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Is a pregnant woman's chance of giving birth to a boy 50 percent?

Marc Weisskopf, a research associate at the Harvard School of Public Health, explains.

In most industrialized countries about 105 boys are born for every 100 girls, for a ratio of 1.05, known as the secondary sex ratio, or SSR; the primary sex ratio is the ratio at conception. This is often expressed as the percentage of boys among all births, or about 51.2 percent. Thus, the short answer to the question is: "On average, no." The percentage of males among all births is not fixed, however. Since the 1950s and 1960s the overall SSR has been declining in the U.S., Canada and several European countries, but some groups display different trends. In the U.S., the SSR is declining for whites, whereas among African-Americans and other races, the SSR has been increasing since the 1960s. Currently the SSR among African-Americans in the U.S. is only about 50.7 percent. There are also both personal and environmental factors that affect the average sex ratio.

The chance of having a boy appears to decline with the mother's age, the father's age and the number of children the family already has. These effects are small. One study in Denmark found that the SSR of children born to fathers younger than 25 was 51.6 percent, which decreased to 51.0 percent among children of fathers at least 40 years of age. Therefore it is unlikely that the declining SSR in many countries results solely from large-scale changes in such personal factors.

With regard to environmental factors, improved prenatal and obstetrical care during the first part of the 20th century is largely responsible for an increased SSR over this period in many countries. The male fetus is more susceptible to loss in the womb than is the female fetus, so with more conceptions reaching term, proportionally more males are born.

It is difficult to discern how much of the decrease in sex ratio since the 1950s arises from contaminants in the environment. What is known is that drug use, high occupational exposures and environmental accidents can affect SSR. For example, hopeful mothers taking clomiphene citrate (Clomid) for infertility bore babies with an SSR of only 48.5 percent. Workers producing 1,2-dibromo-3-chloropropane (DBCP), a chemical used to kill worms in agriculture, experienced even larger decreases in the number of male babies they welcomed into the world. Effects of DBCP on sperm quality were discovered incidentally when male workers found that they were unable to father children. After the exposure ended, male workers experienced some recovery of sperm quality and 36 children were born to 44 workers. Of these 36 children only 10 were boys--an SSR of just 27.8 percent. Decreases in the SSR of offspring from fathers exposed to dioxin and dioxinlike chemicals occurred following an explosion in an herbicide factory in Seveso, Italy, in 1976 and contamination of rice oil used for cooking in Yu-Cheng, Taiwan. The decreases were most extreme among the children of fathers who were exposed at earlier ages: an SSR of 38.2 percent was recorded for fathers exposed before age 19 in Seveso, and fathers exposed before age 20 in Yu-Cheng experienced an SSR of 45.8 percent.

These dramatic changes resulting from extreme exposures raise the concern that chemicals in the environment at lower concentrations may also change the SSR by exposing people over longer periods of time. For example, there are reports that parental exposure to polychlorinated biphenyls (PCBs) and mercury, each of which is widely distributed in the environment, can affect the sex ratio. Confirming such effects will take careful work on large populations, but the results may be quite important for other reasons as well. In the general population, sperm quality deteriorated and testicular cancer and abnormalities of male genitalia increased over the same period that SSR declined. Furthermore, for men who go on to develop testicular cancer, both their semen quality and the SSR of their children are significantly reduced, suggesting a possible biological link between these male reproductive characteristics. Thus, effects of environmental contaminants on the sex ratio may be only the tip of the iceberg.

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