In an analysis of 277 national surveys, the authors, led by Gretchen Stevens from the World Health Organization, estimated the levels and trends of infertility in 190 countries from 1990 to 2010. They found that in 2010, 1.9% of women aged 20 years who wanted to have children were unable to have their first live birth (primary infertility), and 10.5% of women who had previously given birth were unable to have another baby (secondary infertility) -- a total of 48.5 million couples.

The authors found that the levels of infertility were similar in 1990 and 2010, with only a slight overall decrease in primary infertility (0.1%, but with a more pronounced drop in Sub-Saharan Africa and South Asia) and a small increase in secondary infertility (0.4%).

The authors found that primary infertility rates among women wanting to have children also varied by region, ranging from 1.5% in Latin America and the Caribbean in 2010, to 2.6% in North Africa and the Middle East. Furthermore, with a few exceptions, global and country patterns of secondary infertility were similar to those of primary infertility.

The authors say: "Independent from population growth and worldwide declines in the preferred number of children, we found little evidence of changes in infertility over two decades, apart from in the regions of Sub-Saharan Africa and South Asia."

The authors continue: "In the absence of widespread data collection on time to pregnancy, the methods used and results presented here provide valuable insights into global, regional, and country patterns and trends in infertility."