Hard Work Decreases Chance of Pregnancy, Researchers Find

By William J. Cromie

Gazette Staff

Women who do hard physical work decrease their chances of becoming pregnant even if they eat a good diet, according to Harvard scientists. A high expenditure of energy interferes with production of hormones needed to successfully bear children, the researchers conclude from a study of women in rural Poland.

"We did the study with women who work in a 19th-century-like agricultural setting," says Grazyna Jasienska, a postdoctoral fellow in anthropology. "But we would expect the same results in women everywhere who work in arduous occupations, such as firefighters, soldiers, and janitors."

It was previously shown that women in the United States and other industrialized countries suppress their ability to become pregnant by participating in sports or in intense exercise. Hard labor by women in poorer countries who do not get enough to eat also hinders reproduction. However, Jasienska claims, "this is the first study to reveal that physical work interferes with reproduction even without weight loss."

Jasienska and Peter Ellison, professor of anthropology, believe this finding reflects an evolutionary response that inhibits childbearing at times when energy resources are needed for other survival tasks. "It allowed women in pre-agricultural periods to wait for times when less work or more food would increase the chances of successful pregnancy," Jasienska speculates.

High energy expenditures may also decrease the risk of breast, ovarian, and uterine cancers in societies where women must work hard to feed and care for themselves and their families. "We believe this is due to a decrease in exposure to estrogen, a hormone that raises the risk for reproductive cancers," Jasienska says. "There is evidence that women with high energy expenditures have a lower risk of breast cancer compared to women in sedentary occupations."

Hormones Make the Difference

Ellison, Jasienska's adviser, had done studies showing that women who work hard physically and don't eat properly suppress their reproductive functions. In some women, levels of key hormones tied to ovulation decrease; other women fail to produce eggs. (Highly competitive female athletes may experience a total cessation of menstruation.)

Ellison wanted to disentangle the effects of work from those of poor diet by studying a uniform group of women who work very hard but do not suffer from malnutrition. Jasienska knew of such a group in her native Poland. As a biology student at Jagiellonian University in Krakow, she did research in a small village called Chyszówki. Women there work in the fields during harvest and haying times. They use no tractors or other mechanized equipment, nor do they have many labor-savings devices for housework and child care.

"The women live much the same way as agricultural societies in the last century," Jasienska told Ellison. "They eat enough to replace the calories they burn at work, and do not lose weight when they perform strenuous fieldwork."

Jasienska returned to the village and measured seasonal changes in energy expenditure, food intake, body fat, and ovarian function in 20 women aged 24 to 39 years old. To determine ovarian function she measured the amount of progesterone in daily samples of their saliva. High levels of this hormone indicate ovulation and contribute to implantation of any fertilized eggs.

The result was clear: Hard-working women have much lower levels of progesterone than women who are less active.

Jasienska and Ellison explain this as an evolutionary adaptation. A person can only metabolize so much food; therefore, there's a limit on energy intake. In pre-agricultural societies what was available had to be allocated among biological processes needed for staying alive, working, and reproducing. When there's not enough energy to go around, suppressing reproduction until a time when more food is available would provide a natural survival advantage.

Our ancestors ate food with a lower energy density than what we now consume -- roots, fruit, and lean meat instead of steak, candy bars, and high-protein snack foods, Jasienska points out. "Also, the period of nursing was as long as three years," she says.

In other words, reproduction incurred a much higher energy cost, and calories to pay for it were harder to come by. Under such circumstances a natural birth-control mechanism would make women more fit to survive and successfully produce children later.

Jasienska and Ellison published their findings in the Oct. 7 issue of the Proceedings of the Royal Society, London.

Jasienska plans to study the advantage that suppressed reproduction might give women in modern times. Females in societies who work hard physically boast low rates of reproductive cancers. Jasienska believes these low rates involve less production of steroid hormones, particularly estrogen.

"Women who live in rural areas have much lower rates of reproductive cancers, such as breast cancer, than those who live in cities," she notes. "It's widely believed that pollution accounts for some of this effect, but I think difference in lifestyle accounts for a large part of it."