Impact of elective single embryo transfer on the twin pregnancy rate

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1 Human Reproduction Vol.18, No.7 pp. 1449±1453, 2003 DOI: /humrep/deg301 Impact of elective single embryo transfer on the twin pregnancy rate A.Tiitinen 1, L.Unkila-Kallio, M.Halttunen and C.Hyden-Granskog Department of Obstetrics and Gynaecology, Helsinki University Central Hospital, P.O.Box 140, HUS, Finland 1 To whom correspondence should be addressed. aila.tiitinen@hus. BACKGROUND: It is unclear how the implementation of elective single embryo transfer in clinical practice would affect clinical pregnancy and delivery rates and multiple birth rates. METHODS: This retrospective study analysed 1871 IVF/ICSI cycles carried out from 1997 to 2001 in the IVF programme of a single university infertility clinic. RESULTS: The number of elective single embryo transfers increased from 11 to 56%. At the same time the clinical pregnancy rate was relatively stable; mean 34.0% (range 28±42). The number of embryos per embryo transfer decreased from 1.8 to 1.3. The multiple pregnancy and delivery rates dropped markedly from 25 to 7.5% and from 25 to 5% respectively. CONCLUSIONS: An elective single embryo transfer programme can be adopted in daily practice that decreases the twinning rate to <10% and does not affect the overall pregnancy rate. Key words: assisted reproductive technology/multiple pregnancy/single embryo transfer Introduction One of the main challenges in assisted reproductive treatment programmes is to avoid twin pregnancies without there being a signi cant decrease in the overall pregnancy rate (PR). In many countries >2% of newborns are born after assisted reproductive treatment (ESHRE, 2002). Traditionally, assisted reproductive treatment has been associated with a 20-fold rate of multiple pregnancies compared with spontaneous twin pregnancies (Ozturk et., 2001). In Europe, the overall rate of multiple pregnancies in 1999 was 26.3% (twins 24%, triplets 2.2%, quadruplets 0.1%) (ESHRE, 2002). This means that as many as half of the children born after assisted reproduction may have originated from multiple pregnancies. Data for the USA reveal even higher numbers (Toner et al., 2002). Neonatal outcome after IVF is worse than that in the general population of similar maternal age, parity and social standing, but this is mainly due to the large proportion of multiple births after IVF (Koivurova et al., 2002). However, an excess risk of low birthweight among singletons conceived with assisted reproductive treatment has been reported in two large population-based studies (Bergh et al., 1999; Schieve et al., 2002). After elaborate matching for many maternal characteristics, a distinct difference between IVF and control pregnancies was still noted (Koudstaal et al., 2000). In this particular study, gestational age at delivery was 3 days shorter in the IVF group (275 versus 278 days) and the proportion of small-forgestational age children was higher (16.2 versus 7.9%). The perinatal outcome of IVF singletons was signi cantly worse than that of spontaneously conceived pregnancies mainly because of the increased rate of preterm birth, but the outcome for twins was comparable (Dhont et al., 1999). In our hands, when comparing singleton IVF pregnancies with carefully chosen controls, the overall outcome is good (Isaksson et al., 2002). The only method to limit the number of twins is to transfer only one embryo. Many investigators have tried to identify subsets of subjects at a high risk of multiple birth, and hence those suitable for single embryo transfer (Hunault et al., 2002). We started our elective single embryo transfer (eset) programme in 1997 (Vilska et al., 1999). The main indications at the beginning were various medical reasons for the avoidance of twins. Since 1997, the proportion of eset has gradually increased, partly because of couples' own wishes to avoid twins. In 2000 we changed our policy to single embryo transfer, and two-embryo transfer is carried out only for speci c reasons. The aim of this report is to show how this policy has affected both the overall and multiple PR in our IVF programme and to further urge IVF centres to move on to eset. Materials and methods During the years 1997 to 2001 we performed a total of 1871 IVF/ICSI cycles at the Infertility Clinic of Helsinki University Central Hospital, Finland. Our IVF/ICSI programme includes a long protocol for ovarian stimulation (except for seven women treated with a GnRH antagonist) and standard IVF/ICSI culture methods, as previously described (Tiitinen et al., 2001). During this time period we performed 1699 embryo transfers: eset in 470 cases (27.7%), two-embryo transfers in 1024 cases (60.3%) cycles, and in 205 (12.1%) cases only one embryo was available. The treatment cycle did not result in fresh ã European Society of Human Reproduction and Embryology 1449

