Reply: Pregnancy outcome after blastocyst transfer as compared to early cleavage stage embryo transfer
Department of Obstetrics and Gynecology, University of Innsbruck, Austria
1 To whom correspondence should be addressed. Email: margherita.auer{at}uibk.ac.at
Sir,
We appreciate the interest of van der Ven and Montag in our study and their useful comments.
The primary aim of our retrospective cohort study was neonatal outcome. Hence, the purpose of our study was not to demonstrate superior performance of embryo transfer at blastoccyst stage (BS-group) over cleavage stage (CS-group), but that there is no difference in fetal outcome. This is why we investigated only IVF cycles that led to a successful pregnancy. However, there is convincing evidence that higher pregnancy rates after blastocyst transfer are not due to a selection bias (Coskun et al., 2000
), but due to the opportunity to choose more viable and genetically normal embryos.
Van der Ven and collegues also appreciated that maternal age was not significantly different in both study populations. Our discussion does point out that patients in the BS-group were slightly younger because with time we have enlarged the population of patients entering the BS-group. However, elevated maternal age certainly increases the risk for trisomies but not for other chromosomal abnormalities such as Turner syndrome and triploidies. The risk for trisomy 21 at term is 1:776 at the age of 31 and 1:446 at the age of 34, which is still considered to be low risk using a cut-off level of 1:250 (Snijders et al., 1999).
It is also true that the prevalence of dizygotic twins increases with maternal age, but the prevalence is still only 2% at the age of 35; a twin rate of 45% as described in the CS-group of our study population cannot be explained by higher maternal age alone.
We believe that our data give evidence that it is safe to cultivate embryos to the blastocyst stage in terms of neonatal outcome and fetal abnormalities. Moreover, we think that decreasing the multiple pregnancy rate in ART by transfering fewer but more competent embryos is important since preterm delivery is a major cause of fetal death and handicap in obstetrics.
References
Coskun S, Hollanders J, Al-Hassan S et al. (2000) Day 5 versus day 3 embryo transfer: a controlled randomized trial. Hum Reprod 15, 1947–1952.
Snijders R, Sundberg K, Holzgreve W et al. (1999) Maternal age and gestation-specific risk for trisomy 21. Ultrasound Obstet Gynecol 13, 167–170.[Medline]
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