A 50/50 Chance: Sex Determination

Predict the Sex?

If you already have one child, and are trying to conceive (TTC) another one, you may be hoping for one of the opposite sex. You may be wondering if it's possible to predict the sex of the next one.

In most of the industrialized world, there are some 105 boys born for every 100 girls. This is called the secondary sex ration (SSR); the primary sex ratio would refer to the ratio at the moment of conception. So, in most places, the SSR stands at a ratio of 1.05. In terms of how that translates to percentages, it means that 51.2 males account for all births.

However, beginning in the 1950's, the SSR began to decrease in countries like the U.S., Canada, and many European countries. Some groups, on the other hand, show variant patterns. For example, in the U.S. the SSR has declined for whites, but has been increasing among people of color since the 1960's. For now, the SSR of African Americans stands at 50.7%.

The reason for the disparity might be due to sociodemographic or environmental factors that have an affect on sex ratio. This can be seen by looking at the statistics of individual groups within the general populace. For instance, the chances of having a boy baby declines with the age of the parents and how many children they already have, though the effect of these factors is slight.

The SSR of children born in Denmark, to fathers under the age of 25, stands at 51.6%. This figure drops to 51.0% when fathers are 40 years of age or older. This slight change in the SSR due to a significant age gap leads researchers to the conclusion that the changes in SSR are not due in the main to personal factors such as age.

Better Prenatal Care

What can be stated with certainty is that the improved prenatal and obstetrical care that arrived in the early part of the 20th century has had a great deal of impact in increasing the SSR in many countries. The female fetus is stronger and less vulnerable to loss in utero, but with more pregnancies coming to term, the proportion of male births has risen.

It may be that some of the decrease in the sex ratio that began in the 1950's resulted from toxins and contaminants in the environment, though it is impossible to state with any certainty how much this might be true. We do know that environmental accidents, occupational exposures to chemicals, and drug use do affect the SSR. An example of this can be seen in women taking the fertility drug known as Clomid. Those who conceived and gave birth as a result of their treatment had an SSR of only 48.5%.

Those workers involved in the manufacture of the chemical known as 1,2-dibromo-3-chloropropane (DBCP), used to kill worms that affect agriculture, had an even more significant decrease in the number of male babies they birthed. This was discovered quite by accident, when many of the male workers found they were unable to father children. Once their exposure to the chemical ended, sperm quality improved and 36 children were born to 44 of the workers. Out of these 36, 10 were males; which is an SSR of only 27.8%.

Similar decreases in the SSR for fathers exposed to dioxin and like chemicals have been demonstrated in both Italy and Taiwan. The younger the fathers were at the time of exposure, the more extreme the decrease in the numbers of males fathered.


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