Human Reproduction vol.13 no.4 pp.783–788, 1998 Safety of embryo cryopreservation: Statistical facts and artefacts Episcientific aspects of the epigenetic
environmentally induced disturbance (Walker et al., 1996). factors in artificial procreation
Nevertheless it is important to note that adult liveweightsappear not to be affected and birthweight appears not to be an
inheritable trait. In contrast, certain epigenetic events in theearly mouse embryo can affect the adult phenotype; in nucleo-
INSERM Unite´ 355, 32 rue des Carnets, 92140 Clamart,
cytoplasmic hybrids, transcriptional repression of certain genes
and Laboratory for Medically Assisted Procreation,
has been observed as well as mouse growth deficiency resulting
American Hospital of Paris, 63 Boulevard Victor Hugo,
in reduced adult body weight (Reik et al., 1993).
The point is that these experimental results were reported
This debate was previously published on Webtrack on
at a time when MAP was already responsible for Ͼ100 000
human births. Although IVF, embryo transfer and embryofreezing–thawing procedures seem unable to induce anomalies
For 20 years artificial methods for helping sterile couples to
in babies and be capable only of transmitting parental anomalies
procreate have been expanding rapidly. In addition to hormonal
(notably the genetic anomalies which are frequent in male
treatments for induction of numerous mature oocytes at each
infertility treated by ICSI), continued investigations in animals
stimulated menstrual cycle, laboratory techniques have been
are legitimate. This cautious approach is especially important
proposed, including principally in-vitro fertilization (IVF),
because humans are long-lived mammals, with late puberty,
which may include sperm microinjection into the oocyte
and no babies have yet been born to a man or woman conceived
(ICSI), embryo culture in medium alone or in co-culture with
feeder cells, and embryo cryopreservation. These artificial
So, looking for normality in newborns does not suffice and
procedures have to be analysed, not only with respect to their
recent studies demonstrating, for example, the capacity to
clinical efficacy (percentage of successful attempts) but also
reproduce and have normal offspring of mice born following
in terms of their eventual impact on various aspects of human
immature sperm injection (Kimura and Yamagimachi, 1995),
life. In addition to potentially altering the health of patients or
are welcome. Similarly, studies on the future of animals born
their offspring, these techniques may have economic, social
from freeze–thaw embryos are of interest for embryologists
and psychological consequences. Animal experiments are
and physicians involved in MAP. However the results of such
necessary prior to the clinical application of any new technique.
studies are potentially disruptive in that they could seriously
However their results are only indicative since there are species
disturb parents who have already benefitted from MAP or
specific factors and extrapolations to humans are more or less
those who intend to, not to mention the political and ethical
tentative. Moreover experimental animals are usually healthy
authorities. For those reasons the design of such studies and
whereas medically-assisted procreation (MAP) techniques con-
the analysis of their results must leave no place for ambiguity
cern individuals with abnormal performances stemming at
times from genetic alterations which may interfere with off-spring characteristics.
Although several studies reveal lower developmental rates
The case of embryo freezing
for IVF and cultured mammalian embryos (Massip et al.,
Several major series of frozen–thawed embryo transfers have
1984), no anomalies have been reported, even with frozen–
now been carried out. The French national registry (FIVNAT,
thawed embryos. The only differences found between experi-
1996) gives us the opportunity to compare Ͼ15 000 embryo
mental and control animals were the higher birthweight of
transfers following freezing and nearly one hundred thousand
certain newborns in bovines produced by IVF (Behboodi et al.,
transfers of fresh embryos. From these data only transfers of
1995) and the decreased viability of bovines or ovines in case
single embryos are summarized in Table I.
of in-vitro procedures or nuclear transfer (Willadsen et al.,
Significant differences were found for several parameters.
