1 Epidemic Intelligence Service, Epidemiology Program Office, Division of Reproductive Health and 2 Assisted Reproductive Technology Epidemiology Team, Women's Health and Fertility Branch, Division of Reproductive Health, Centers for Disease Control and Prevention, Atlanta, GA 30341-3724, USA
3 To whom correspondence should be addressed at: Epidemic Intelligence Service, Epidemiology Program Office, Division of Reproductive Health, Centers for Disease Control and Prevention, 4770 Buford Highway NE, MS K-34, Atlanta, Georgia 30341-3724, USA. Email: dkissin{at}cdc.gov
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Abstract |
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Key words: blastocyst/extended culture/IVF/multiple pregnancy
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Introduction |
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The desire to decrease multiple-birth risk (MBR) while maintaining a high live-birth rate has prompted researchers to investigate techniques that will improve the ability to identify high-quality embryos with greater potential for implantation, thus allowing acceptable rates of live birth to be maintained while transferring fewer embryos. One technique currently proposed as a method that can reduce MBR by achieving high pregnancy rates while transferring fewer embryos is the use of extended embryo culture (Gardner et al., 1998; Vidaeff et al., 2000
).
The recent commercial availability of sequential culture media has greatly increased assisted reproductive technology providers' ability to extend the duration of embryo culture to 5 days, corresponding with the blastocyst stage of development. The technique allows selection of embryos that have improved viability and proven developmental capacity. It also allows for increased synchronization between the embryo and endometrium at the time of embryo transfer. Although the clinical advantages of extended embryo culture continue to be debated, several clinical studies report higher success rates with blastocyst transfers than with cleavage stage transfers (Milki et al., 2000; Karaki et al., 2002
; Van der Auwera et al., 2002
; Frattarelli et al., 2003
). Day 5 transfer has become a routine procedure in many clinics, necessitating research on the association between blastocyst transfers and MBR (Wright et al., 2004
). Recent studies have suggested that transferring fewer blastocysts does not lead to a reduction in multiple births, as expected (Coskun et al., 2000
; Levron et al., 2002
; Emiliani et al., 2003
). Those studies, however, were limited by small sample sizes, which precluded detailed analyses of patient and other treatment factors.
Although we previously published an analysis of MBR based on the population of patients receiving assisted reproductive technology in the USA in 1996 (Schieve et al., 1999), at that time, nearly all embryo transfers were limited to cleavage stage embryos. The current popularity of transferring blastocysts necessitates a re-examination of MBR with particular focus on the effects of the stage of embryo development at transfer. We sought to assess MBR and factors associated with it among the population of patients undergoing IVF in the USA in 2001. We also assessed pregnancy and live-birth rates associated with these procedures. This study was approved by the Centers for Disease Control and Prevention (CDC) institutional review board.
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Materials and methods |
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Our analyses include assisted reproductive technology cycles that were initiated in 2001. We used data from 384 clinics, an estimated 91% of the clinics that were in operation in 2001 (Centers for Disease Control and Prevention, 2003). Although we do not have complete information on the reasons for non-reporting on the part of some clinics, anecdotal reports suggest that many non-reporting clinics were smaller in size than average practices. Thus, we estimate that our data represent >91% of assisted reproductive technology cycles performed in the USA in 2001.
From the 107 587 assisted reproductive technology cycles initiated in 2001, we selected those that progressed to the transfer of 1 embryo (89 162 cycles). Of those, we selected the most common assisted reproductive technology cycle: IVF with freshly fertilized embryos, derived from patient oocytes (64 289 cycles). IVF cycles, both with and without ICSI, were included. Due to sample size limits imposed by key stratification factors we included only cycles among patients aged 2040 years at the time of cycle initiation (56 908 cycles). Cycles performed for patients aged <20 (10 cycles) and patients aged >40 (7371 cycles) were excluded. We further excluded cycles in which embryos were transferred on days 2, 4 or 6 (11% of cycles). After the exclusions, a total of 50 819 IVF cycles remained for analysis: 43 265 cycles in which embryos were transferred on day 3 of culture and 7554 cycles in which embryos were transferred on day 5.
