While singleton human births are the most common, there are also several different forms of twinning that may arise in the early weeks (first two weeks) of development.
The two major twinning forms are monozygotic (from one fertilised egg and a single spermatazoa) or dizygotic (from two eggs fertilised by two different spermatazoa).
In addition to the zygosity, the additional twinning classifying terms refer to the type of placenta and fetal membranes, either separate or shared by the twins. Twinning has both a higher incidence of mortality in twins, due mainly to preterm delivery, and of incidence of birth defects.
Australian pregnancies resulting in multiple births
(data - Australian Bureau Statistics)
A serious additional abnormality associated with monozygotic twinning is conjoining, where the two twins have a variable degree of physical attachment to each other, depending upon the point at which the twinning event has occurred. (More? Conjoined Twins)
Monozygotic twins have proved to be a valuable clinical research tool (More? WWW Links), sharing identical genetic material (nature) and only differing in their environment (nurture). Although the different growth rate (slower) of twin fetuses may also be a complicating factor (More? see Fetal Origins Hypothesis).
Twinning is often first detected using ultrasound techniques. (More? Human 7 Week Twins Ultrasound)
Page Links: Introduction | Some Recent Findings | Reading | Objectives | Development Overview | Twinning Prevalence | Australian Twinning Prevalence | Folic Acid and Twinning | OMIM Database | References | WWW Links | Glossary
Medically conceived twins have an increased risk of preterm birth "Compared with naturally conceived twins, twins resulting from subfertility treatment have a slightly decreased gestational age at birth and an increased risk of preterm birth." (BMJ 19 Nov) Verstraelen H, Goetgeluk S, Derom C, Vansteelandt S, Derom R, Goetghebeur E, Temmerman M. Preterm birth in twins after subfertility treatment: population based cohort study. BMJ. 2005 Oct 25 [Epub ahead of print]
Recent review by Hall JG. Twinning. Lancet. 2003 Aug 30;362(9385):735-43.
Monozygotic (MZ) Twins
The prevalence of spontaneous livebirth monozygotic twinning is relatively constant, with variability in dizygotic twinning around the world.
Monozygotic conjoined twins - 1 in 100,000 births (more female)
(data from Twinning, Hall, J.G., Lancet 2003; 362: 735-43)
United States of America - 2.7% of all confinements resulted in a multiple birth in 1996 (U.S. Census Bureau, 1999, p.80)
New Zealand - 1.6% in 1998 (Statistics New Zealand, 2000, p.70)
Australia - 1.5% in 1998 (ABS, see below)
Data from the Year Book Australia (2002) looking at pregnancies (confinements) shows the number resulting in a singleton live birth has been declining while the number resulting in multiple births has been increasing. This has been attributed to increased number of births to older women and the increasing use of assisted conception technologies.
"While the number of confinements resulting in multiple births remains relatively low, there has been a steady increase since the 1970s."
Multiple Births
1980 - 1.0% (2,249 of 223,318; 2,219 twins, 30 triplets or higher)
1990 - 1.2% (3,168 of 259,435; 3,074 twins, 94 triplets or higher)
2000 - 1.6% (3,900 of 245,700; 3,800 twins, 100 triplets or higher)
"Among older women this trend is more pronounced. In 1980, there were 730 confinements resulting in multiple births to women aged 30 years and over, constituting 1% of all confinements among women over 30. By 2000, this number had increased to 2,300 (2%)."
(Data- Australian Bureau of Statistics Year Book Australia 2002)
Some clinical studies point to an increase in occurance with folic acid supplementaion in older women, the quality of data on this area of research is variable. The significant alternative is an increase in neural tube deffects without supplementation (More? Folic Acid and Neural Tube Defects)
For example : "Use of IVF is a strong confounder because it is associated with both use of folic acid and twinning. Even when misclassification of IVF was reduced to 5%, this bias persisted in the adjusted model. Using a 40% misclassified surrogate to adjust for IVF, as reported in the Swedish study, probably led to a false finding that folic acid increased dizygotic twinning." Berry RJ, Kihlberg R, Devine O. Impact of misclassification of in vitro fertilisation in studies of folic acid and twinning: modelling using population based Swedish vital records. BMJ. 2005 Apr 9;330(7495):815.
