In Utero Handedness

The uterus is a roughly egg shaped object, and in the fundus of a growing pregnant uterus the shape can almost be spherical. We refer to it as a sac, not a box or rectangle. So on the left and the right is generally thought of as no different from the top and bottom, or anterior and posterior. But lots of references to the right and left have started popping up in the medical literature, not just for right handed and left handed fetuses (which hand was your baby sucking on it's thumb), but for things like babies staying on the right or the left of the uterus. In a recent retrospective studying of still images one author concluded that at 6 week ultrasound 97% of the baby boys had placentas on the left of the uterus, the girls had placentas on their left.   the doc that did the study called the method after himself, Ramzi's method of early prediction of fetal gender. We always caution moms to not rely on a single study, no matter how well done, so we will see if this is confirmed. Biologically it's not clear what would be the mechanism or the reason for this. And I didn't see if girl boy twins directed their placental traffic in that way, so I wasn't clear. But another story of left and right has been developing, and it has to do with the shape of the skull. Head flattening towards the end of pregnancy is more common as the baby's position both gets fixed and molded into shape against the pelvis. So right sided head flattening is a bit more common. Apparently we care, because the back sleeping babies can exaggerate head asymmetries that even can affect face shape and features eventually. And if your baby is getting that flat shape, take to the water, it may help!

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