2 A.Tiitinen et al. Table I. Clinical characteristics of women in the embryo transfer programme, with the number of oocytes retrieved and the number of cryopreserved embryos Group n Age (years) (mean 6 SD) Age range (years) Primary Oocytes per Cryopreserved infertility retrieval embryos n (%) (mean 6 SD) (mean 6 SD) SET ± (62.0) eset ± (55.5) ET ± (59.8) No ET ± (59.9) Data available on 89% of all cycles. SET= only one embryo available; eset = elective single embryo transfer; 2ET = two-embryo transfer; No ET = no embryo transfer; n = number of cycles. Table II. The number of single and two-embryo transfers during 1997±2001 and the resulting clinical pregnancy rates (PR) per embryo transfer and the twin rates per pregnancy Group Year Total Twin rate (N3/N2) embryo transfer in 172 (9.2%) cases. The percentage of ICSI was 35.5% in all cycles and 36.6% in cycles with embryo transfer. The mean 6 SD age of the women undergoing embryo transfer was years (range 20.5±41.9) and the mean 6 SD body mass index (BMI) was kg/m 2 (range 16.3±41.4, for the last 955 cycles). Infertility was primary in 1000 cycles (58.8%). The clinical characteristics of the different embryo transfer groups are given in Table I, with the number of oocytes retrieved and the number of embryos cryopreserved. The indications for IVF included tubal factor in 22%, endometriosis (moderate to severe) in 20%, other female factors in 6%, male factor in 19%, combined in 6% and unexplained infertility (including mild endometriosis) in 27% of the transfer cycles. The embryos were graded according to the number of blastomeres, the degree of fragmentation, regularity of blastomeres and presence of multinucleated blastomeres (Tiitinen et al., 2001). Embryo selection was based on morphological criteria on the transfer day, which was most often day 2 after oocyte retrieval. Assisted hatching was not applied. Embryos were suitable for transfer or cryopreservation if they had <50% fragmentation and no multinuclear blastomeres. Elective transfer of one embryo was performed only if on day 2 at least one embryo was available with four or more cells, <20% fragmentation and no multinuclear blastomeres. In 13 cycles eset was performed for medical reasons with embryos with less than four cells or >20% fragmentation. All the spare embryos were frozen (Testart et al., 1986; Tiitinen et al., 2001). At the beginning, the main indication for eset was a medical reason such as diabetes or other chronic disease of the woman, previous hysterotomy, uterine malformation, risk of severe ovarian hyperstimulation or indication for prenatal diagnosis (Vilska et al., 1450 SET PR (%) N2/N1 10/46 9/49 8/43 6/38 6/29 39/205 1/39 eset PR (%) N2/N1 11/40 16/50 33/79 58/152 44/ /470 2/162 2ET PR (%) N2/N1 80/271 96/ /250 63/150 30/86 376/ /376 SET = only one embryo available; eset = elective single embryo transfer; 2ET = two-embryo transfer; N1= number of embryo transfers; N2 = number of pregnancies; N3 = number of twin pregnancies. 1999). Later, the number of indications was gradually increased, to include previous IVF delivery, selection in a randomized study (Martikainen et al., 2001) and the couple's wish to avoid twins. In 2000 we were ready to change our policy to eset as the rst option, while two-embryo transfer was considered in the following conditions: two unsuccessful IVF cycles or poor embryo quality, but no medical contraindications for twin pregnancy. We have now retrospectively analysed the clinical implantation rate, the clinical PR, the delivery rate per embryo transfer and the multiple birth rate in our IVF/ICSI programme. Results The overall clinical PR per embryo transfer during this time period was 34.0% (577/1699), the multiple PR 20% (116/577), the delivery rate 25.5% (434/1699) and the multiple birth rate 18.4% (80/434). Transfer of the only existing embryo resulted in clinical pregnancy and delivery rates of 19.0% (39/205) (Table II) and 15.1% (31/205) respectively. One of these deliveries was a monozygotic twin pregnancy (3.2%, 1/31). The number of cycles where only one embryo was available remained stable over the years (Figure 1). The number of eset increased gradually over the years, representing 56% of the cycles in 2001 (Table II, Figure 1). The clinical PR in eset was 34.5% (162/470) and the delivery rate 27.2% (128/470) during these 5 years. Monozygotic twins were born on two occasions (1.6%, 2/128).

