Twins

multiple birth

multiple birth, bringing forth of more than one offspring at birth. Although many smaller mammals bear several young at a time, multiple births are relatively uncommon in humans and other primates. Twinning, the process that leads to the production of more than one offspring, results in twins, and with decreasing frequency, triplets, quadruplets, quintuplets, sextuplets, septuplets, and octuplets.

In the one-egg, or identical, type of twinning, a single fertilized ovum divides to form two complete organisms. Such twins are always of the same sex, are usually extraordinarily similar in physical appearance, and have identical blood-group types. Twinning to form one-egg identical twins usually takes place early in pregnancy. If considerable development has taken place before the twinning occurs, there may be an incomplete separation of the two embryos resulting in conjoined twins. Despite their similarities, identical twins have epigenetic differences that influence how their common DNA is used and expressed.

Fraternal twins, which are more common than identical twins, are those that develop from two separate ova, each fertilized by a sperm. Fraternal twins may or may not be of the same sex and need not resemble each other more than do any other two offspring of the same parents. In the United States twins occur once in approximately 40 births. In rare cases, non-identical embryos can fuse in the womb to produce a condition called chimerism, in which some of the individual's cells come from one of the embryos and the rest of the cells come from the other, genetically distinct embryo.

The incidence of multiple-egg births is in part genetically determined, varying according to race and family tendencies; and it is also influenced by external factors, i.e., the incidence increases with increasing age of the mother and the number of children she has already borne. One-egg, or identical, twinning occurs with the same frequency in all women, regardless of race, age, or other factors. There is evidence from comparative biology that deleterious factors in the environment of the newly fertilized ovum, such as a reduction in oxygen, increase the likelihood of one-egg twinning. Fertility drugs such as clomiphene, which are used when the cause of infertility is lack of released ova, sometimes cause several ova to be released and fertilized simultaneously. The use of these drugs has led to a rise in the incidence of multiple births, including sextuplets, septuplets, and octuplets.

The Columbia Encyclopedia, 6th ed. Copyright© 2014, The Columbia University Press.

Selected full-text books and articles on this topic

Twins: A Practical and Emotional Guide to Parenting Twins
Katrina Bowman; Louise Ryan.
Allen & Unwin, 2003
Infancy to Early Childhood: Genetic and Environmental Influences on Developmental Change
Robert N. Emde; John K. Hewitt.
Oxford University Press, 2001
Librarian’s tip: This book recounts the findings of the MacArthur Longitudinal Twin Study.
School Placement and Separation of Twins: A Review of Research
Lacina, Jan.
Childhood Education, Vol. 86, No. 3, Spring 2010
PEER-REVIEWED PERIODICAL
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
Twin Loss: Implications for Counselors Working with Surviving Twins
Withrow, Rebecca; Schwiebert, Valerie L.
Journal of Counseling and Development : JCD, Vol. 83, No. 1, Winter 2005
PEER-REVIEWED PERIODICAL
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
The Sibling Bond
Stephen P. Bank; Michael D. Kahn.
Basic Books, 1997
Librarian’s tip: "Twins: The Special Case of Close and Intense Siblings" begins on p. 39
Alcoholism in Identical Twins
Matosic, Ana; Marusic, Srdan; Martinac, Marko; Vidrih, Branka; et al.
Alcoholism, Vol. 42, No. 2, July 1, 2006
Schizophrenia in Adolescent Twins: A Case Study
Gothelf, Doron; Brand-Gothelf, Ayelet; Apter, Alan; Sever, Yehonatan; et al.
The Israel Journal of Psychiatry and Related Sciences, Vol. 41, No. 1, January 1, 2004
PEER-REVIEWED PERIODICAL
Peer-reviewed publications on Questia are publications containing articles which were subject to evaluation for accuracy and substance by professional peers of the article's author(s).
Males, Females, and Behavior: Toward Biological Understanding
Lee Ellis; Linda Ebertz.
Praeger Publishers, 1998
Librarian’s tip: Chap. 3 "Evidence from Opposite-Sex Twins for the Effects of Prenatal Sex Hormones"
Schizophrenia and Manic-Depressive Disorder: The Biological Roots of Mental Illness as Revealed by the Landmark Study of Identical Twins
E. Fuller Torrey.
Basic Books, 1994
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