1991). In fact, enhanced fetal growth can result from in-
Firstly the pregnancy rate was lower after transfer of a frozen
vitro culture of embryos, asynchronous embryo transfer, or
embryo, as could be anticipated given that only two out three
progesterone treatment of the mother soon after ovulation.
frozen embryos survive the damage associated with freezing
There is evidence to suggest that cell lineage differentiation
and thawing procedures (Testart et al., 1987). However, from
in the manipulated embryo is altered, resulting in preferential
the time of implantation, pregnancies induced with a frozen
allocation of cells to the trophectoderm and aberrant fetal
embryo were no longer at a disadvantage compared with those
growth with a larger than normal placenta. Changes in the
induced with a fresh embryo. On the contrary, frozen embryo
regulation of early gene expression could result from such
single pregnancies were subject to fewer medical problems
European Society for Human Reproduction and Embryology
J.Testart Table I. Comparative results of fresh and frozen–thawed embryo transfers (from FIVNAT, 1996)
Percentage pregnancies/transfers (one embryo)
Percentage pathologies in single pregnancies:
**Significant difference (P ); NSϭ not significant.
that, without cryopreservation procedures, the potential of
Table II. Various relationships in a study on the long-term effects of mouse
supernumerary embryos could not be exploited.
embryo freezing (from Dulioust et al., 1995)
Although no differences have been reported between children
born from fresh versus frozen embryos (Olivennes et al.,1996), it is not possible to ascertain whether no differences
will be found in the future. In the light of a study involving
mouse embryos (Dulioust et al., 1995), we can anticipate
epigenetic effects due to freezing and that these will be
observed only in adults or in very old animals. As observed
by Wood (1997) certain aspects of the experimental design of
this study are a cause for concern. Notably the outbred foster
mothers provided non-uniform uterine environments that could
have contributed to the observed differences. Apart from this
there are other possible biases which could also become
increasingly common in published scientific papers.
ϩ ϭ significant; – ϭ not significant.
Table II summarizes the results published by Dulioust et al.
(1995). They observed the statistical effects of embryo freezingon mouse behaviour, mandible morphometry and weight. Sur-
Table III. Number of studied data to assess the effects of mouse embryo
prisingly, the weight differences emerged only in very old mice.
freezing (from Dulioust et al., 1995)
Moreover this phenomenon was not observed in females and it
occurred in males of only one of the two tested strains. As shown
in Table II, similar or even more dramatic differences were foundby comparing groups of control mice, from non frozen embryos:
Preweaning development 2 genotypes ϫ 2 sexesϫജ9 criteria
the studied parameters differed depending on both mouse sex
and genotype. Given these conditions it becomes less clear that
2 genotypes ϫ 2 sexes ϫ 4 periodsϫജ5 tests
cryopreservation procedures have an epigenetic effect. Mandible morphometry 2 genotypes ϫ 2 sexesϫജ11 measurements
Studies involving multiplication of comparisons lead inevit-
ably to the detection of statistical differences between groups
drawn from the same population. For example, given rejection
of the null hypothesis at the typical 5% probability level, compar-ison of two experimental groups from the same homogeneous
and the newborns were in better condition, with higher weight
population with respect to 100 parameters will reveal differences
at birth. This could be explained by the greater age of frozen
embryo recipients and the fact that fresh embryo transfer was
Returning to the Dulioust et al. (1995) study, numerous data
often the origin of a first pregnancy. From the above study
obtained in mice grown from frozen versus control embryos,
and all recently published series it appears that the clinical
were compared (Table III). By accumulating the parameters
risk of freezing human embryos is only to slightly decrease
studied, at least 172 different data pairs were compared and this
the pregnancy rate. Complementing this observation is the fact
may account for the discovery of certain differences. Moreover
Safety of embryo cryopreservation
ically valid, artifactual differences between experimental andcontrol births, while the abuse of statistics and absence of con-firmatory experiments may reveal false differences.The recentavailability of automatic recording of experimental results,coupled with rapid, computerized data analysis in the search forstatistical differences, increases the chances of finding resultsgood enough for publication but not confirmation: the episcient-ific effect.
