UNSW Embryology

Week 1 Twinning

© Dr Mark Hill (2008)

Acknowledgements

Introduction

Fertilization in the first week and development in the second week are critical times in the generation of twins. Initially there are 2 major fertilization options. Firstly, if two separate fertilization events have occurred, by two eggs being fertilized by two separate sperm, then dizygotic twins are generated. Secondly, if a single fertilization event have occurred, by one egg being fertilized by one sperm, then monozygotic twins are generated. The other subsequent descriptions of twins refers to the type of fetal membranes and placentation that occurs in the second and subsequent weeks of development

This topic is now covered in detail on the page Abnormalities - Twinning

Dizygotic Twins

With the introduction mothers being treated with ovulation-inducing hormones and in vitro fertilization techniques, which employ multiple fertilized eggs being presented for possible implantation, there has been an increased possibility of multiple pregnancies that are dizygotic in nature (Imaizumi, 2003). Monozygotic rates have remained relatively constant.

In a recent (European) study (Helmerhorst etal., 2004) of perinatal outcome of singleton and twin pregnancies between natural and assisted conceptions it was found that: "Singleton pregnancies from assisted reproduction have a significantly worse perinatal outcome than non-assisted singleton pregnancies, but this is less so for twin pregnancies. In twin pregnancies, perinatal mortality is about 40% lower after assisted compared with natural conception."

Monozygotic Twins

These twins are an amazing medical research resource. As both twins share identical genetic material, comparisons in development, health outcomes are a powerful tool for medical research that allows many different questions to be studied. For this reason, many contries have established a twin registry.

References

Reviews

Hall JG. Twinning. Lancet. 2003 Aug 30;362(9385):735-4

Boomsma D, Busjahn A, Peltonen L. Classical twin studies and beyond. Nat Rev Genet. 2002 Nov;3(11):872-82. Review.

Morley R. Can we generalise from findings in twins? Paediatr Perinat Epidemiol. 2005 Jan;19 Suppl 1:54-9. Review.

Hankins GV, Saade GR. Factors influencing twins and zygosity. Paediatr Perinat Epidemiol. 2005 Jan;19 Suppl 1:8-9. Review.

Kaufman MH. The embryology of conjoined twins. Childs Nerv Syst. 2004 Aug;20(8-9):508-25. Epub 2004 Jul 27. Review.

Helmerhorst FM, Perquin DA, Donker D, Keirse MJ. Perinatal outcome of singletons and twins after assisted conception: a systematic review of controlled studies. BMJ. 2004 Jan 31;328(7434):261. Epub 2004 Jan 23. Review.

Imaizumi Y. A comparative study of zygotic twinning and triplet rates in eight countries, 1972-1999. J Biosoc Sci. 2003 Apr;35(2):287-302. Review.

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