Menstruation (mammal)
Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.
Pygmy sperm whale
Although firm details concerning pygmy sperm whale reproduction are limited, they are believed to mate from April to September in the Southern hemisphere and March to August in the Northern hemisphere. These whales become sexually mature at age 4-5, and like virtually all mammals, are iteroparous (reproducing many times during their life). Once a female whale is impregnated, the average gestation period lasts nine to eleven months and, unusually for cetaceans, the female gives birth to a single calf head-first. Newborn calves are about in length, weighing 50 kg, and are weaned at around one year of age. They are believed to live up to age 23.