References
There is little scientific evidence to document frequency or relevance of spontaneous reductions to postnatal development of this undetected twin loss.
"The disappearance of gestational sacs or embryos after documented fetal heart activity in multiple pregnancies is not unusual. When it occurs, the process is known as the vanishing twin phenomenon." Landy HJ, Keith LG. The vanishing twin: a review. Hum Reprod Update. 1998 Mar-Apr;4(2):177-83.7 Review.
"One in 10 IVF singletons originates from a twin gestation. Spontaneous reductions that occur at > 8 weeks gestation are one of the causes for the higher risk of adverse obstetric outcome in IVF singletons. ..... We found no excess risk of neurological sequelae in survivors of a vanishing co-twin versus the singleton cohort or of cerebral palsy" Pinborg A, Lidegaard O, la Cour Freiesleben N, Nyboe Andersen A. Consequences of vanishing twins in IVF/ICSI pregnancies. Hum Reprod. 2005 Jun 24
Online Mendelian Inheritance in Man (OMIM) database is a catalog of human genes and genetic disorders (2005: 31 entries on "twinning").
Use the links below to view specific entries.
Search Pubmed Now: Twinning[TITL] (2005: 709 Articles, 42 Reviews)
Reviews on Twinning
Articles
International Society for Twin Studies (ISTS) "is an international, nonpolitical, nonprofit, multidisciplinary scientific organization. Its purpose is to further research and public education in all fields related to twins and twin studies, for the mutual benefit of twins and their families and of scientific research in general. The Society was founded in Rome in 1974."
Australian Twin Registry " The Australian Twin Registry is a national registry of twin pairs who are willing to consider participating in health related research."
St Thomas' UK Adult Twin Registry "Information about twins and related genetic & epidemiology projects for: Twins/General Public , Scientific Community"
Netherlands Twin Register (NTR) "was founded on February 1st 1987 at the Vrije Universiteit in Amsterdam to conduct scientific research. The study of twins and their family members can provide insights into the way differences between individuals are determined by hereditary and environmental influences."
A Social History of Conjoined Twins Based on "Body Doubles: Siamese Twins in Fact and Fiction", an exhibit constructed by Laura E. Beardsley at the Mütter Museum, Philadelphia, PA, Spring 1995.
William Smellie: A sett of anatomical tables, with explanations, and an abridgment, of the practice of midwifery A set of historical drawings (1754) bu William Smellie which include images of twins in utero.
Virtual Childrens Hospital Twins: A Parents' Guide
TWINS Magazine Web Site for Parents of Twins
Look at types of Abnormal Development that can occur during development.
Alternatively, look at normal development. Development Notes
For those wanting to see dynamic processes of development (and have a reasonably quick connection) then the Movies pages are good for watching changes occur.
The study of human development has relied extensively on studying the process in other model animals. For those wanting to see the process of development in other species then the other embryos pages are a good start.
Home Page
Embryo stages
Fetal Dev
Dev Notes 1
Dev Notes 2
System Notes
Acknowledgements
Abnormal
Twinning
Monozygotic Twinning
Dizygotic Twins
Conjoined Twins
Associated Abnormalities
Twin-twin transfusion syndrome
Movies
Audio
Class Notes
Serial Images
K12 Students
Abnormal
Embryology in the News
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School of Med Sci
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Dev Biol (Gilbert)
This page introduces the abnormality of twinning and didymology (the science of twins).
Most twinning occurs in the first two weeks of development and is not rare, the spontaneous rate of twinning is about 1 in 80 livebirths.
See also the Prenatal Diagnosis page NSW Pamphlet Checking your baby's health before birth.
Please email Dr Mark Hill if you wish to make a comment about this current project.