When two fetuses are carried simultaneously and delivered by a woman, it is termed a twin birth. Most multiple births are twins. In 1997, 94% of multiple births reported were twins. However, most of these twin births were fraternal or nonidentical twins.
Twins referred to as identical or paternal twins (also known as monozygotic or uniovular) are much less common than nonidentical twins. They are derived from a single fertilized egg. During fertilization, only one ovum (or egg) is impregnated by only one sperm. Afterwards, the egg divides into two separate embryos. Each embryo has the same chromosomes, they are mostly identical in appearance, and are always the same sex. In rare cases, the division is not complete and results in conjoined twins. The degree of union may be great or slight, and the twins may be conjoined at any part of their bodies. The majority (70%) of conjoined twins are females. Most conjoined twins do not survive long after birth, although there are some who have lived as long as 63 years.
The term identical twins is somewhat of a misnomer and is not generally used in professional medical language; monozygotic is more often the term used. The main difficulty is that identical twins are not necessarily identical. Frequently one twin has a physical feature (i.e., mole) that the other does not, for example. More dramatically, some identical twins are “mirror images” of each other, even though they have the same DNA. In such rare cases, one twin’s heart is on the left side (normal) and one twin’s heart is on the right side (mirror image). The parents of twins can usually tell them apart by subtle visual cues. While their fingerprints are generally similar, they are not identical. Because physical appearance is influenced by environmental factors and not just genetics, identical twins can actually look very different. The only true means of determining whether or not same-sex twins are fraternal or paternal is through DNA testing.
The rate for identical, or monozygotic, multiples is random and universal; it is the same in all populations regardless of race, heredity, or other factors, and it has remained constant over time. The chance of having identical twins is about 1 in 285. Improvements in the early detection of twin gestations through the use of ultrasound techniques, improved prenatal care, and medical advances in the care of neonates have increased the survival rate of twins.
All multiple pregnancies are automatically considered “high risk.” Twins are eight times more likely than singletons to be born at a weight of less than 1,500 grams or very low birth-weight (VLBW). More than one half of all twins are born with low birth-weight (LBW) (less than 2,500 grams). This compares with only 6% of singletons born with LBW. Additional risks for the infants include prematurity, underdeveloped lungs that can lead to respiratory distress syndrome (RDS), cerebral palsy, hearing or vision problems, developmental delays, or learning disabilities. Risks for the mother include preeclampsia (a rapid rise in blood pressure, protein in urine, and fluid retention), anemia (low red blood count), preterm bleeding, preterm labor, gestational diabetes, blood clots, and miscarriage. There are thought to be many more twin conceptions than births, as often one or both fetuses are lost during the first trimester to miscarriage or vanishing twin syndrome. As would be expected, families of twins face increases in financial burdens, marital stress, childcare issues, and a multitude of other adjustments.
Some identical twins have been known to switch places with each other in school to have their twin take tests for them, or to play tricks on others. An interesting part of many identical twins development is the emergence of idioglossia, or “twin language.” Identical twins also are thought to be able to read each other’s minds, and to develop a special bond unlike any other interpersonal relationship. There is great interest in researching the unique and intriguing aspects of identical twins.
- Bryan, (1995). Twins, triplets and more; Their nature, development and care. London: Multiple Birth Foundation. Center for the Study of Multiple Birth, http://www.Multiple Birth.com
- Keith, G., Oleszczuk, J. J., & Keith, D. M. (2000). Multiple gestation: Reflections on epidemiology, causes and consequences. International Journal of Fertility, 45(3), 206–214.
- Keith, G., Papiernik, E., Keith, D. M., & Luke, B. (Eds.). (1995). Multiple pregnancy—Epidemiology, gestation, and perinatal outcome (pp. 163–190). New York: Parthenon.
- Martin, J. , & Park, M. M. (1999). Trends in twin and triplet births: 1980–97. National Vital Statistic Reports (Vol. 47, No. 24). Hyattsville, MD: National Center for Health Statistics.
- National Organization of Mothers of Twins Clubs, http://www.nomotc.org
- Paternity Angel, http://www.paternityangel.com
- Twin Stuff, http://www.twinstuf.com
- Volpe, P. (1993). Biology and human concerns (4th ed.).Dubuque, IA: William C. Brown.