Increasing obesity threatens to undermine the health of women in nations previously not known for a growing percentage of overweight citizens, claim the researchers.
And as the amount of available food grows in most countries, the condition is becoming a greater issue for poorer women than for richer ones.
The study underlines the need for better education and affordable healthy foods to help stop obesity from rising further.
In the past, health experts and others believed that populations with large numbers of overweight people were confined chiefly to wealthier countries such as the United States, England, Germany, Italy and Japan.
But scientists at the University of North Carolina at Chapel Hill and the University of Sao Paulo in Brazil say that an analysis of information from almost 149,000 women in 37 countries show a major shift in the burden of disease.
Obesity makes heart disease, stroke, diabetes, bone and joint problems, some cancers and some other illnesses more likely, doctors say.
The researchers reported at a conference on obesity this week that obesity starts to fuel health inequities in the developing world when the gross national product of a country reaches a value of about $2,500 per capita.
Author Dr Carlos Monteiro said: "In the absence of concerted national public actions to prevent obesity, economic growth will greatly expand the list of developing countries where this occurs."
"Worldwide, the burden of obesity increasingly rests on the poor and less educated even in many developing nations we never thought of as having an obesity problem," added Dr Barry M. Popkin, co-author.
One possible reason for the inverse association between women's obesity and their family income in some lower-middle income countries is that after a certain level of economic growth, high-energy expenditures and lack of food are no longer as common in those countries.
Another is that poorer people have less education and knowledge about health and more difficulty obtaining more expensive, low-energy dense foods such as fruits, vegetables and whole grain cereals. They also have less time and inclination to exercise.