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Cohort Study of Monozygotic Twinning in IVF

D.A. Vaughan, E. Cleary, A. Penzias, and D. Sakkas

August 19, 2017
Emma Holt

What is Monozygotic Twinning?

Why does it matter to the researchers and why it should matter to every IVF patient 

The incidence of MZT has greatly increased among IVF patients compared to the general population, but the reason for this is unclear. Monozygotic twins are a result from one zygote that separated into independently growing cell aggregations at the early stage of development. These twins are always of the same sex, have the same genetic constitution, possess identical blood groups, and closely resemble each other in physical, psychological, and mental characteristics. Freaky.

Researchers from this study sought to investigate the incidence of and risk factors for MZT. The purpose of this study is to educate and inform patients undergoing IVF about what monozygotic twins are and independent risks that may result.

Details of the Research

Who was included and when

The authors, D.A. Vaughan, E. Cleary, A. Penzias, and D. Sakkas put together the retrospective, multicenter, cohort study from over the course of 17 years.  Ten large IVF clinics from January 2000 to January 2017 were reviewed and the research included fresh and thaw cycles in the analysis. Monozygotic pregnancies were identified by the number of fetal heart beats when they’ve exceeded the number of embryos transferred. The researchers performed a binary logistic regression to discover risk factors for MZT. 

Results & Conclusion

What are the risk factors for MZT? 

Throughout the entire study, there were about 124,000 clinical pregnancies identified in the database. 50,215 pregnancies resulted from fresh transfers and 11,709 pregnancies resulted from thaw transfers. 352 cycles were confirmed as MZ twins with 198 in the fresh cycle and 152 in the thaw cycle group. Independent risk factors for MZT were found to be prolonged embryo culture, thaw cycles and increasing numbers of oocytes retrieved in the index cycle. However, increasing numbers of embryos transferred seemingly decreased the risk of MZT. While the average age of the patient who had MZ twins was 36.4, age nor micromanipulation techniques seemed to extend additional independent risk. 

As of today, this is the largest investigation of monozygotic twinning in IVF. The authors have proven that prolonged embryo culture is a major risk factor for MZT. Not only that ,but thaw cycles are also twice as likely to result in MZT.


Risk factors for monozygotic twinning in IVF: a multicenter cohort study
Fertility and Sterility - September 2017, Volume 108, Issue 3, Supplement, Page e92


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