Identical twins occur when one egg is fertilized by one sperm and then divides producing two embryos. Identical twins are virtually genetically identical and are always the same sex. They are not necessarily “mirror image” identical, but usually look very similar physically. Their fingerprints, for example, are not identical. The frequency of identical twins is about 4 per 1000 births. Scientists are still unsure as to exactly why an egg splits.

Fraternal twins are the result of two eggs being fertilized by two sperm. Fraternal twins may be boy-boy, girl-girl, or boy-girl. There is no set pattern to how they will look when compared to each other. Think of them as siblings who happened to be born at the same time. Traditionally, fraternal twinning is hereditary from mother to daughter and frequently skips generations. However, with the increasing use of fertility treatments fraternal twins and higher order multiples are now being born at five times the rate of fraternals in the 1970’s, about 22 per 1000 births. It is possible that fraternal twins can be conceived up to two weeks apart

Twins occur naturally in 1 out of 80 births; triplets in 1 out of 8000; quadruplets in 1 out of 700,000; and quintuplets in 1 out of 65,610,000. The average gestation for a twin pregnancy is 36 weeks and in less than half of those pregnancies both babies are born head first. One third of twin pregnancies are identical, one third fraternal same sex, and one third boy-girl

...The term "twin" derives from the ancient German word "twine" meaning "two together"?

...multiple births occur naturally in less than 1% of all births?

...once you have had multiples, your chances of having multiples again are 4 times better?

...a woman is more likely to bear fraternal twins if she herself is a fraternal twin?

...in the United States, women bear a set of twins once in every 80 births?

...in Nigeria, women bear a set of twins once in 22 births?

...in Japan, women bear a set of twins once in 155 births?

...triplets naturally occur only once in 9,400 births?

...the term "Siamese twin" originated with Chang & Eng Bunker, born in Siam in 1811? They were conjoined at the chest and eventually married sisters and before their death at age 63 fathered a total of 22 children!

...the first quintuplets (5) to ever reach adulthood were the famous Dionne Quints born in Canada in 1934? Did you also know they are believed to be the only identical quintuplets?

...the largest number of children ever born to one woman was 69? The woman was pregnant 27 times in a period of 40 years, giving birth to 16 sets of twins, 7 sets of triplets, and 4 sets of quadruplets !!!

...the highest order of multiple birth to date is nonuplets (9), but only 7 of the infants survived initally, and 2 more died weeks later.

...in 1983 medical history was made in England when sextuplets (6) were born and all survived?

...in 1997 medical history was made in the United States with the arrival of the first ever surviving septuplets (7)?