Women"s fertility is determined in large part at birth. They are born with their total number of reproductive cells, which normally influences the age at which menopause—the shutting down of female reproductive system—begins. But in the 1990s, researchers proposed that if a child"s energy is depleted by malnutrition, disease, or other factors, he or she would be less fertile as an adult. By using the natural experiment of migration, researchers demonstrated how differences during childhood do alter the course of reproduction in adult women. Biological anthropologist Gillian Bentley of Durham University in the UK and colleagues compared levels of reproductive hormones in 250 Bangladeshi women, including women who migrated from Sylhet, Bangladesh to London; women who stayed in Sylhet; and Bangladeshi women bom in London. In the first stage of their study, they found that women who migrated from Bangladesh as children had higher levels of reproductive hormones in their saliva than women who lived in Sylhet, but less than women born in London. This had a direct effect on fertility: Migrant women in London had an 11% higher rate of ovulation—discharging of mature ovum—during their lives than did women in Sylhet, the team reported in 2007. The team has now studied 900 women between the ages of 35 and 60 to see if the beginning of menopause varies between migrants and women in Sylhet. Bentley presented preliminary results from their measurement of hormones that regulate the maturation of reproductive cells and are indirect indices of how many ova they can still produce. Her team found that migrants enter menopause later than did women who stayed in Bangladesh but earlier than did those born in London. "The adult migrants seem to be sensitive to improved conditions," says Bentley. The group is trying to find out which environmental factors in Bangladesh lower growing girls" fertility. All the Bangladeshi women in the study came from middle-class, land-owning families, who grew up with adequate calories. However, girls growing up in Bangladesh were probably exposed to more infectious diseases during crucial developmental years. So, they may have had to make tradeoffs among using energy to grow, to maintain their bodies, or to maximize their reproductive potential as adults. Bentley plans to test that idea next year when her team returns to Bangladesh to see if girls there suffer from more diseases than do those in London. "In other words," says Bentley, "where you spend your childhood influences adult reproductive function."
单选题
According to the text, which of the following factor in childhood will probably cause an adult to be less fertile?