Macrostomia is an enlargement of the mouth due to a rare congenital defect (birth defect). Macrostomia is estimated to occur once in every 60,000 to 300,000 births. It is caused by a failure of the facial bones to fuse properly during embryonic development, leading to a facial cleft. Other abnormalities of the soft tissues or bones of the face can also be present in affected infants. Unilateral macrostomia refers to the presence of the defect on one side of the mouth, while bilateral macrostomia refers to the presence of the abnormality on both sides.
Simonse, Eva, et al. "Unilateral macrostomia in the newborn: a rare congenital anomaly of the oral commissure." BMJ Case Rep 2016: bcr2016216752. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5093832/>.