A study of thumb-sucking among fetuses shows the tendency to be left-handed may start in the womb, researchers reported this week.

About 10 percent of the adult population is left-handed, and scientists have long debated whether that preference stems from genetic factors or is caused by some kind of injury, perhaps occurring during birth. Now, Irish researchers said they have found evidence some fetuses display a preference for sucking their left thumbs as early as 15 weeks after conception.Using ultrasound to examine the fetuses of 224 women who had normal pregnancies, the researchers found 12 of the 224 fetuses - or about 5.4 percent - favored their left thumb. The positions of the fetus in the womb had no effect on thumb preference.

Follow-up observations of 17 fetuses indicated "preference for a particular thumb is maintained during pregnancy," the researchers said.

In a letter published in the British journal Nature, Peter Hepper and his colleagues from Queen's University of Belfast said their study "demonstrates for the first time the existence of behavioral asymmetries before birth."

Furthermore, the work raises questions about theories that left-handedness is a result of injury and "supports genetic explanations of handedness," the researchers said.

Dr. Stanley Coren, a University of British Columbia psychologist who has studied left-handedness for 20 years, called the findings "a cute, little study" that provides the first direct observations hinting that handedness may exist prior to birth. However, he said he thinks the conclusions drawn by Hepper's team are too broad.

"My major concerns are whether thumb-sucking really grows into handedness and the fact they have so few left-handers," said Coren, noting that he smokes his pipe with his left hand even though he is right-handed.

The Canadian researcher said about 15 percent of young children are left-handed, compared to the 5 percent of fetuses that sucked their left thumb. Stresses during pregnancy or difficult deliveries may account for that difference, he added.

In their letter, the Irish researchers speculated that while hand preference may initially arise from genetically determined variations in neuromuscular development, fetal thumb-sucking may reinforce those differences.