3 Elective single embryo transfer out in 56 selected good prognosis cycles (12.9%), only one embryo was available in 48 cases and two embryos were transferred in 269 cycles (72%). In women aged <37 years, the clinical PR with eset was 34.8% (144/414) and it was 32.1% (18/56) in those aged >37 years. If only one embryo was available for transfer, the respective clinical PR were 21.0% (33/157) but only 12.5% (6/48) in the older group. With twoembryo transfer the PR were 39.9% (301/755) and 27.9% (75/ 269) respectively. The mean number of embryos per transfer and the mean delivery and multiple delivery rates during the study period are given in Figure 2. Figure 1. Percentages of one- and two-embryo transfers in an elective single embryo transfer programme (eset; total of 1699 embryo transfers, see text) from 1997 to 2001 in the IVF clinic of Helsinki University Central Hospital, with corresponding pregnancy and multiple pregnancy rates per embryo transfer (ET). Figure 2. Mean number of embryos transferred (right-hand axis) and the corresponding delivery rates and multiple delivery rates per embryo transfer (%) from 1997 to 2001 in the eset programme of the IVF clinic of Helsinki University Central Hospital, Finland. When two embryos were transferred, the overall clinical PR was 36.7% (376/1024) (Table II, Figure 1) and the delivery rate 26.9% (275/1024). The multiple birth rate was 27.6% (76/275) after two-embryo transfer, including 75 pairs of twins and one set of triplets. Out of the 421 cycles in women aged >37 years (upper range 42.2 years), 373 resulted in embryo transfer: eset was carried Discussion Our results show that single embryo transfer clearly reduces the risk of twin pregnancy and still results in very acceptable pregnancy and delivery rates. Two-embryo transfer became the policy in our clinic in 1993, and in 1994 the rst elective oneembryo transfer was performed. After 1996 we have not performed three-embryo transfers, and in 1997 our eset programme was introduced. After 2000 the primary choice has been single embryo transfer, and two embryos are transferred only with speci c indications. The strategy of eset was gradually implemented in our daily practice from 1997, resulting in a 5-fold increase in the percentage of eset by Today eset is performed in over half of the cycles (56%) when at least two embryos are available. The introduction of assisted reproductive treatment in the late 1980s increased the incidence of multiple pregnancies in many countries. In a large population-based cohort in the USA, the proportion of multiple births attributable to ovulation induction or assisted reproductive treatment was 33% (Lynch et al., 2001). In triplets and higher-order multiple pregnancies this rate must be even higher. In our country the precise impact of all infertility treatments on the rate of multiple pregnancies is not known, since there are no national statistics on the number and results of various ovulation induction protocols. In contrast, the IVF statistics have been collected from the early beginning of assisted reproductive treatment and the nationwide annual IVF statistics in Finland have been reported since 1992 (Gissler and Tiitinen, 2001). The number of IVF/ICSI cycles in our country has stabilized at ~4500 cycles per year. Due to the relatively high availability of treatment, nearly 2.5% of all children born in Finland are nowadays born with the help of assisted reproductive technology. The number of transferred embryos decreased during the 1990s, but the proportion of multiple pregnancies has remained close to 25% (Gissler and Tiitinen, 2001). According to the Finnish Birth Registry, the rate of triplets in Finland was 1:6200 in 2000, which is close to the natural triplet rate. The twin rate has stayed between 1.63% (in 2000) and 1.77% (in 1998). It has been shown that triplets can nearly be avoided by replacing two embryos without signi cantly decreasing the overall PR (Staessen et al., 1993; Templeton and Morris, 1998). However, this policy does not prevent twin pregnancies. The elevated rate of pregnancy complications and neonatal morbidity among IVF children is mainly due to the increased 1451

