If one looks up Head Defect in a well-written dictionary of cat breeding* one can read:

”Head-deformation on the Burmese breed (meningoencefalocele).
This recessive defect is a form of incomplete split, which results in the area around the upper chin is formed in a double edition. The rest of the head above the chin is thus deformed. Eyes and ears are malformed and the front part of the scull is open. The kittens are born alive, but do not survive. Cats carrying the defect may have a deviant headshape, split lip, dermoids (skin alterations) or disformation in one or both eyelids.”

Further you can read explanations about inheritable diseases :

“Diseases caused by a recessive gene. These diseases are only apparent when the gene occurs in a double setting, meaning in a homozygous state. Heterozygous individuals are not affected by the disease but they can hereditary it to its offspring. These individuals are called “healthy carriers”. Inbreeding causes the number of affected cats to rise.
To prevent the diseases, knowledge of the breeding cats pedigree is crucial. Parents to the affected kittens are excluded from breeding”.

When one have read the text above it is not hard to understand why the number of Bombay breeders has decreased over the last 10 years in the US. The kittens die and the queen has often a hard labour and has to be neutered. That decreases the number of Breeding animals. Therefore the breed is nowadays “exclusively rare”.

* ”Practical guide for cat breeding” by Élise Malandain, Susan Little, Grégory Casseleux, Lorraine Shelton, Pascale Pibot och Bernard-Marie Paragon.