Noun
concealed ovulation (uncountable)
(biology) A form of ovulation, in certain species, where the female shows no perceptible signals indicating that she is fertile.
Unlike chimpanzees, whose females have bright red swollen buttocks when ovulating, females in humans evolved to have concealed ovulation, and do not have any perceptible signs that indicate their fertility.
Concealed ovulation and menopause in women both also occur in a few other primates however, but are uncommon in other species. Source: Internet