How Ultrasound Changed the Human Sex Ratio

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A technology originally developed for maritime navigation and detection has become the dominant method for sex selection
By Mara Hvistendahl | June 11, 2011

Editor's Note: The following is an excerpt from Mara Hvistendahl's book, Unnatural Selection: Choosing Boys over Girls and the Consequences of a World Full of Men.

The technology that ultimately became the dominant method of sex selection around the world began as a tool for navigation. The story of ultrasound dates to 1794, when an Italian biologist curious about how bats find their way in the dark discovered sonar, or the fact that distance can be determined by bouncing sound waves off a faraway object and measuring how long it takes for the waves to ricochet back. Centuries later, when the growing prowess of German submarines during World War I convinced the Allies that to win the war they needed a way to navigate underwater, scientists put sonar to use. The American, British, and French governments jointly funded research into the phenomenon. The effort succeeded, and by 1918 the Allies were using acoustic echoes to correctly pinpoint the location of German U-boats.

After the war, doctors guessed sonar might have medical applications as well. They first used ultrasound in surgery, where it turned out sound waves could heat and destroy tissue, making them helpful for everything from treating ulcers to performing craniotomies. Then in 1949 a chemist stationed at the Naval Medical Research Institute in Bethesda, Maryland, employed the new technology to locate gallstones in dogs, and ultrasound became a diagnostic tool as well. Physicians began navigating the human body as World War I submarines had navigated dark waters, bouncing sound waves off the internal organs.

Ultrasound proved surprisingly versatile. It could clean teeth, treat cysts, and dissolve kidney stones. It may have been with one of these applications in sight that in 1959 Scottish obstetrician Ian Donald used the new technology on a woman who happened to be pregnant and noticed that the fetus returned echoes as well.

Back then, ultrasound offered the simple promise of learning more about a pregnancy. Doctors could not perform x-ray exams on pregnant women because of the risk of damaging the fetus, so Donald’s discovery raised the prospect of an alternative form of prenatal imaging, giving physicians hope of monitoring high-risk pregnancies. If Donald suspected that knowledge would translate into fetal selection and subtraction, he probably envisioned women attempting to avoid debilitating sex-linked diseases like hemophilia. (When the first sex-selective abortions had been performed in Denmark using amniocentesis four years earlier, indeed, they were done for that reason --and discriminated against males as a result.) He could have hardly guessed that ultrasound would one day contribute to a sex ratio imbalance involving over 160 million "missing" females in Asia and elsewhere.

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