In my opinion, a replica of the Dulioust et al. (1995) experi-
ment, with exactly the same procedures and measures, and eventhe same research team, would lead to the demonstration of othereffects of freezing, different from those already reported. Byplaying with statistics, several of my colleagues and myself havebeen able to find correlations between the pregnancy rate afterIVF/embryo transfer and such vitally significant parameters asthe first letter of the patients names or the position of the sun or
Figure 1. Hypothesis for a selection bias in artificial procreation.
moon at the time of egg recovery. Imposition of two conditionson experimental research practises may limit misinterpretations.
it is well known that most published papers do not relate all
Firstly, rather than a politics of fishing for significant differences
the measures recorded, selecting only those revealing statistical
with a sort of statistical net, research destined for publication
differences. For example, in the paper under discussion, the
should involve hypothesis testing. These hypothesis may be
mean weight of the mice was indicated in two mouse strains, for
based on rational considerations. They may also be empirical,
males and females, and at three different ages. One can postulate
resulting from the sort of statistical fishing practised by Dulioust
that the weight of the mice was measured at other ages, giving
et al. (1995). This is the second point: the production of such
more chances of finding significant differences in the abundant
empirical hypotheses is a legitimate and laudable activity. How-
ever they remain to be tested and, perhaps, confirmed. Until
While it is true that other studies suggest that the increased
these simple rules are respected, we can expect the epigenetic
weight of in-vitro derived calves is a reproducible effect of
effects associated with embryo manipulation to be confounded
embryo manipulations (Behboodi et al., 1995; Walker et al.,
with the episcientific effects associated with the unfortunately
1996; Kruip and den Daas, 1997), they do not suffice to demon-
apt dictum: ‘publish or perish’.
strate that the anomaly arises from the procedure itself. Theselection of embryos of different genotypes from a non-homo-geneous pool may well involve a bias. Such biased selection may
be innate in human artificial procreation procedures (Figure 1).
Behboodi, E., Anderson, G., Bondurant, R. et al. (1995) Birth of large calves
that developed from in vitro-derived bovine embryos. Theriogenology, 44,
From 100 recovered oocytes we currently obtain Ͼ50 embryos
which give rise to about five babies. Although we are unable to
Dulioust, E., Toyama, K., Busnel, M.C. et al. (1995) Long-term effects of embryo
discriminate between different mature oocytes, it is obvious
freezing in mice. Proc. Natl. Acad. Sci. USA, 92, 589–593.
that they have different characteristics even where they have
FIVNAT (1996) Bilan des transferts d’embryons congele´s de 1987 a` 1994. Contracept. Fertil. Sexual., 24, 700–705.
comparable spontaneous developmental potential. It is possible
Kimura, Y. and Yanagimachi, R. (1995) Mouse oocytes injected with testicular
that the artificial procedures, for example freezing, disfavour
spermatozoa or round spermatids can develop into normal offspring.
certain oocytes or embryos, here of types A and B (see Figure
Development, 121, 2397–2405.
Kruip, Th. and den Daas, J.(1977). In vitro produced and cloned embryos: effects
1), but are without effect on other oocytes, namely types E and
on pregnancy, parturition and offspring. Theriogenology, 47, 43–52.
D in the example. Under such conditions, the babies born come
Massip, A., Van der Zwalmen, P., Puissant, F. et al. (1984) Effects of in vitro
mostly from a particular cohort of oocytes. They may have
fertilization, culture, freezing and transfer on the ability of mouse embryo to
particular characteristics, such as a different mean weight, com-
implant and survive. J. Reprod. Fertil., 71, 199–204.