4 A.Tiitinen et al. proportion of multiple pregnancies and partly due to maternal characteristics regarding infertility (Koivurova et al., 2002). This is true also in twin pregnancies. The ultimate goal of IVF is the birth of a single healthy child, and the way ahead must lie with single embryo transfer (Ozturk et al., 2001). The fear of reducing existing PR is probably the main reason behind the slow adoption of eset. Today a major change towards a single embryo transfer policy is taking place, at least in parts of Europe. Indeed, the rate of eset has been reported to be between 20 and 31% in selected assisted reproduction treatment programmes (Gerris et al., 2002; De Sutter et al., 2003). The following recommendations were formulated during the ESHRE Campus Course. A twin PR of >25% is not acceptable and the aim should be to reduce the incidence to perhaps around 10%. At the same time, an ongoing PR of >30% per started treatment cycle is very acceptable (ESHRE Campus Course Report, 2001). Our opinion is that the most appropriate way forward is by means of an effective eset programme and a good cryopreservation programme to allow satisfactory cumulative PR. We have now shown that this strategy can be introduced into IVF programmes. Indeed, the multiple PR in our IVF programme in 2001 was only 7.5% and the multiple delivery rate very low, 5%. At the same time the overall ongoing PR had fallen slightly, but was still >30%, which is even more than the natural conception rate. Further, our preliminary results for the rst 9 months of 2002 show that the clinical PR has risen to 34% with a multiple PR of 6.4%, and the proportion of eset was 54%. It is to be remembered that we treat only a few women aged >40 years (3.2%), as the age limit for our 1 year waiting list for IVF/ ICSI treatment is 38 years, and the maximum number of fresh treatment cycles offered is usually three. The policy of limiting the number of embryos transferred is not possible without a good cryopreservation programme. In our randomized study, we estimated that maximally 5±10% more pregnancies would be achieved after two-embryo transfer instead of eset, taking into account the cumulative PR after the use of available cryopreserved embryos (Martikainen et al., 2001). When eset is combined with freezing of extra embryos for later use, the cumulative delivery rate per oocyte retrieval can be as high as 53% (Tiitinen et al., 2001). This strategy results in many advantages for both the couple and the physician (Schnorr et al., 2001). Today, most of our subjects are ready to accept our eset programme when they are properly counselled on the risk of twin pregnancy. Indeed, during the randomization study many couples did not want to take part because they wished for one-embryo transfer (Martikainen et al., 2001). Nowadays one-embryo transfer is progressively being introduced in all Finnish IVF clinics (Martikainen et al., 2001). According to a recent healtheconomic estimation, there is no difference in the cost per child born between single and double embryo transfer (De Sutter et al., 2002). Laboratory expertise is highly important in an eset programme, especially in terms of embryo culture, embryo selection, and freezing and thawing techniques. The question of how to select an embryo with a very high implantation potential is yet to be studied. At least the cleavage stage, degree 1452 of fragmentation and absence of multinuclear blastomeres is important (Van Royen et al., 2001). Early cleavage, i.e. complete rst division within 25±27 h post-insemination, also seems to be a strong indicator (Salumets et al., 2003). Variation in zona pellucida thickness, location of fragments and equality of blastomere size may be of importance (Palmstierna