Identical or monozygotic twins are formed from one fertilized egg (one ovum and one sperm) which splits after conception into two identical halves which develop separately. Each identical child (always the same sex) will have the same chromosones and many of the same physical attributes. Identical twins will also share the same blood type, eye and hair color. According to research done by the National Organization of the Mothers of Twins Clubs, slightly more than half of all twins born are male and approximately one in four pairs of identical twins are mirror-image twins. About 1/3 of all twin births are identical.

Fraternal or dizygotic twins are formed from two fertilized eggs (two ovums and two sperms) and can result in two siblings of the same or opposite sexes. Your likelihood of having fraternal twins is dependent upon the woman carrying a fraternal twin gene and can also be affected by heredity, race, marital age and number of children previously borne. One-third of all twin births result in same sex fraternal twins and one-third are different sex fraternal twins.

And despite there being just the two types of twins, there is a growing scientific belief of a possible third type of twinning, Polar-Body Twinning. This type (although not fully accepted as an "official" type of twins by scientists yet) theorizes that twins aren't exactly identical and aren't exactly fraternal, but half identical/half fraternal. It can happen when the oocyte (primary egg cell) divides twice on its way to maturity, yielding egg cells and polar bodies at different stages along the way. Usually these smaller polar bodies don't play a meaningful role in reproduction. But now scientists believe that some twins could be the result of two of these egg cells and larger polar bodies being pregnated by two sperm. The twins would most likely share all of their mother's genes but only half of their father's genes.

Rates of twins vary by society but according to the latest CDC National Vital Statistics Report (2002), the approximate twin birth rate (number of twin births to total live births) in the U.S. is about 31.1 per 1,000 live births. Even though twins and other multiples are more common today than they once were, you're still pretty unique if you are born as a twin--only about one out of every 32.1 live babies born is a twin. These odds used to be much higher, when the twin authors of this essay were born in 1964, there were 41,259 sets of twins born in the United States, with 10.4 per 1,000 births meaning about one of every 96 babies in 1964 was a twin.

As the tables below show, the numbers of twin births has risen slightly in the United States each year from 1996 to 2002, reaching a record-number 125,134 twins born in the most recent year for which data is available. The numbers of triplet, quadruplet and quints-and-higher births has dropped slightly in three of the past four years. The total number of births in the U.S. was virtually unchanged in 2002 from the previous year (4,021,726 births in 2002 vs. 4,025,933 births in 2001) but the overall birth rate (13.9 births per 1,000 total population) did decline 1% in 2002 to another all-time record low in the United States.

With the recent overall decrease in the American birth rate coupled with the record number of twin births, it meant back in 2001 for the first time in U.S. recorded history that more than 3 percent of all births were twins. That number rose slightly in 2002 and now 3.1 percent.

Since 1990, the twin birth rate has now risen 38% (from 22.6 to 31.1 per 1,000 live births), while rising 65% since 1980 (from 18.9 to 31.1 per 1,000 live births). As mentioned, the rate for triplets and other higher-order multiples has slowed down slightly from its 1998 high of 193.5, but it has still risen close to 400 percent (from 37.0 to 184.0 per 100,000 live births) since 1980.

Other data released by the National Center for Health Statistics regarding the numbers of multiples born in the U.S. in 2002 indicates that births of multiples is still greatly impacted by the age of the mother and continues to rise each year. Between 2001 and 2002, there was another increase of 10% of women ages 40-and-over of giving birth to twins. Since 1990, the twin birth rate has soared dramatically among women age 45-49 (from 23.8 to 189.7 per 1,000). In 2002, a remarkable 18.9 percent of all births to women age 45-49 were twins.

Other countries also show increases in twinning. A report by M2 Communications reported that twice as many twins were born in Norway in 1999 as were born in that country in 1986. Latest figures in that Scandanavian country show there were 1031 pairs of twins born in 1999, 21 sets of triplets and one set of quadruplets. According to the book The Two-Headed Boy & Other Medical Marvels by Jan Bonderon (Cornell University Press, 2000), there were 112 sets of quadruplets, 16 sets of quintuplets, six sets of sextuplets and 1 set of septuplets born in England and Wales between 1952-88. Contrast that with numbers from the Office of National Statistics (UK) in 1999 which shows 8,636 sets of twins, 267 sets of triplets and four sets of quads-or-higher born in either England or Wales out of a total of 622,000 live births. In the United Kingdom, there were eight sets of quadruplets or higher, 323 sets of triplets and 9,893 pairs of twins born in 1998.

The rates in Australia have also risen slightly. In 1978, about 1% of all pregnancies in Australia resulted in multiple births (according to the Australian Bureau of Statistics); in 1998 that number had increased to 1.5%. In 1998, there were 245,898 total births in Australia with 3700 multiple births (sets of multiples). In the region of New South Wales, there were 1218 multiple births (1191 sets of twins and 27 sets of triplets) among the 85,449 total births in 1998.

While it is difficult to see historical data on the chances of having twins and higher-order multiples for the early 1900's or other centuries, the book 'Multiple Human Births' by Horatio Hackett Newman (Doubleday, 1940) does make reference to some numbers. One study detailed in that book is by an American biologist named Greulich, who looked at birth records of 21 (undisclosed) countries between 1915-25. That data showed that among 120,000,000 births during that period, there were 1,408,912 twin births, 15,738 triplet births and 179 quadruplet births. This equates out to a ratio of 1:85.2 for twins, 1 to (87.2)² for triplets and 1 to (87.5)³ for quadruplets.

As women are giving birth at older ages, there is much more of a likelihood of these births resulting in multiples. In 1990, there were 39 sets of twins born in the United States to women age 45-49. In 1997, there were 444. Between 1980-1996, there were no recorded cases of a woman age 50-54 giving birth to either twins or triplets in the U.S., in 1997, there were 50 such births of twins and 13 sets of triplets. In 2001, there were 912 twins and 111 triplets-or-higher born to women between the ages of 45-54.

But the increase in fertility treatments continues to be the major reason that the birth rate of multiples has risen so dramatically. A recent study estimated that 80% of all triplet births was the result of fertility treatments.

The increase in the births of multiples has also had a dramatic impact on lower birthweights and the increase in preterm infants. Multiple births are at at much greater risk of being born at a lower birthweight and prior to full term, two factors which also lead to greater infant mortality rates. Statistically, twins are 10 times and triplets-plus are 34 times more likely than singletons to be born at weight of less than 1,500 grams (very low birthweight). For 2002, 58.2 percent of twins and 92.4 percent of triplets were born preterm (less than 37 weeks gestation) and 55.4 percent of twins and 94.4 percent of triplets with low bodyweight (less than 2,500 grams).

In 2002, the average twin pregnancy gestation in the United States was 35.3 weeks and triplets average gestation was 32.0 weeks compared to 38.8 weeks for singletons. Although the infant mortality rates declined by about a third between 1990-2000, the risk of early death among twins continues to be nearly five times higher than singletons.

sources: Annual CDC U.S. Vital Statistics & Trends in Twin and Triplet Births: 1980-98, by Joyce A. Martin, M.P.H. and Melissa M.