Olivennes, F., Schneider, Z., Remy, V. et al. (1996) Perinatal outcome and
pared with babies born following natural procreation but such
follow-up of 82 children aged 1–9 years and conceived from cryopreserved
differences have been probably selected, rather than induced, by
embryos. Hum. Reprod., 11, 1565–1568.
the artificial procreation procedures. In the Dulioust et al. (1995)
Reik, W., Romer, I., Barton, S. et al. (1993) Adult phenotype in the mouse can
study the effect of mouse embryo cryopreservation on prewean-
be affected by epigenetic events in the early embryo. Development, 119, 933–942.
ing development, when significant, almost exclusively con-
Testart, J., Lassalle, B., Belaisch-Allart, J. et al. (1987) Human embryo viability
cerned one of the two studied genotypes (C3D2). On the contrary
related to freezing and thawing procedures. Am. J. Obstet. Gynecol., 157,
the weight increase between 39 and 67 weeks concerned the
Walker, S., Hartwich, K. and Seamark, R. (1996) The production of unusually
other genotype (B6CBA) as though freezing–thawing procedures
large offspring following embryo manipulation: concepts and challenges.
were able to select different individual mice according to the
Theriogenology, 45, 111–120.
Willadsen, S., Janzen, R., McAlister, R. et al. (1991) The viability of late morulae
In conclusion, we postulate two ways in which the effects of
and blastocyts produced by nuclear transplantation in cattle. Theriogenology, 35, 161–170.
the diverse MAP procedures lend themselves to misinterpreta-
Wood, M.J.(1997) Embryo freezing: is it safe? Hum. Reprod., 12 (Natl. Suppl.),
tion. The selective effect (Figure 1) could explain certain statist-
M.Ludwig et al. No impact of cryopreservation and
after IVF or IVF/ICSI (2.1 and 2.4 respectively) and frozen–
thawing on embryo developmental potential – one more example for the problems of retrospective, Experience from cycles with threatened OHSS non-controlled data
The second group of studies included those with cycles, inwhich all embryos have been frozen, due to threatened OHSS
Michael Ludwig1, Safaa Al-Hasani,
or other problems. In all of these studies but one (Awonuga
Ricardo Felberbaum and Klaus Diedrich et al., 1996) the clinical pregnancy rate and live birth rate
Department of Gynecology and Obstetrics, Medical
were satisfactory, and the authors were convinced, that this
University of Lu¨beck, Ratzeburger Allee 160, 23538
could be a real treatment alternative for their patients in
1To whom correspondence should be addressed
In the analysis of cycles at high risk for OHSS from
This debate was previously published on Webtrack on
Awonuga et al. (1996) the clinical pregnancy (35 versus 17%;
P Ͻ 0.03) and the live birth (27 versus 12%; P Ͻ 0.05) ratesin patients receiving fresh embryo transfer was significantly
In his debate article Testart (1998) claims that the statistical
higher than in those who had elective cryopreservation of all
significance between pregnancy rates in transfer cycles using
embryos. However, the study was not prospectively random-
frozen–thawed embryos and fresh embryos does not reflect
ized. Therefore a selection bias of patients cannot be excluded.
the different implantation rate of those embryos. He proposed
A pregnancy rate of 38.6% was achieved by Shaker et al.
that other factors are present, which might explain the obvious
(1996) under similar conditions in 13 patients. In another study
difference in success rates, like the rate of high quality embryos
among 23 patients at increased risk of OHSS, 15 clinical
and the number of transferable embryos.
pregnancies after transfer of two to three frozen–thawed
The question is difficult to answer from the general experi-
embryos in natural cycles, with a 32.6% pregnancy and 22.7%
ence in freeze–thaw cycles, because the mentioned problems
implantation rate could be achieved by Tiitinen et al. (1995).
will always bias those data. In our mind, there are three
In 96 patients a pregnancy rate after transfer of frozen–thawed
possible ways to find a reliable answer: (i) the German
embryos of 25.2% per transfer, with a cumulative pregnancy
experience from the transfer of frozen–thawed oocytes at the
rate of 40.6% was reported by Pattinson et al. (1994). Wada
pronuclear stage, which cannot be selected by morphology;
et al. (1992) reported their experience from 78 patients, who
(ii) the experience from cycles, in which all embryos are
had had their embryos frozen and underwent 125 frozen–
frozen to avoid serious medical problems for the patient, e.g.
