The risk of monozygotic twinning in an in vitro fertilization cycle (IVF) may be intrisinically related to oocyte quality

Abstract

Objective: To determine the prevalence of monozygotic twinning (MZT) in IVF pregnancies. Additionally, to evaluate what clinical and laboratory factors may predispose embryos to the development of MZT gestations. Design: IRB-approved retrospective review of 4700 IVF cycles done between October 1999 and March 2003. Materials and Methods: The records of all patients attempting conception through IVF were reviewed. Parameters selected for evaluation included age, ovarian response to stimulation, number of oocytes retrieved, number of oocytes which fertilized, number of embryos still developing on day 3, distribution of embryo quality on day 3, number of embryos still developing on day 5 (for those patients whose embryos were placed in extended culture), the distribution of blastocyst quality on day 5 (where appropriate), day of transfer, the number of embryos transferred, number of clinical implantations, number of ongoing implantations, and overall ongoing pregnancy rate. Data were analyzed by complex Chi-square tests. Results: A total of 12912 embryos were transferred during 4700 treatment cycles during the study interval. There were a total of 2758 pregnancies with a total of 4483 implantations being identified. There were 69 documented monozygotic twins ( 2.5% of pregnancies and 1.5% of implantations) documented during the study interval. All were identified in the first trimester and as such were all monochorionic. Dichorionic monozygotic twins would not be evident in the clinical data and as such would not be captured in this analysis. Several clinical parameters were evaluated in relationship to the prevalence of MZT and the following findings were documented: a)The prevalence of MZT increased with increasing number of oocytes retrieved. ( eg in those case 1–5 oocytes retrieved - MZT occured in 3/436 (0.7%); in those cases where > 20 oocytes retrieved - MZT occured in 25/1014 pregnancies (2.5%) [p < 0.02].b)MZT rates were negatively related with age with the youngest patients having the highest prevalence of MZT [p < 0.01].c)MZT was significantly related to the number of high quality embryos on day 3 (independent of day of transfer) [p < 0.02].d)MZT was higher with day 3 transfer of good quality embryos than in day 3 transfer of lower quality embryos [p < 0.02].e)MZT was higher with blastocyst transfer than with day 3 transfers [p < 0.0001].f)MZT was higher following transfer of high quality blastocysts in comparison to the transfer of lower quality blastocysts [p < 0.02].g)MZT was higher following the transfer of blastocysts with high quality inner cell masses in comparison to those with lesser quality inner cell masses [p < 0.02]. Conclusion: The prevalence of MZT is relatively low, but remains higher than that typically reported in the general population. While the risk of MZT was higher in those patients undergoing blastocyst transfer, these data indicate that the risk relates principally to the quality of the embryos and not the culture system or developmental day of transfer. In essentially all comparisons, higher quality embryos as judged by traditional morphological criteria on day 3 or day 5 had significantly higher MZT rates than did those with lower quality. This strongly suggests that the risk for MZT relates prinicipally to the patient and the intrinsic quality of the oocytes provided, and not to the culture system employed or the duration of in vitro culture.