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It’s not that hip to be square!
Karina Karenina and Andrey Giljov of the Department of Vertebrate Zoology at Saint Petersburg State University, Russia, are interested in, when babies are held by adults, which side of the body the adult prefers, which happens to be, particularly in the case of women, on the left.
Think about the last time you carried a baby to the change table; chances are that, if you’re a female, it was on your left. Researchers have known for quite some time that this is the case, but not why it is the case. It has been shown that more than three out of four women and girls show a preference for holding infants, or dolls, on the left side of their body. Women prefer to hold babies on there left. They also know that males are not so inclined, such that, whilst fathers prefer to hold an infant on their left, men who are not fathers are far more likely to not to have a preferred side, at all.
This left side preference phenomenon is not limited to humans. These Russian researchers studied a number of different mammals including wild horses, reindeer, walruses, kangaroos and whales, and concluded that not only did mothers keep their babies to the left, but when able to make their own choices, infants prefer to be to the left of their mothers.
Apparently, when a baby is held on the left, the right side of the mother’s brain is stimulated, which helps her bond with her baby. 
The research concluded that, when the left side of the body is touched the right side of the brain, the side that is responsible for such things as communicating and interpreting emotional signals, responds. It follows that, when a baby is held on the left, the mother is better able to interpret the infant’s physical and emotional cues.
It should almost go without saying that there is something important in this for educators who work with babies: if you feel that you prefer to hold a baby on your left hip, it’s perfectly natural, it’s good for your relationship with the infant and the baby prefers it as well. For those who don’t favour the left, maybe they should try it and see if it enhances the bonding relationship with the baby – that is, of course, if there is no sensible reason to keep baby on the right, such as a muscular or skeletal problem.
Reference: Karenina, K., Giljov, A., Ingram, J., Rowntree, V. J. & Malashichev, Y. Lateralization of mother–infant interactions in a diverse range of mammal species. Nat. Ecol. Evol. 1, 0030 (2017).