What is the word that describes the human female breast’s shape?
Mammillar and Mamillary are (apparently) used to refer to things that are breast or nipple shaped (but are not breasts or nipples).
The non-OED Oxford dictionaries (for mammillary):
Shaped like or resembling a breast or nipple
The free dictionary (for mammillar):
(Biology) resembling a breast or nipple
The term you're looking for is breast-shaped.
Examples:
Winter Park may buy breast-shaped building on Lee Road (Orlando Sentinel, 2014)
During the colonial and early American periods, the mountain was known as "Mamelle" mountain. "Mamelle" is a name commonly applied in the French-speaking parts of the world to a breast or any breast-shaped hill. (Encyclopedia of Arkansas History & Culture, 2016)
Is the breast-shaped shadow on St. Mary’s Cathedral an accident or a clever prank? (KALW, 2018)