thawed embryo replacements. An implantation rate of 11%
ovarian hyperstimulation syndrome (OHSS) or due to negative
and pregnancy rates of 19 and 29% was achieved in cycles
predictors of success; and (iii) prospective, randomized studies,
with either hormonal replacement therapy or hormonal stimula-
to compare directly fresh and frozen–thawed embryo transfers.
tion for the frozen–thawed transfers. Finally, Frederick et al. (1995) also reported on a retrospective series of 36 patients,whose embryos were all cryopreserved, due to threatened
German experience with cryopreservation of oocytes at the
OHSS or the absence of an optimal sonographic endometrial
pattern at the day of transfer. After thawing a pregnancy rate
It is well known, that embryo quality can be assessed by
of 33.3% and a live birth rate of 28.6% per cycle was achieved.
morphological criteria (Staessen et al., 1995), but that oocytes
The implantation rate per embryo was 9.1%, with an average
at the pronuclear stage (PN) can not. To date we do not know
number of 4.2 embryos replaced per cycle.
any publication, which deals with morphological criteria of
All but one of these publications show that there was no
PN, which are helpful to assess the implantation chance.
harm to the embryos, with regard to the capacity to implant.
Since in Germany, due to legal restrictions, only PN can be
However, prospective randomization of fresh transfer versus
cryopreserved, a selection bias on implantation rates is not
all cryopreserved embryos has been carried out in only one
possible. We reported recently a pregnancy rate of 17% and
study (Shaker et al., 1996). Since these authors reported no
18% per transfer after in-vitro fertilization (IVF) and IVF/
pregnancies in fresh transfer cycles with threatened OHSS,
intracytoplasmic sperm injection (ICSI) respectively, for
and a pregnancy rate of 38% after freezing–thawing, these
frozen–thawed PN (Al-Hasani et al., 1996). This is obviously
data are not really representative of day-to-day experience.
lower than the pregnancy rate after the transfer of fresh, inthe PN stage, selected embryos (26% per transfer). Experience from prospective, randomized studies
The German IVF Registry (Deutsches IVF Register, 1996),
Selick et al. (1995) designed a prospective study, in which pooled
which includes 2452, 11969, and 14866 transfer cycles for
fertilizable oocytes from young oocyte donors were allocated
frozen–thawed PN, IVF and IVF/ICSI cycles, confirms a lower
after fertilization either to fresh or frozen–thawed embryo trans-
pregnancy rate per transfer for frozen/thawed PN cycles
fer cycles. This study was controlled for male, oocyte and endo-
(10.4%), compared with fresh embryo transfers after conven-
metrial factors. A total of 87 transfer cycles were included in
tional IVF or IVF/ICSI (24.1 and 23.7% respectively). The
this analysis. Implantation rate per embryo and delivery rate per
mean number of replaced embryos was similar in fresh cycles
transfer of 12.6 and 26.2% respectively, from fresh transfer
Safety of embryo cryopreservation
cycles were not significantly different when compared with a
procedure on human embryos seems to be wrong. However,
per embryo implantation rate of 8.1 % and per transfer delivery
the numbers in the cited studies were almost always very low.
rate of 13.3% from frozen–thawed transfer cycles. The lower
Therefore, to really confirm this thesis, we have to await
implantation and delivery rates in the frozen–thawed group were
attributed to a statistically significant difference in the number
This should teach us once more, that one can rely only on
of embryos per transfer, the mean number of cells per embryo,
well designed studies, and that retrospective analysis can only
and the rate of high quality embryos.
help to raise questions, and not to answer them.
