When problems occur in the growth process of the face, the face parts may not fuse causing a structural deficit.
At about 7 weeks after fertilization, lumpy growths have appeared at the head of the growing cell mass that is the baby.
They expand rapidly from above and from the sides splitting into several segments.
The segments quickly join and fuse, like a well machined puzzle, to form the face.
Hence, the upper lip and gum ridge, for example, is formed from three different segments.
If something interferes with the timing of this process, the face may fail to join or fuse.
In this case the child may be born with a cleft lip (right, left or both)--or worse. This occurs in 1 of 1000 births.
Regardless, at this point, baby has no roof to his mouth. The next growths to occur appear on each side of the oral cavity.
NOTES: More information on cleft lip and palate