Ms. Coffey calculated that the average woman in India weighs less at the end of her pregnancy than the average woman in sub-Saharan Africa did at the beginning, an astonishing finding.

Image A premature infant in Gurgaon, India. The poor health of children in India, even after decades of economic growth, is a perplexing public health issues linked to their mothers’ relatively poor health. Credit... Kuni Takahashi for The New York Times

“In India, people are richer, better educated and have fewer children than those in sub-Saharan Africa, so it’s really surprising that Indian children are shorter and smaller than those in sub-Saharan Africa,” Ms. Coffey said in an interview. “But when you step back and look at the state of Indian mothers, it’s not such a surprise after all.”

Research has shown that genetics play no role in the size differences, leaving environmental factors as the only explanation, Ms. Coffey said.

The reasons for Indian mothers’ relatively poor health are many, including a culture that discriminates against them. Sex differences in education, employment outside the home, and infant mortality are all greater in India than in Africa.

“In India, young newly married women are at the bottom of household hierarchies,” Ms. Coffey said. “So at the same time that Indian women become pregnant, they are often expected to keep quiet, work hard and eat little.”

Mothers also suffer from the same sewage-borne infections that so often kill their babies, made endemic by the primitive sanitation in much of the country, Ms. Coffey said.

“It is likely that infectious disease is responsible for a signification portion of India’s pre-pregnancy underweight problem,” she said.