Abnormal Development - Twinning

From Embryology

Introduction

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.

Twinning Prevalence

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

  • 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)

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

Hatching

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
Monoygotic

Twin Type

Diamniotic

Dichorionic

Diamniotic

Monochorionic

Monoamniotic

Monochorionic

Conjoined

Table based upon recent Twinning Review.[1]

References

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Reviews

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Cite this page: Hill, M.A. (2020, March 31) 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