TWINS BY THE NUMBERS
In the United States 2004 Data*
Number of multiples born during the year 139,494
Number of twins 132,219
Number of triplets 6,750
Number of quadruplets 439
Number of quintuplets and higher order multiples 86
(These numbers refer to individual babies, not sets.)
Total number of U.S. births 4,112,052
Percentage of all births that are twins 3.22%
Birth rate for twins 32 per 1,000
Birth rate for triplets, quads, quints, etc. 2 per 1,000
*Source: CDC, Monthly Vital Statistics Report (2004)
There are 125 million people in the world who are multiples (twins, triplets and higher-order multiples)
The National Center for Health Statistics reported that the US birth rate for multiples rose 28% between 1990 and 1998 to 29 births per 1000. The rate rose still more to 32 births per 1000 in 2004. The triplet rate and higher-order multiples rate more than doubled in 1999 for several reasons; 1) women waiting longer to have babies; 2) increased use of fertility drugs; and 3) in vitro fertilization. The birth rate for triplets or more declined slightly between 2001 and 2004.
FAMOUS CELEBRITY TWINS
(pro baseball player)
(actor, Days of Our Lives)
(actor, Law & Order)
(former pro football player)
What are fraternal (dizigotic) and identical (monozygotic) twins?
There are two main categories of multiples—fraternal and identical. Similarity in looks does NOT determine whether twins are identical versus fraternal.
What are fraternal twins?
Fraternal twins are the result of two separate eggs (ovum, or zugotes) becoming fertilized by two separate sperm, resulting in two completely distinct pregnancies in the womb at the same time. They are known as non-identical or dizygotic twins. Dizygotic twins account for about 2/3rds of all twins.
What are identical twins?
Identical, or monozygotic, twins occur when a single fertilized egg splits into two embryosaround the time the fertilized egg is becoming implanted in the womb (usually between the fourth and 12th day after fertilization). Monozygotic twins make up 1/3rd of all twins.
How do you tell look-alike fraternals
from true identical twins?
A DNA test is required to determine with a high degree of certainty whether twins are identical or not. Blood-typing is a less certain realiable but sometimes used. Sometimes physicians will predict whether twins are identical based upon an examination of the placenta(s), but this is much less accurate.
What are mirror-image twins?
Identical twins, always monozygotic, can be different in one fascinating way—they may exhibit mirror-image features or behaviors. For example, they may have opposite hair whorls or opposite dominant hands or even mirror-image fingerprints. Mirror-imaging is related to the timing of the splitting of the fertilized egg. As many as 25% of all identical twins exhibit some kind of mirror-imaging.
Are all same-sex twins (boy/boy or girl/girl) identical (monozygotic)?
No, definitely not. This is tricky—onozygotic/identical twins are nearly always the same sex either boy/boy or girl/girl. Until very recently, in fact, experts thought monozygotic/identical twins had to be same-sex. But now we know that there are certain rare instances* when monozygotic/identical twins can actually be different-sex individuals; when this has occurred and been documented, the twins are genetically identical in every way other than a slight chromosomal difference.
Roughly one-third of all twins are boy/boy sets; another third are girl/girl sets; and the remaining one-third, roughly speaking, are mixed-sex boy/girl twins.
But make no mistake, not every set of boy/boy or girl/girl twins is monozygotic/identical. In fact, many same-sex sets of twins are dizygotic/fraternal twins. Roughly two-thirds of all twins in a same-sex set are dizygotic/fraternal. They sometimes look very much alike, but their physical appearances are not what makes a set of twins monozygotic/identical at all!
Very often, same-sex twins look very different, indeed, sometimes right from birth. They all have different-color hair and eyes, be of different height and have different physical builds, and display very different mannerisms.
This leaves about one-third of all same-sex sets of twins who are monozygotic/identical. The same percentages are thought to hold true for monozygotic/identical sets of girls and boys.
When it comes to triplets, quads, quints, and even larger higher-order multiples, the vast majority of these sets include at least one pair of monozygotic/identical twins, with one or more dizygotics/fraternals included in the set.
*The rare instance in which a set of boy/girl monozygotic/identical twins occurs is the result of Turner's Syndrome, in which both individuals are actually XY boys, but one child loses the Y chromosome, yielding a baby who is XO. One twin appears externally to be a girl, but grows up infertile, short, and usually with a couple of other recognizable physical characteristics. Miscarriages occur frequently in such pregnancies, which is why these sets of twins are rare.
Basic Types of Conjoined Twins
Thoracopagus: Ventral or frontal union at the chest, often with a shared heart; most common form of conjoined twins; about 35% of all conjoined twins.
Omphalopagus: Ventral or frontal union at the abdomen, often with shared liver tissue; the highest survival rate; about 30% of all conjoined twins.
Pyopagus: Dorsal or rear union at pelvis; about 19% of all conjoined twins; never involves the heart or umbilicus.
Ischiopagus: Ventral or frontal union at the pelvis, often with shared intestines, bladders, genitals and kidneys; about 6% of all conjoined twins.
Parapagus: Lateral or side union with a variety of third and fourth limbs; conjoinment extends a variable distance upward; about 5% of all conjoined twins.
Craniopagus: Dorsal or rear union at the head; only about 2% of all conjoined twins; never involves the heart or umbilicus.
Rachipagus: Dorsal or rear union at the spine.
Cephalopagus: Ventral or frontal union including the head and chest; two faces on the opposite sides of the head; do not survive; extremely rare.
Dicephalus: This refers to one body and two heads.
Famous Parents of Twins
George W. Bush
(former U.S. Secretary of State)
Ingrid Bergman and Roberto Rossellini
(actress and director)
Corbin Bernsen and Amanda Pays
David Birney and Meredith Baxter
Kerry Kennedy Cuomo
(lawyer, daughter RFK)
(Former Gov. of Michigan)
and Andre Previn
(actress and musician)
Michael J. Fox and Tracy Pollan
(former King of Jordan)
and Garry Trudeau
(journalist and cartoonist)
Holly Robinson Peete
(singer, The Pointer Sisters)
(former U.S. Vice President)
(former Prime Minister of Great Britain)
Richard Thomas —
Mary Alice Williams