Our baby is now close to 6 weeks old and likes to turn the head to the left - while being held in a carrier, sleeping on my chest, or while lying on the back and looking around.
A friend said that this habit could potentially cause the development of an asymmetric posture / skeleton / muscles therefore we should encourage turning to the right to be even.
Is this a valid concern? How could we encourage turning to the other side?
If you are afraid this may be causing issues, just place something interesting (a toy or yourself) on the other side. You can then see if she prefers the other side or if you misjudged that and the most interesting things just happened to be on the left.
You can do that with anything:
This helped wonders with our child when it seemed to prefer one side. If that does not seem to solve the problem for you, talk to your pediatrician about your concerns. Mostly, it should not be an issue and if it is, he may help you.
Disclaimer: I'm in no way an expert on this.
There is some evidence that the shape of the head can be influenced by sleeping positions (see, for example, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3116745/ and http://www.uchicagokidshospital.org/specialties/neurosurgery/patient-guides/head-shape/). The back of the head seems to be especially soft, so if baby sleeps on the back and the same spot on the back of the head keeps touching the bed, that spot might flatten a bit because the skull is still fairly soft in the first months of life. In order to counter that, you're supposed to make sure that the head isn't always in the same position when the baby sleeps, and not keep them in car seats etc for longer than necessary.
However, I very much doubt that this usually leads to serious asymmetries that might lead to problems later. First, it's mostly an aesthetic thing. Second, if serious asymmetries were likely, wouldn't the world be full of people having such problems? Wouldn't we all walk through the streets thinking to ourselves "jeez, what an ugly asymmetric head that guy over there has" all the time?
You can't watch your baby sleep the whole night. When it prefers one side and turns back an hour after you go to bed, how do you fix that? What happens to all the babies of people who have never heard about the shape of the skull being influenced by the sleep position of the baby? Why aren't these babies all growing up with horribly deformed heads?
Personal obervation does suggest that slight deformations at the back of the skull are a possibility. With my oldest child, we took the advice to keep turning his head very seriously, because we were first-time parents and very concerned about basically everything. He happens to have a very nicely shaped head. With the following babies, we didn't have the time to do that. They do have skulls that are somewhat flatter at the back. That might just be coincidence, or a result of our "negligence". Whatever the cause, it's never been a problem.
could potentially cause the development of an asymmetric posture / skeleton / muscles
I think that everything besides the shape of the head is myth.
As Anthony comments, there can be serious consequences of positional skull deformation (although, as I've said, that's rare - usually it's harmless). This also includes skull asymmetries when a baby keeps putting the head on one side (which I wasn't originally aware of). See my first link or his answer for further details. So keeping an eye on your baby's head shape is certainly a good idea, so you can involve a doctor as early as possible if you notice developing asymmetries or notable flattening of the back of the skull.
Obligatory disclaimer that I am not a doctor or expert and you take a risk with my advice blah blah blah
My personal experience: 2 out of my 3 kids strongly prefered looking to one side when they were newborns. It's pretty common. We took them to a family chiropractor to get them adjusted and it fixed them each time (we had to do it 2 or 3 times each kid). Once they reached about 4 to 5 months old it wasn't an issue anymore. The Chiro's reasoning was that the neck muscles and vertebrae undergo a lot of stress during a delivery.
My sister's son favored one side strongly, so she tried to adjust his sleeping position, which ended up causing him to favor sleeping on his back. This went unnoticed for several months, which caused the back of his head to become very flat. That condition is called Brachycephaly. It completely screwed up his vision and now he has to wear a very expensive helmet to try to reshape his head. His vision is most likely impaired for life, but the skull shape has slowly become more normal. It's not as extreme of an example as you would like to think unfortunately, but it's also not common either.
Here's a page with some more info.
This type of thing has happened for as long as people have been people, but it's more common today than 500 years ago because we are more aware of other issues with how infants sleep. As a result, people are starting to use devices that restrain movement at night, which often prevents infants from switching sides and naturally negating the effects of laying too long on a single head surface.
Lastly, many adults have misshapen heads and don't know it because either their hair obscures their head shape or it's not very pronounced.
