When it comes to pushing down on baby’s chin to get a wider latch, sometimes it works but often it doesn’t… Many a baby will naturally resist the downward pressure and close his mouth further if you push down on his chin.
I’ve been told that occupational and physical therapists learn that if you “force” a muscle to go the way you want it to, it will tend to go the opposite way. So if baby is clenching his jaw, it can be helpful to do the opposite of what you want him to do – gently push baby’s jaw closed (before nursing, of course!) then quickly remove your finger to allow him to open his mouth. You might try doing this several times before nursing.
If you do try pushing down on baby’s chin, use a very gentle pressure back and down (more like simply resting your finger in the “notch” between baby’s lower lip and chin). Do this after baby is already latched on to widen the latch a bit. If baby resists this, then stop and try something else.
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See also: Latching and Positioning Resources