Most are in sub-Saharan Africa and South Asia, with one in 10 girls in Bangladesh, Chad, Guinea, Mali, Mozambique and Niger reporting having a child before the age of 15, the UN Population Fund (UNFPA) said.
An estimated 70,000 girls aged 10 to 19 die each year from complications during pregnancy and childbirth, according to the Fund's annual State of the World Population report.
Many more are left incontinent and in pain by giving birth before their bodies are ready, while they also face a tough future if they are pressured to leave school early.
The UNFPA warned that this is a "huge global problem" that demands attention -- but urged governments and civil groups to realise that the girls are not solely responsible.
Issues of poverty, poor education, a lack of contraceptive advice and the practice of child marriage -- nine out of 10 adolescent births occur within a formal union -- all play their part, as does sexual violence.
"Too often, society blames only the girl for getting pregnant," said UNFPA executive director Babatunde Osotimehin ahead of the launch of the "Motherhood in Childhood" report in London.
"The reality is that adolescent pregnancy is most often not the results of a deliberate choice, but rather the absence of choices and of circumstances beyond a girl's control."
Health surveys suggest 19 percent of women aged 20 to 24 in developing countries had their first live birth before they turned 18, equating to 36.4 million women based on 2010 population figures.
Of these, an estimated 17.4 million are in South Asia, 10.1 million are in sub-Saharan Africa, and 4.5 million in Latin America and the Caribbean, the UNFPA says.
From these figures, the authors calculated that 7.3 million girls each year become mothers while they are still children themselves -- two million of them aged 14 and under.
The equivalent of 2.9 million girls in South Asia reported having had their first child before they were 15, 1.8 million in Sub-Saharan African and 0.5 million in Latin America and the Caribbean.
According to the report, these girls are at greatly increased risk of maternal death or obstetric fistula, a debilitating condition resulting from prolonged and obstructed labour.
In most case the baby dies and, without surgery, the mother is left incontinent.
There is also an economic cost to having children too young, with estimates of foregone annual income over a girl's lifetime ranging from one percent of annual GDP in China to 30 percent in Uganda.
There has been a slight decline in the percentage of women reporting adolescent births across developing countries, the report said, with the exception of sub-Saharan Africa.
The highest proportion of child mothers are in Niger (51 percent), followed by Chad (48 percent), Mali (46 percent), Guinea (42 percent) and Mozambique (40 percent).
Adolescent pregnancies also occur in the developed world but these represent just five percent of the total -- about 680,000, of which nearly half are in the United States.
Across all countries, girls who are poor, badly educated and living in remote areas are more likely to become pregnant, and these factors are key to helping them.
The UNFPA urged efforts to keep girls in school, teach them about sexual health, stop child marriage, change attitudes to gender roles and equality and better support young mothers.
"We must reflect on and urge changes to the policies and norms of families and governments that often leave a girl with no other choice but a pathway to early pregnancy," Osotimehin said.