Horn et al. (1997) report on two randomly selected groups
of patients. In the first group only two PN were allowed todivide, all other PN were cryopreserved (PN group). In the
second group, a selection of the two best embryos was done
Al-Hasani, S., Ludwig, M., Gagsteiger, F. et al. (1996) Comparison of
in the cleavage stage, all other embryos were frozen (EC
cryopreservation of supernumerary pronuclear human oocytes obtained after
group). The livebirth rate per fresh embryo transfer in the EC
intracytoplasmic sperm injection (ICSI) and after conventional in-vitro fertilization. Hum. Reprod., 11, 604–607.
group (27.4%) was significantly higher than that for the PN
Awonuga, A.O., Pittrof, R.J., Zaidi, J. et al. (1996) Elective cryopreservation of
group (11.1%). Embryo survival following thawing was similar
all embryos in women at risk of developing ovarian hyperstimulation
for the PN (74.4%) and EC (77.4%) stages. Although not
syndrome may not prevent the condition but reduces the live birth rate. J. Assist. Reprod. Genet., 13, 401–406.
significant, the livebirth rate following the transfer of thawed
Deutsches IVF Register (1996) Jabrbuch 1995. Deutsches IVF Register.
embryos was higher in the PN group (25%) than in the EC
group (10.5%). Following one fresh and two freeze–thaw
Frederick, J.L., Ord, T., Kettel, L.M. et al. (1995) Successful pregnancy outcome
embryo replacements, the observed cumulative viable preg-
after cryopreservation of all fresh embryos with subsequent transfer into an unstimulated cycle. Fertil. Steril., 64, 987–990.
nancy rates were comparable for patients in both the PN (40%)
Horne, G., Critchlow, J.D., Newman, M.C. et al. (1997) A prospective evaluation
of cryopreservation strategies in a two-embryo transfer programme. Hum. Reprod., 12, 542–547.
Pattinson, H.A., Hignett, M., Dunphy, B.C. and Fleetham, J.A. (1994) Outcome
of thaw embryo transfer after cryopreservation of all embryos in patients at
risk of ovarian hyperstimulation syndrome. Fertil. Steril., 62, 1192–1196.
Only the German experience, published by the Deutsches IVF
Selick, C.E., Hofmann, G.E., Albano, C. et al. (1995) Embryo quality and
Register seem to support the general idea, that cryopreservation
pregnancy potential of fresh compared with frozen embryos—is freezing detrimental to high quality embryos? Hum. Reprod., 10, 392–395.
has a severe impact on the implantation rate of embryos even
Shaker, A.G., Zosmer, A., Dean, N. et al. (1996) Comparison of intravenous
after exclusion of the possibility for selection. However, these
albumin and transfer of fresh embryos with cryopreservation of all embryos
data are the result of a retrospective, multicentre database, and
for subsequent transfer in prevention of ovarian hyperstimulation syndrome. Fertil. Steril., 65, 992–996.
are not controlled for embryo quality. Additionally, the impact
Staessen, C., Nagy, Z.P., Liu, J. et al. (1995). One year’s experience with elective
of different methods and the reliability of the data, have at
transfer of two good quality embryos in the human in-vitro fertilization and
least to be discussed. Finally, a much better indicator would
intracytoplasmic sperm injection programmes. Hum. Reprod., 10, 3305–3312.
be the implantation rate per embryo or PN. However, these
Testart, J. (1998) Episcientific aspects of the epigenetic factors in artificial
procreation. Hum. Reprod., 13, 783–785.
numbers are not available from the Deutsches IVF Register
Tiitinen, A., Husa, L.M., Tulppala, M. et al. (1995) The effect of cryopreservation
in prevention of ovarian hyperstimulation syndrome. Br. J. Obstet. Gynaecol.,
The data from cycles, in which all embryos are cryopreserved
Wada, I., Matson, P.L., Troup, S.A. et al. (1992) Outcome of treatment subsequent
due mainly to a threatening OHSS, show satisfactory results,
to the elective cryopreservation of all embryos from women at risk of the
which are in the range of those known from fresh transfers.
ovarian hyperstimulation syndrome. Hum. Reprod., 7, 962–966.
However, these data are not randomized, as are those fromAl-Hasani et al. (1996) and the Deutsches IVF Register (1996).