et al., 1998). Whether strategy which is based on oocyte or zygote morphology is helpful remains to be studied further (Salumets et al., 2001). Some groups have found the use of pronuclear stage scoring helpful in their programmes (Ludwig et al., 2000). Several embryo scoring systems have been developed, being proposed to predict pregnancy (Terriou et al., 2001). However, most of the reports are based on cycles where more than one embryo has been transferred. Hence, it is dif cult to draw de nite conclusions, because one cannot ascertain which of the embryos actually implanted. Some of the earlier reports were based on single embryo transfers, where only one embryo was available, which is also not the ideal situation for proper embryo classi cation (Giorgetti et al., 1995). In our opinion, appropriate embryo scoring can only be performed with eset. Even though embryo quality is the most important single factor predicting pregnancy and birth, other factors should be remembered when individualizing the transfer strategy. Endometrial receptivity is of importance, and some markers, such as endometrial thickness and texture as documented by means of transvaginal ultrasonography, as well as uterine vascularity, determined by colour Doppler ultrasonography, are available (Tan, 1999). The age of the woman matters, as shown by the lower implantation and delivery rates in older women. However, with strict evaluation, eset can be a good option for women aged >37 years, as shown by our results. The outcome of ovarian stimulation may be predictive: a good ovarian response to FSH stimulation, leading to several mature oocytes, could be a marker of good reproductive function as such (Hunault et al., 2002). This leads to more numerous embryos and allows more choice for selection of one good embryo for transfer. Correct counselling is very important, as infertile couples are known sometimes to desire multiple pregnancies (Gleicher et al., 1995). It is likely that such couples are not really aware of the real risks associated with multiple pregnancies. When women were presented with scenarios of differing pregnancyassociated risk magnitudes between single and multiple pregnancies, their desire for twin pregnancy decreased (Grobman et al., 2001). This means that good counselling should include realistic information, not only on the risks of twin gestation but also on later burdens with multiple birth. It is to be remembered that twin pregnancies cannot totally be abolished with eset programmes. We had two pairs of twins out of 162 pregnancies in our group. A >2-fold increase in monozygotic twinning is indeed associated with assisted reproductive treatment (Schachter et al., 2001). When counselling the couples, such information should be made clear. We have also had two sets of triplets with two-embryo transfers during the last 5 years. We now report a 1.2% incidence of monozygotic twinning, which is in accordance with the results of other studies (Schachter et al., 2001).

5 The need to prevent twin pregnancies is widely accepted (ESHRE Campus Course Report, 2001), although transfer of two embryos is standard policy in many IVF centres. However, increasing the number of embryos transferred does not increase the chance of a birth, but it increases the chance of a multiple birth (Engmann et al., 2001). In another study, judicious application of eset reduced the twinning rate (from 29.5 to 16.3%) while maintaining the overall PR at 33.5% (Gerris et al., 2002). Another group has obtained similar results: the twinning rate dropped from 30 to 21% but the overall pregnancy rate remained the same, 35 and 34% per transfer, respectively (De Sutter et al., 2003). Our eset programme shows that even better results, a multiple PR <10%, can be achieved with acceptable pregnancy and delivery rates. The additional impact of a wellfunctioning cryopreservation programme on the overall cumulative PR per oocyte retrieval is very important to bear in mind. All this should arouse more interest in eset in all countries. References Bergh, T., Ericson, A., HillensjoÈ, T., Nygren, K.