We examined total pregnancy rate, live-birth rate, multiple gestation risk (MGR) and MBR. Pregnancy was defined as a clinical intrauterine gestation detected on ultrasound, generally during the first trimester, and live-birth delivery was defined as the delivery of 1 liveborn infant. A live-birth delivery was classified as a multiple-birth delivery if
2 infants were delivered and
1 of them was liveborn. We examined the percentage of IVF cycles resulting in pregnancies and live-birth deliveries and the percentage of live-birth deliveries that were multiple births. Additionally, because procedures resulting in pregnancies with >2 fetuses are more likely to involve spontaneous or medical reductions in the number of fetuses, we examined the percentage of pregnancies that had been multiple gestations as the total potential for multiple births. A multiple gestation was defined as a pregnancy in which >1 fetal heart was detected on early ultrasound or that resulted in the birth of >1 infant. Because all multiple births are associated with substantially higher rates of infant morbidity and mortality than singletons, we did not separate either MGR or MBR by the number of fetuses or infants. All indices were assessed after stratifying the data according to patient age, number of embryos transferred, and cryopreservation of supernumerary embryos [all of which are factors known to be important predictors of MBR (Templeton and Morris, 1998
; Schieve et al., 1999
)], as well as duration of embryo culture.
Patient age at cycle initiation was categorized as 2029, 3034, 3537 and 3840 years, and the number of embryos transferred was categorized as 1, 2, 3, 4 and 5. Because clinical information on embryo quality is not collected as part of the national Assisted Reproductive Technology Registry, embryo availability was used as a proxy measure. Embryo availability indicates that supernumerary embryos from the same treatment cycle were cryopreserved for future use; in such cases, the clinician would have chosen the embryos with the seemingly highest quality for transfer in the IVF cycle included in the study. Day of embryo transfer was used as a proxy measure for stage of embryo development, because specific data were not available for laboratory assessment of embryo developmental stage or for type of culture medium. Day of transfer was calculated by subtracting the oocyte retrieval date from the embryo transfer date. Typically, embryos transferred on day 3 correspond to the cleavage stage of development, whereas embryos transferred on day 5 correspond to the blastocyst stage.
To assess whether stratification by key variables was sufficient to control for any differences among patient and treatment groups, we repeated our analyses restricting to even more select patient groups, defined as the best and the worst prognosis groups, based on available variables consistently shown to be predictors of pregnancy and live-birth rates in our population-based registry. The best patient prognosis group consisted of cycles meeting all of the following criteria: (i) 10 oocytes retrieved, (ii) no assisted hatching used, (iii) no ICSI performed in the absence of male factor infertility diagnosis, (iv) documented day 3 FSH level that was not elevated, (v) no diagnosis of diminished ovarian reserve, and (vi) either first assisted reproductive technology attempt or a history of live birth(s). In total, 6839 transfer cycles met the criteria for the best patient prognosis group. This number is likely smaller than the true number of best prognosis procedures because FSH data were missing for 35% of all transfers; thus none of these procedures could be classified as best prognosis. The worst patient prognosis group consisted of cycles in which at least one of the following was true: (i) <5 oocytes retrieved, (ii) diagnosis of diminished ovarian reserve, or (iii) elevated FSH level. A total of 8316 transfer cycles was included in the worst patient prognosis group.
Differences in patient and treatment characteristics, pregnancy and live-birth rates, MGR and MBR were evaluated using 2-tests.