In conjunction with the results from the cycles, in which all
Aspects of epigenetic factors in artificial
embryos are cryopreserved, the prospective studies are of the
greatest importance in answering the question. The lowerpregnancy rate in the EC group after freezing–thawing, and a
similar pregnancy rate after freezing–thawing in the PN groupcompared with the fresh transfer in the EC group show, that
Reproductive Biology Unit, Royal Women’s Hospital,132 Grattan Street, Carlton, Victoria 3053, Australia
in fact the results of transfers after cryopreservation are biased
by excluding the best embryos (Horne et al., 1997). Also
To whom correspondence should be addressed
Selick et al. (1995) reported no detrimental effect of freezing–
This debate was previously published on Webtrack onWeb2 on January 27, 1998 as a letter
thawing on the implantation and pregnancy rates.
However, since a loss rate of 20–40% due to freezing–
For every question there is an answer that is simple,
thawing has to be taken into account, and fresh and freeze–
straightforward and wrong (H.L.Menken).
thaw cycles differ statistical significantly in the number oftransferable embryos, their cell number and the rate of high
Testart (1998) makes a number of points which need frequent
quality embryos (Selick et al., 1995), a fresh transfer should
emphasis. It is now easier than ever for non-statisticians to
do vast numbers of statistical tests looking for statistical
The idea of a detrimental effect of the freezing–thawing
significance. The discerning reader will look behind the written
word and recognize the many unreported non-significant com-parisons inherent in a study design. Dr. Testart alerts us to thiswith regard to the mouse study by Dulioust et al. (1995). Todemonstrate the same effect I reported a study of the letters inthe surnames of pregnant and non-pregnant in-vitro fertilization(IVF) patients (Speirs, 1991). Pregnancy was strongly linkedwith ‘GYN positive’ (having G, Y or N in the surname!) P Ͻ0.01. The naive reader might not realize how readily theundertaking of many statistical tests may be disguised or theway that many comparisons eventually produces a ‘significant’result. This inevitability arises from the very meaning of astatistical P value. Incorrectly rejecting the ‘null hypothesis’because of an impressive P value when samples are tested isdescribed as a type I error.
It should also be recalled that statistical significance implies
that an observed difference is unlikely to have arisen bychance. It should not be taken to mean causation. Because itis plausible, Dr. Testart seems to have accepted that thawedembryos produce a lower pregnancy rate than do fresh transfersbecause of the freeze–thaw process. This is by no means theonly explanation. Single embryos transferred fresh will oftenbe the best embryo of several available whereas a singlethawed embryo will not have been selected on this basis. Thethawed embryo will often be single because it is the last oneavailable, even if nothing like the best of those originallyavailable.
Yet another bias could be introduced by looking at who
might elect to have a single embryo transferred fresh when sooften many would be available. Some will be couples whoparticularly want to avoid twins because they have alreadybeen successful with past IVF and are returning for one morebaby. Couples with past IVF success have a somewhat betterchance of further pregnancy. It is not entirely the single freshembryo that is better than the average (sometimes final) thawedembryo but their reason for electing to have a single embryotransferred when they (often) had many to choose from.
It is, of course, not possible to know from these data the
extent to which any or all of these influences producedthe observed difference between fresh and thawed embryotransfers. References Dulioust, E., Toyama, K., Busnel, M.C. et al. (1995) Long-term effects of
embryo freezing in mice. Proc. Natl. Acad. Sci. USA, 92, 589–593.
Speirs, A.L. et al. (1991) When predictions don’t predict. Aust. N.Z. J. Obstet.Gynaecol., 31, 346.
Testart, J. (1998) Episcientific aspects of the epigenetic factors in artificial
procreation. Hum. Reprod., 13, 783–785.
Original- und Übersichtsarbeiten ó Schwerpunkt: Kontroversen in der Kardiologie óó Pro & Contra: Therapie des Vorhofflimmerns Die Ablationstherapie wird der neue Goldstandard LARS LICKFETT, BONN1 Abstract óóóó Vorhofflimmern ist die häufigste anhaltende Herzrhythmus- störung. Die Prävalenz, die in der Gesamtbevölkerung bei 0,4—1% óó Die Indikation zu den Sinusrh
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