After the widespread introduction of hormonal fertility treatments in the 1960s, the rate of multiple births—twins, triplets, quadruplets, etc.—in humans was in comparative flux. Since the mid-1970s, however, twinning rates have steadily increased.
Fertility treatments affect only the occurrence of fraternal twins. Fraternal twins (or triplets, quadruplets, etc.) are the result of the fertilization of multiple ova during conception, producing multiple viable zygotes.
Hormonal fertility treatments make dizygotic (fraternal) multiple births more likely, as can factors of heredity and age. Women who conceive later in life are more likely to give birth to fraternal twins, as are certain ethnic groups. Generally speaking, African women experience the highest twinning rates; Asian women experience the lowest.
The incidence of identical (or monozygotic) twins—the result of a single fertilized ovum splitting into multiple zygotes—is relatively constant. Moreover, for the majority of mammals (human or otherwise), fraternal twins are far more likely to occur than identical twins.
However, one species of mammal invariably produces identical multiple births, and it almost always gives birth to quadruplets.
WHAT MAMMAL ROUTINELY PRODUCES IDENTICAL MULTIPLE OFFSPRING?
What species of mammal routinely produces monozygotic multiple births, almost always identical quadruplets?
The nine-banded armadillo, Dasypus novemcinctus, is unique among mammals in that it habitually produces identical quadruplet offspring, rather than the conventional fraternal litters common to many other mammal species. The nine-banded armadillo is the only species of armadillo native to North America, and its entire conception process is somewhat unusual.
North American nine-banded armadillos typically breed in July, but females don't truly become pregnant until November because fertilized ova delay implantation for three to four months. After implanting, zygotes always divide, typically into four genetically identical embryos that will be born in early spring.
While scientists are currently at a loss to explain the nine-banded armadillos' unique polyembryony, recent research suggests that you shouldn't handle these animals with any regularity. Research has shown that exposure to armadillos can increase your chances of contracting Hansen's disease—otherwise known as leprosy. In fact, the armadillo is the only animal besides human beings known to carry the disease.