Human Reproduction, Vol. 17, No. 4, 1129-1130,
April 2002
© 2002 European Society of Human Reproduction and Embryology
Letter to the editor |
How homogeneous are comparison groups in any study evaluating techniques of embryo transfer?
1 Center for Fertility and Reproductive Endocrinology, Virginia Mason Medical Center, and 2 Department of Obstetrics and Gynecology, University of Washington, Seattle, WA, USA
Dear Sir,
There is an increasing appreciation that multiple facets of the technique of embryo transfer may impact the success of an IVF cycle. Factors that contribute to the single best technique of embryo transfer continue to be debated. Catheter types and materials, amount of culture media, cervical irrigation and the experience of the clinician are among several important variables impacting success (Letterie et al., 1999
). In the September 2001 issue of Human Reproduction Prapas et al. address the important issue of ultrasound guided embryo transfer (Prapas et al., 2001). This well-designed study carries with it the power of a large sample size and an attempt to ensure homogeneous populations in each comparison group, no small task given the number of variables to consider. Patients were matched using well-defined and commonly accepted clinical and laboratory criteria including age, oocyte number, fertilization rate, number of embryos transferred and embryo score.
However, we would like to put forth the contention that, complete as they may appear, these criteria fail to control for the very important variable of vaginal microflora and its impact on the likelihood of success regardless of technique of transfer (Franchin et al., 1998). Though multifaceted and still evolving, certain trends are emerging that suggest that the vaginal microbiological environment may influence the outcome of embryo transfer techniques. Two lines of evidence make a compelling case that vaginal flora may be as significant a factor as any of the aforementioned variables. The first is the observation that contamination of the embryo transfer catheter tip with streptococci veridans is associated with decreased clinical pregnancy rates, increased first trimester loss rates and ultimately a decrease in live birth rates (Moore et al., 2000). The second is the observation that a shift in the dominance of the vaginal flora away from lactobacillus to increased concentrations of anaerobes and G. vaginalis, as occurs with bacterial vaginosis, is also associated with decreased rates of success (Hillier et al., 1993; Moore et al., 2000). The picture is further complicated by a failure to improve success rates with doxycycline therapy suggesting a complex picture requiring further study.
The emerging data (and we emphasize that this is preliminary and evolving) is one that strongly implicates the vaginal microenvironment as an important variable determining success with IVF. Until the details of this important variable are delineated and controlled in any study comparing embryo transfer techniques, we would contend that the homogeneity of the comparison populations in the Prapas study (and all studies comparing techniques published thus far) is spurious and that in fact these populations have the potential to be quite disparate in spite of controlling for age and cycle dynamics. This possibility makes any conclusions regarding advantages or disadvantages of one technique over another open to question. Clearly, further detailed study is required to answer these important issues.
Notes
3 To whom correspondence should be addressed. E-mail: Gerard.Letterie{at}vmmc.org 
References
Franchin, R., Harmas, A., Benaoudia, F., Lundkvist, U., Olivennes, F. and Frydman, R. (1998) Microbial flora of the cervix assessed at the time of embryo transfer adversely affects in-vitro fertization and embryo transfer. Fertil. Steril., 70, 866–870.[Web of Science][Medline]
Hillier, S.L., Krohn, M.A., Rabe, L.K. Klebanoff, S.J. and Eschenbach, D.A. (1993) The normal vaginal flora, hydrgogen peroxide producing lactobacilli, and bacterial vaginosis in pregnant women. Clin. Infect. Dis., 16, (Suppl. 4) S273–S281.
Letterie, G.S., Marshall, L. and Angle, M. (1999) A new coaxial catheter system with an echodense tip for ultrasonographically guided embryo transfer. Fertil. Steril., 72, 266–268.[Web of Science][Medline]
Moore, D.E., Soules, M.R., Klein, N.A., Fujimoto, V.Y., Agnew, K.J. and Eschenbach, D.A. (2000) Bacteria in the transfer catheter tip influence the live-birth rate after in-vitro fertilization. Fertil. Steril., 74, 1118–1124.[Web of Science][Medline]
Prapas, Y., Prapas, N., Hatziparasidou, P., Vanderzwalmen, P., Nijs, M., Prapa, S. and Vlassis, G. (2001) Ultrasound-guided embryo transfer maximizes the IVF results on day 3 and day 4 embryo transfer but has no impact on day 5. Hum. Reprod., 16, 1904–1908.
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