough to consent. Yet only months before, the parents told birth registration officials she was born in October 2001.
The wedding was called off; the parents spent a night in jail.
The progress in India results from a decades-long initiative. Health workers, midwives, teachers and village councilors in remote areas have all been empowered to report births.
Chhitaranjan Khaitan, a census official, said 15 of India's 29 states now report a 100 percent birth registration rate.
An added motivation is India's effort to stem its skewed gender ratio, due largely to families' preference for sons. By requiring health workers and village officials to register all births, authorities hope fewer newborn girls will be killed by their families.
"It was the first thing I did after my daughter was born," said Verma, 28. "My parents did not register my birth. It was not considered important or necessary in those days." Verma, who dropped out of school in 10th grade, has had repeated problems with proving his identity, particularly in getting a government ration card entitling him to cheap rice and sugar.
"My daughter will not have to face such hassles," he said.
Chhattisgarh was recording just 55 percent of births in 2011. In 2013, with help from UNICEF, the state government launched a registration campaign and today it registers virtually every birth.
There's also the massive problem of children without birth certificates or other identification who comprise a significant portion of the millions of displaced people worldwide, fleeing war, famine, persecution and poverty. Birth registration can be crucial to enabling refugee children to return home or to reunite after being separated from their parents.
Claudia Cappa, author of the upcoming UNICEF report, says such separations can be heartbreaking for a parent.