-G. and Wennerholm, U.-B. (1999) Deliveries and children born after in-vitro fertilization in Sweden 1982±95: a retrospective cohort study. Lancet, 354, 1579±1585. De Sutter, P., Gerris, J. and Dhont, M. (2002) A health-economic decisionanalytic model comparing double with single embryo transfer in IVF/ICSI. Hum. Reprod., 17, 2891±2896. De Sutter, P., Van der Elst, J., Coetsier, T. and Dhont, M. (2003) Single embryo transfer and multiple pregnancy rate reduction in IVF/ICSI: a 5 year appraisal. Reprod. Biomed., Online, in press Dhont, M., De Sutter, P., Ruyssinck, G., Martens, G. and Bekaert, A. (1999) Perinatal outcome of pregnancies after assisted reproduction: a case±control study. Am. J. Obstet. Gynecol., 181, 688±695. Engmann, L., Maconochie, N., Tan, S.L. and Bekir, J. (2001) Trends in the incidence of births and multiple births and the factors that determine the probability of multiple birth after IVF treatment. Hum. Reprod., 16, 2598±2605. ESHRE (2002) The European IVF-monitoring programme (EIM), for the European Society of Human Reproduction and Embryology (ESHRE). Assisted reproductive technology in Europe, Results generated from European registers by ESHRE. Hum. Reprod., 17, 3260±3274. ESHRE Campus Course Report (2001) Prevention of twin pregnancies after IVF/ICSI by single embryo transfer. Hum. Reprod., 16, 790±800. Gerris, J., De Neubourg, D., Mangelschots, K., Van Royen, E., Van de Meerssche, M. and Valkenburg, M. (1999) Prevention of twin pregnancy after in-vitro fertilization or intracytoplasmic sperm injection based on strict embryo criteria: a prospective randomized clinical trial. Hum. Reprod., 14, 2581±2587. Gerris, J., De Neubourg, D., Mangelschots, K., Van Royen, E., Vercruyssen, M., Barudy-Vasquez, J., Valkenburg, M. and Ryckaert, G. (2002) Elective single day 3 embryo transfer halves the twinning rate without decrease in the ongoing PR of an IVF/ICSI programme. Hum. Reprod., 17, 2626±2631. Giorgetti, C., Terriou, P., Auquier, P., Hans, E., Spach, J.L., Salzmann, J. and Roulier, R. (1995) Embryo score to predict implantation after in-vitro fertilization: based on 957 single embryo transfers. Hum. Reprod., 10, 2427±2431. Gissler, M. and Tiitinen, A. (2001) IVF treatments and their outcomes in Finland in the 1990s. Acta Obstet. Gynecol. Scand., 80, 937±944. Gleicher, N., Campbell, D.P., Chan, C.L., Karande, V., Rao, R. and Balin, M. (1995) The desire for multiple births in couples with infertility problems contradicts present practice patterns. Hum. Reprod., 10, 1079±1084. Grobman, W.A., Milad, M.P., Stout, J. and Klock, S.C. (2001) Patient perceptions of multiple gestations: an assessment of knowledge and risk aversion. Am. J. Obstet. Gynecol., 185, 920±924. Hunault, C.C., Eijkemans, M.J.C., Pieters, M.H.E.C., te Velde, E.R., Habberna, J.D.F., Fauser, B.C.J.M. and Macklon, N.S. (2002) A prediction model for selecting patients undergoing in vitro fertilization for elective single embryo transfer. Fertil. Steril., 77, 725±32. Isaksson, R., Gissler, M. and Tiitinen, A. (2002) Obstetric outcome among women with unexplained infertility after IVF: a matched case±control study. Hum. Reprod., 17, 1755±1761. Koivurova, S., Hartikainen, A.-L., Gissler, M., Hemminki, E., Sovio, U. and JaÈrvelin, M.