The hand you prefer to use as a 10-week-old foetus is the hand you will favour for the rest of your life, research suggests.
Scans have shown foetuses sucking their thumbs
A team from Belfast's Queen's University studied foetuses in the womb, and after birth.
Their findings challenge the widely held view that a child does not develop left or right-handedness until it is at least three years old.
The research is reported by New Scientist magazine.
In one part of their study, the Belfast team identified 60 foetuses who sucked their right thumb in the womb, and 15 who sucked their left thumb.
When the babies were examined again between the ages of 10 and 12, the researchers found all 60 of the right thumb suckers were now right-handed.
Two-thirds of the left thumb suckers were left handed, the rest apparently having switched their dominant hand.
In another investigation the researchers found that nine out of 10 foetuses at just 15 weeks old preferred to suck their right thumb - mirroring the percentage of right and left-handed people in the general population.
They also produced evidence suggesting foetuses begin to favour one hand over another at an even earlier stage.
At ten weeks old foetuses are too young to have begun to suck their thumb, but they do wave their hands about.
The Belfast team found the majority tend to wave their right arm more than their left.
Lead researcher Professor Peter Hepper said at 10 weeks movements are not under brain control or conscious control.
Nervous connections to the body from the brain are not thought to start developing until around 20 weeks' gestation.
Instead, Professor Hepper believes the arm movements are probably the result of local reflexes involving the spinal cord.
One side may be favoured over the other, because it develops slightly faster, he believes.
Most experts believe that whether a person is left or right handed is a reflection how their brain develops.
The brain is effectively a double organ with two sides, each taking control of different functions.
But the Belfast team say their scans challenge this theory because they show that handedness appears long before the brain has any control over movement.
Professor Hepper speculates the reverse may be true: the foetus's body movements may somehow influence the way the brain develops, rather than the other way round.
He points out that the nerve connections from the body to the brain develop before
the connections that allow the brain to control the body's movement.
Professor Hepper said: "Previous studies have linked our strong tendency to be right or left handed with brain asymmetries.
"The argument is that because the brain has specific functions associated with its left and right sides - for instance, speech centres are usually in the left brain, music and mathematics on the right - this somehow makes us unevenly one-handed.
"But our new work looking in the womb shows that foetuses prefer to use one arm or the other long before these movements come under brain control.
"This early activity is probably under muscle control, or controlled by nerves in the spinal cord.
"So is it body movement that shapes our brain development, not the other way round?"
Dr Stephen Wilson, a developmental biologist at University College London, is sceptical.
He said the movements seen in the womb are not necessarily an indication of which side of the body was likely to become dominant.
He said: "The movements you see in a foetus don't have to be influencing brain asymmetries."
Details of the research were presented at the Forum of European Neuroscience in Lisbon.