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Results |
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The percentage distribution of patient and treatment characteristics among the four presumed embryo quality groups is presented in Table I. A comparison of the general patient profiles for each group suggests that patients in group I have the poorest prognosis in terms of factors associated with successful embryo implantation (i.e. higher proportion of patients: in the two oldest age categories; with a diagnosis of diminished ovarian reserve or both male and female infertility; with prior assisted reproductive technology cycles; and with elevated level of day 3 FSH). In contrast, the patient profile for group IV suggests the best prognosis [i.e. higher proportion than other groups: in the two youngest patient age categories; with diagnoses of tubal factor infertility and ovulatory dysfunction; with no prior assisted reproductive technology cycles (despite having similarly high rates of previous pregnancies and births as the other three groups); and with normal day 3 FSH level]. Additionally, among cycles included in group I, notably fewer oocytes were retrieved, a higher proportion of ICSI use without male factor infertility diagnosis took place and more frequent use of assisted hatching occurred than in other groups; again group IV had the largest contrast on each treatment factor. Groups II and III had similar values for many key patient and treatment factors, which tended to be intermediate between groups I and IV.
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Of 50 819 transfer cycles, 22 279 (43.8%) resulted in a clinical intrauterine pregnancy. Substantial variation in pregnancy rates was found across strata in terms of both patient age and embryo quality (Table II). In general, pregnancy rates were lowest for the group with the poorest presumed embryo quality (group I), intermediate for embryo quality groups II and III, and highest for the highest presumed embryo quality group (group IV). Moreover, an inverse association was found between patient age and pregnancy rate for the poorest presumed embryo quality group (group I): the pregnancy rate associated with 2 embryos transferred was nearly 44% for women aged 2029 years, compared to 22.0% for women aged 3840 years. As embryo quality increased, the age effect lessened, such that no clear trend in pregnancy rate was seen for the highest presumed embryo quality group (group IV). For group IV, the pregnancy rate associated with transferring 2 embryos was 64.1% for women aged 2029 years, whereas the corresponding rates for women aged 3034, 3537, and 3840 years were 65.5, 56.9 and 59.7% respectively.
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The overall live-birth rate was 36.6%. Although lower than pregnancy rates in all but a few instances, live-birth rates had a pattern of results similar to pregnancy rate results (Table III). Generally, highest live-birth rates were achieved in the highest presumed embryo quality group (group IV), whereas the lowest rates were associated with the poorest presumed embryo quality group (group I). Additionally, increasing the number of transferred embryos from 2 to 3 did not increase the chance of a live birth for most patient age and presumed embryo quality groups.
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Discussion |
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Day 5 transfer has become an increasingly viable assisted reproduction treatment option since 1999, and many providers have looked to this promising technique as a feasible opportunity to reduce the number of embryos transferred. Our population-based data show that cycles using day 5 embryos were, in fact, associated with fewer embryos transferred than day 3 transfer cycles. The reduction in number of embryos transferred, however, did not translate into decreased risk for multiple gestations or births. Although in most day 5 IVF cycles, only 2 embryos were transferred, the increased implantation rate associated with day 5 embryos led to an MGR nearly or exceeding 30% in all age groups and nearly or exceeding 40% in the two youngest age groups.
These analyses demonstrate that the chance of a successful assisted reproduction treatment cycle, as measured by pregnancy and live-birth rates, does not always increase linearly with each additional embryo transferred (see Tables II and III). For many cycles, transferring >2 embryos did not increase pregnancy or live-birth rates. Note that comparisons of various numbers of embryos transferred is perhaps most reasonable for the group of women known to have extra embryos available and, thus, known to have electively selected fewer embryos for transfer than were available. Among all such cycles included in this 2001 study population (both day 3 and day 5 transfers in every age group), rates of pregnancy and live birth did not improve when the number of embryos transferred increased from 2 to 3. In fact, the rates did not significantly improve from 1 to 2 embryos transferred for some groups (day 5 transfers, patients aged 3537 years). As noted, however, limiting transfer to 2 embryos still posed a substantial MGR and MBR. Thus, success rates of single embryo transfer are important to consider. Although a few groups in this study did appear to benefit from transferring >2 embryos (i.e. those in the oldest age group with poorest presumed embryo quality), pregnancies in these groups still resulted in high MGR and MBR when high numbers of embryos were transferred.