-R. (2002) Neonatal outcome and congenital malformations in children born after in-vitro fertilization. Hum. Reprod., 17, 1391±1398. Koudstaal, J., Braat, D.D.M., Bruinse, H.W., Naaktgeboren, N., Vermeiden, J.P.W. and Visser, G.H.A. (2000) Obstetric outcome of singleton pregnancies after IVF: a matched control study in four Dutch university hospitals. Hum. Reprod., 15, 1819±1825. Ludwig, M. SchoÈpper, B., Al-Hasani, S. and Diedrich, K. (2000) Clinical use of pronuclear stage score following intracytoplasmic sperm injection: impact on pregnancy rates under the conditions of the German embryo protection law. Hum. Reprod., 15, 325±329. Lundin, K., Bergh, C. and Hardarson, T. (2001) Early embryo cleavage is a strong indicator of embryo quality in human IVF. Hum. Reprod., 16, 2652±2657. Lynch, A., Mcduf e, R., Murphy, J., Faber, K., Leff, M. and Orleans, M. (2001) Assisted reproductive interventions and multiple birth. Obstet. Gynecol., 97, 195±200. Martikainen, H., Tiitinen, A., TomaÂs, C., Tapanainen, J., Orava, M., Tuomivaara, L., Vilska, S., Hyden-Granskog, C., Hovatta, O. and the Finnish ET Study Group (2001) One versus two-embryo transfer after IVF and ICSI: a randomized study. Hum. Reprod., 16, 1900±1903. Ozturk, O., Bhattacharya, S. and Templeton, A. (2001) Avoiding multiple pregnancies in ART. Evaluation and implementation of new strategies. Hum. Reprod., 16, 1319±1321. Palmstierna, M., Murkes, D., Csemiczky, G., Andersson, O. and Wramsby, H. (1998) Zona pellucida thickness variation and occurrence of visible mononucleated blastomeres in preembryos are associated with a high pregnancy rate in IVF treatment. J. Assist. Reprod. Genet., 15, 70±75. Salumets, A., Hyden-Granskog, C., Suikkari, A.-M., Tiitinen, A. and Tuuri, T. (2001) The predictive value of pronuclear morphology of zygotes in the assessment of human embryo quality. Hum. Reprod., 16, 2177±2181. Salumets, A., Hyden-Granskog, C., MaÈkinen, S., Suikkari, A.-M., Tiitinen, A. and Tuuri, T. (2003) Early cleavage predicts the viability of human embryos in elective single embryo transfer procedures. Hum. Reprod., 18, 821±825. Schachter, M., Raziel, A., Friedler, S., Strassburger, D., Bern, O. and Ron-El, R. (2001) Monozygotic twinning after assisted reproductive techniques: a phenomenon independent of micromanipulation. Hum. Reprod., 16, 1264±1269. Schieve, L.A., Meikle, S.F., Ferre, C., Peterson, H.B., Jeng, G. and Wilcox, L.S. (2002) Low and very low birth weight in infants conceived with use of assisted reproductive technology. N. Engl. J. Med., 346, 731±737. Schnorr, J.A., Doviak, M.J., Muasher, S.J. and Jones, H.W. (2001) Impact of a cryopreservation program on the multiple PR associated with assisted reproductive technologies. Fertil. Steril., 75, 147±151. Staessen, C., Janssenswillen, C., Van Den Abbeel, E., Devroey, P. and Van Steirteghem, A.C. (1993) Avoidance of triplet pregnancies by elective transfer of two good quality embryos. Hum. Reprod., 8, 1650±1653. Tan, S.L. (1999) Clinical applications of Doppler and three-dimensional ultrasound in assisted reproductive technology. Ultrasound Obstet. Gynecol., 13, 153±156. Templeton, A. and Morris, J.K. (1998) Reducing the risk of multiple births by transfer of two embryos after in vitro fertilization. N. Engl. J. Med., 339, 573±577. Terriou, P., Sapin, C., Giorgetti, C., Hans, E., Spach, J-L. and Roulier, R. (2001) Embryo score is a better predictor of pregnancy than the number of transferred embryos or female age. Fertil. Steril., 75, 525±531. Testart, J., Lassalle, B., Belaisch-Allart, J., Hazout, A., Forman, R., Rainborn, J.D. and Frydman, R. (1986) High pregnancy rate after early human embryo freezing. Fertil. Steril., 46, 268±272. Tiitinen, A., Halttunen, M., HaÈrkki, P., Vuoristo, P. and Hyden-Granskog, C. (2001) Elective single embryo transfer: the value of cryopreservation. Hum. Reprod., 16, 1140±1144. Toner, J.P. (2002) Progress we can be proud of: U.S. trends in assisted reproduction over the rst 20 years. Fertil. Steril., 78, 943±950. Van Royen, E., Mangelschots, K., De Neuborg, D., Laureys, I., Ryckaert, G. and Gerris, J. (2001) Calculating implantation potential of day 3 embryos in women younger than 38 years of age: a new model. Hum. Reprod., 16, 326±332. Vilska, S., Tiitinen, A., Hyden-Granskog, C. and Hovatta, O. (1999) Elective transfer of one embryo results in an acceptable pregnancy rate and eliminates the risk of multiple birth. Hum. Reprod., 14, 2392±2395. Submitted on February 6, 2003; accepted on April 10, 2003 Elective single embryo transfer 1453