Abnormal Development - Twinning

From Embryology


thumb|Historic drawing of twins by William Smellie (1697-1763) 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.

Abnormality Links: abnormal development | abnormal genetic | abnormal environmental | Unknown | teratogens | ectopic pregnancy | cardiovascular abnormalities | coelom abnormalities | endocrine abnormalities | gastrointestinal abnormalities | genital abnormalities | head abnormalities | integumentary abnormalities | musculoskeletal abnormalities | limb abnormalities | neural abnormalities | neural crest abnormalities | placenta abnormalities | renal abnormalities | respiratory abnormalities | hearing abnormalities | vision abnormalities | twinning | Developmental Origins of Health and Disease |  ICD-11
Historic Embryology  
1915 Congenital Cardiac Disease | 1917 Frequency of Anomalies in Human Embryos | 1920 Hydatiform Degeneration Tubal Pregnancy | 1921 Anencephalic Embryo | 1921 Rat and Man | 1966 Congenital Malformations

Some Recent Findings

  • Increased prevalence of cardiovascular defects among 56,709 California twin pairs.[1] "An increased prevalence was observed in twins compared to singletons in all 16 cardiovascular categories. Seven of the cardiovascular categories had at least double the prevalence in twins compared to singletons. Like-sex twins, as a proxy of monozygosity, had an increased prevalence of cardiovascular defects compared to unlike sex twins. Probabilities of concordance for flow lesions were higher among monozygotic than dizygotic twins."
  • Maternal immunologic rejection: lessons from discordant dizygotic twin placentas.[2] "We describe a series of dizygotic twin placentas where the more severe the chronic villitis, the more affected the placenta and fetus. Since the maternal environment was constant for each of these twins, differences in villitis severity appears to be attributable to differences in the ability of each placenta to induce a maternal immune response."

Twinning Prevalence

The prevalence of spontaneous livebirth monozygotic twinning is relatively constant, with variability in dizygotic twinning around the world.[3]

  • Asia 6 in 1000
  • Europe/USA 10-20 in 1000
  • African-Americans 26 in 1000
  • Africa 40 in 1000
  • Japan 1 in 250
  • Nigeria 1 in 11

Monozygotic conjoined twins - 1 in 100,000 births (more female)

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)

Australian Twinning Prevalence

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%)." [4]

Dizygotic Twinning

Dizygotic twins (fraternal, non-identical) arise from separate fertilization events involving two separate oocyte (egg, ova) and spermatozoa (sperm).

Monoygotic Twinning

Monoygotic twins (identical) produced from a single fertilization event (one fertilised egg and a single spermatazoa, form a single zygote), these twins therefore share the same genetic makeup. Occurs in approximately 3-5 per 1000 pregnancies, more commonly with aged mothers. The later the twinning event, the less common are initially separate placental membranes and finally resulting in conjoined twins.

Week Week 1 Week 2
Day 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cell Number 1 1 2 16 32 128 bilaminar
Event Ovulation fertilization First cell division Morula Early blastocyst Late blastocyst


Implantation starts X inactivation
Follicle 001 icon.jpg Early zygote.jpg Human embryo day 2.jpg Human embryo day 3.jpg Human embryo day 5.jpg CSt3.jpg Week2 001 icon.jpg

Twin Type








Table based upon recent Twinning Review.[3]

Twin-twin Transfusion Syndrome

Twin-twin transfusion syndrome (TTTS) can occur in both monochorionic and diamniotic twins that results from an unbalanced blood flow from one to the other in utero. Monozygotic twin pregnancies carry a 10-20% risk of twin-twin transfusion syndrome.

  • Twin-to-twin transfusion syndrome, vein of galen malformation, and transposition of the great arteries in a pair of monochorionic twins: coincidence or related association? [5] "The development of TTTS, VGM, and TGA in a single monochorionic pregnancy could be pure coincidence, but there might also be a causative link. We discuss the possible contribution of genetic factors, fetal flow fluctuations, vascular endothelial growth factors, and the process of twinning itself to the development of these congenital anomalies."

Acardiac Twins

Historically called chorioangiopagus parasiticus. Acardia, also called twin reversed-arterial perfusion (TRAP) sequence, is an extreme form of twin-twin transfusion syndrome. In a twinned human fetal development where monozygotic twinning or higher multiple births have an artery-to-artery and a vein-to-vein anastomosis in the monochorial placenta.[6]

The incidence of this condition is 1% of monochorionic twin pregnancies (approx. 1 of 35,000 pregnancies).


  1. <pubmed>19353581</pubmed>
  2. <pubmed>18598117</pubmed>
  3. 3.0 3.1 <pubmed>12957099</pubmed>
  4. Australian Bureau of Statistics Year Book Australia 2002
  5. <pubmed>16808639</pubmed>
  6. <pubmed>10072652</pubmed>


<pubmed>16283408</pubmed> <pubmed>16045531</pubmed>



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Cite this page: Hill, M.A. (2020, July 10) Embryology Abnormal Development - Twinning. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Abnormal_Development_-_Twinning

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© Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G