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HOW DOES IT HAPPEN?
 

MONOZYGOTIC (IDENTICAL) TWINS

Occasionally  a single fertilized egg forms a zygote that divides to form two separate embryos during the first 2 weeks after conception. These twins are referred to as monozygotic or "identical" twins. Depending on the timing of the division each zygote may develop its own membranes and placenta or the embryos may share the membrane and placenta. If the division occurs very late the twins will be conjoined.


PLACENTAL TYPES

Dichorionic (two placentas), Diamniotic (two amnions)
( 18 to 36 % of monozygotic twins )
 
If a single zygote divides into two separate embryos within 72 hours after fertilization then the twins
will develop in two separate amniotic sacs (bag of water) and will have their own, separate,
placentas. However, the placentas may at times appear to be fused [25]..


             
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Monochorionic (one placenta), Diamniotic (two amnions)
( 60 to 70 % of monozygotic twins ) 

If division occurs 3 to 8 days after fertilization then the twins will develop in two separate amniotic sacs (bag of water) and will share one placenta [25].

 

 


LifeART image copyright 2005 Lippincott Williams & Wilkins. All rights reserved.

 


Monochorionic (one placenta),
Monoamniotic (one amnion)
( 1 to 2 % of monozygotic twins )
 
If division occurs 8 to 12 days after fertilization then the twins will share one amniotic sac (bag of water) and will share one placenta [25].

 


LifeART image copyright 2005 Lippincott Williams & Wilkins. All rights reserved.

 

Monochorionic (one placenta),
Monoamniotic (one amnion)
( 0.5 % of monozygotic twins )
 
If division occurs 12 to 13 days after fertilization then the twins will share one amniotic sac (bag of water) and will share one placenta. They will also share body parts [25].

Conjoined twins occur 1 in 40,000 total births.

 

LifeART image copyright 2005 Lippincott Williams & Wilkins. All rights reserved.



ULTRASOUND FINDINGS

It is not uncommon to "miss" a monochorionic twin on  ultrasound examination and diagnose these pregnancies as a singleton early [26].

Ultrasound findings which support the diagnosis of
monozygotic  twins include [11,12]:
  • Single placental site
  • Same-sex fetuses
  • Thin dividing membrane (two layers or <2 mm)
  • A "T" sign [27] or "empty" lambda sign
    • No chorion extending between the membranes .
       



FACTORS INFLUENCING THE OCCURRENCE OF MONOZYGOTIC TWINS

The birth rate of monozygotic twins worldwide appears to be  relatively constant at 3 to 5 per 1000 births [1].

  • Infertility therapy


  • Monozygotic twinning has been reported to increase after ovulation induction techniques and in proportion to the number of blastocysts transferred during in vitro fertilisation [28-30]
     
  • Maternal or paternal history of monozygotic twinning

  •  

    Monozygotic twinning transmitted either through the father or mother has been
    reported [31].

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SEE ALSO

 

REFERENCES

1. MacGillivray I.Epidemiology of twin pregnancy.Semin Perinatol.1986;10:4-8.PUBMED 11.Scardo JA,Prospective determination of chorionicity,amnionicity, and zygosity in twin gestations.Am J Obstet Gynecol. 1995 Nov;173(5):1376-80. PUBMED
12.Finberg HJ.The "twin peak" sign: reliable evidence of dichorionic twinning.J Ultrasound Med.1992;11:571-7.PUBMED
25.Bomsel-Helmreich O and Al Mufi W : The Phenomenon of Monozygosity:Spontaneous Zygotic Spltting. In: Blickstein I ed: Multiple Pregnancy. Epidemiology, Gestation, and Perinatal Outcome, 2nd ed London UK, Taylor and Francis; 2005: 95
26.Doubilet PM,Benson CB ."Appearing twin": undercounting of multiple gestations on early first trimester sonograms. J Ultrasound Med.1998;17:199-203. PUBMED
27.  Matias A,et al .Down syndrome screening in multiple pregnancies. Obstet Gynecol Clin North Am. 2005;32:81-96PUBMED
28.Hankins GV and Saade GR .Factors influencing twins and zygosity.Paediatr Perinat Epidemiol.2005;19 Suppl 1:8-9.PUBMED
29.Henne MB,Monochorionic triplet gestation after in vitro fertilization using donor oocytes: case report and review.Fertil Steril.2005;83:742-8.PUBMED
30.Blickstein I.Estimation of iatrogenic monozygotic twinning rate following assisted reproduction: Pitfalls and caveats. Am J Obstet Gynecol. 2005;192:365-8.PUBMED
31. Twinning, Monozygotic
. Online Mendelian Inheritance in Man, OMIM (TM). Johns Hopkins University, Baltimore, MD. MIM Number: 276410: Date last edited:12/11/2001. World Wide Web URL:http://www.ncbi.nlm.nih.gov/omim/

Created: 11/2/2005

 


 

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