Researchers find a connection between left-handedness and low birth weight

A team of researchers from Finland, the Netherlands and Japan has found a connection between left-handedness and low baby birth weight. In their paper published in Proceedings of the National Academy of Sciences, the group describes their study of two large birth datasets and what they found.

Prior research has suggested a possible link between left-handedness and low birth weight. There have also been studies that suggest handedness develops before birth—fetuses as young as 10 weeks have been seen showing a preference for one thumb or the other on sonograms. In this new effort, the researchers sought to gather more evidence regarding birth weight and handedness. To that end, they obtained birth datasets—one from the Netherlands, the other from Japan. Noting that prior research has shown that left-handedness is more common in twins, and even more common in triplets, the researchers chose to focus their study exclusively on triplets. After filtering for triplets born at 33 weeks, the researchers pared down the datasets to 947 triplets in the one from the Netherlands and 1,305 from the one in Japan.

The researchers found that the average birth weight between left and right handers differed—for the Netherlands it was 1.79 kg on average for lefties, compared to 1.903 kg for right-handers. The results for the Japanese babies were similar—1.599 kg for lefties and 1.727 kg for right-handers. They noted also that the age of the mother did not seem to make any noticeable difference, nor did birth order. The researchers also discovered that those babies born left-handed were slower to reach major motor-skill milestones such as sitting, crawling and standing.

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