This analysis is subject to certain limitations. The unit of analysis was an embryo transfer procedure rather than a patient who underwent assisted reproduction treatment. Therefore, it is possible that women undergoing >1 embryo transfer in a given year (who are also most likely to be those who failed at least one treatment) are included in the analysis more than once. This lack of independence could have resulted in underestimation of the true per-patient pregnancy rates and live-birth rates, for which the denominator is the number of IVF transfers. It is highly unlikely that a woman would achieve >1 live birth during a 1 year period, and therefore we do not think that analyses of MBR are affected.
An important limitation to the data is that women were not randomized for number of embryos transferred or for receipt of day 3 or day 5 embryos. The number of embryos transferred and stage of embryo development at transfer were based on patient and provider choice. If a large number of transfer cycles were from clinics with policies to exclusively transfer embryos at either cleavage or blastocyst stage, our results could be influenced by clinic practices that would be difficult to separate from the effect of the embryo stage. While we do not have data on clinic transfer polices, we found that of the 384 clinics included in these analyses, only a limited number exclusively transferred either day 3 (n=42 clinics) or day 5 embryos (n=5 clinics). Moreover, many of these 47 practices tended to be small: 87% had <100 transfers, and 60% had <50 transfers. Thus, this practice pattern might simply reflect patient and provider decision, rather than actual clinic policy. Regardless, these clinics (n=47) contributed only 5% of transfer cycles to our sample.
Nonetheless, it is likely that multiple personal and clinical factors played a role in the decision to transfer a certain number of embryos at a certain stage of development. Although analyses limiting to cycles with known extra embryos does address some of the heterogeneity present in the population regarding number of embryos transferred, unmeasured differences between various patient and treatment groups likely remained. As observed in our early descriptive analysis, both days in culture and cryopreservation of supernumerary embryos showed variation with the few potential predictors of prognosis available in the dataset. In our analyses we controlled for the two most predictive factors of both success and multiple gestation and birth risk available to us: patient age and presumed embryo quality group. Nevertheless, residual differences likely remained. Our analyses of the best and worst prognosis groups, however, did not show different patterns of MGR and MBR distribution by patient age, presumed embryo quality group, or number of embryos transferred from the original sample. Thus, we believe that our results are robust even though they are observational.
To date, published data do not provide conclusive evidence on whether transferring embryos at the blastocyst stage versus the cleavage stage is advantageous for achieving pregnancy and birth (Blake et al., 2002). Our study supports a positive association. Even though uncertainty remains, blastocyst transfers have become widely available during the past several years. Our population-based study suggests that many clinics may be limiting embryo transfer to 2 blastocysts to avoid high-order multiple gestations. Our study also shows, however, that transferring 2 embryos that have been cultured for 5 days is associated with high MGR and MBR. This finding necessitates consideration of single-blastocyst transfer.
Our study suggests that transferring a single embryo, either cleavage or blastocyst, appears to be the only way to reduce MGR and MBR in all subgroups. Our analyses showed, however, that SET is an extremely rare procedure. Although we could not fully assess the efficacy of SET due to sample size limitations, our data are suggestive that SET may be efficacious for some groups of women. Other research supports the feasibility of SET, with pregnancy rates ranging from 29.7 to 60.9% in specific groups of women (Gerris et al., 1999; Vilska et al., 1999
; Dhont, 2001
; Martikainen et al., 2001
; Kovacs et al., 2003
; Gardner et al., 2004
; Milki et al., 2004
; Thurin et al., 2004
). Further studies on SET efficacy that would address the issue of generalizability of the evidence are strongly encouraged.
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References |
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Submitted on January 10, 2005; resubmitted on February 25, 2005; accepted on March 7, 2005.