Family size is closely linked to reproductive rights, according to the State of World Population 2018 report.
The U.N. report says people in developed countries tend to have lower fertility rates because of greater access to family planning services, modern contraceptives and age-appropriate sex education.
The director of the U.N. Population Fund office in Geneva, Monica Ferro, says in places where reproductive rights are constrained, either due to lack of resources or government mandates, people have a limited ability to choose the size of their families.
"Many sub-Saharan African countries, for example, have fertility rates of four or more births per woman," Ferro said. "At the other end of the spectrum, you have some eastern Asian and European countries with fewer than two births per women. In both cases, individuals face obstacles to the full realization of their reproductive rights."
The world population is expected to increase by 2.5 billion by 2050, to nearly 10 billion people, with sub-Saharan Africa expected to contribute more than half of that growth.
Women in Africa must overcome many legal and social barriers to achieve control of their fertility, Ferro said.
"Women may not have the access to medical services," she told VOA. "They may not have the access to child care. They may not have access to all the institutional and social support that comes with being ready or being able to plan your fertility."
To make freedom of choice a reality, the report urges countries to offer universal access to quality reproductive health care, including modern contraceptives and better education.
It also advocates for a change in men's attitudes toward a woman's right to choose the number, timing and spacing of children.
