Reproductive Medicine Unit, Department of Obstetrics and Gynaecology, The University of Adelaide, The Queen Elizabeth Hospital, 28 Woodville Road, Woodville, South Australia 5011, Australia
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Abstract |
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Key words: frozen embryo transfer/infertility/multiple conception
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Introduction |
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Materials and methods |
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The general procedures associated with the programmes of assisted reproduction have been previously described (Kerin et al., 1984; Sathanandan et al., 1989
; Matthews et al., 1991
; Norman et al., 1991
) and embryo cryopreservation by Wang et al. (Wang et al., 1994
). Following fertilization and the transfer of fresh embryos/oocytes on either day 2 or day 3 after oocyte retrieval, the remaining embryos were assessed and embryos classified grade I or II according to a standard classification system (Mandelbaum et al., 1987
) were cryopreserved. Embryos were generally cryopreserved at the early cleavage stages. Since 1988 a standard freezingthawing protocol based on propanediol as cryoprotectant has been used (Testart et al., 1986
). At the time of frozen embryo transfer, the frozen embryos were thawed and surviving embryos were transferred within 24 h depending on the stage of development at cryopreservation. Survival was defined as at least 50% of the blastomeres being intact. Confirmation of a successful implantation was performed by detecting an increased serum human chorionic gonadotrophin (HCG) concentration (>25 IU/l) 14 days post-frozen embryo transfer. The number of embryonic sac(s) was determined by ultrasound scanning 46 weeks after the thawed embryo transfer. Both ectopic pregnancy and miscarriage prior to ultrasound were considered as having a single embryo implantation. The implantation rate was calculated as the number of successful implanted embryos divided by the total number of embryos transferred.
The major factors analysed included: maternal age at the time of transfer; aetiology of infertility; classified as tubal factor and non-tubal factor (endometriosis, male factor and unexplained infertility); type of assisted reproduction employed in the fresh cycle; outcome of the fresh embryo transfer (pregnant or not); number of previous fresh cycle attempts; embryo storage time defined as from the day of storage (fresh stimulation cycle) to the day of frozen embryo transfer.
Statistical analyses
Instat and SAS programmes were used to perform statistical analyses. The results were analysed using 2 test or Fisher's exact test whenever there was a cell with small number (n <5). A two-sided P value of < 0.05 was considered as statistically significant.
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Results |
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Multiple conception was also markedly higher in the groups with the higher implantation rate. In the three younger groups, 17.7% of subjects had a multiple conception compared with only 4.8% of women aged >40 years old. This result was not significant possibly due to the small sample size.
In addition to age, both the aetiology of the infertility and the outcome of the fresh embryo transfer at the time the embryos were generated also had significant influences on the implantation rate of the frozenthawed embryos (Table II). Younger women (<40 years) with non-tubal infertility had a significantly higher implantation rate than similar-aged women with a tubal cause for their infertility. However, for older women (
40 years) the implantation rate was low, and did not differ between tubal and non-tubal factor infertility. An increased rate of embryo implantation after frozen embryo transfer was evident in younger women who became pregnant in their fresh assisted reproductive technology cycles compared with those who did not. This increase was of the order of 65% (P < 0.001) and was even more marked in older women (>40 years) who established a pregnancy in the fresh cycle for whom a 144% increase was observed. However, this latter observation failed to reach statistical significance (P = 0.055). A slight, albeit non-significant (P = 0.06), increase in the implantation rate was also associated with a longer storage time (8.9% for <12 months, 10.3% for 12 years and 11.4% for >2 years), but this result was probably due to the confounding effect of those women who achieved a pregnancy following the fresh cycle. Patients previously treated by GIFT also had a better implantation rate (19%) but <10% of women were in this category. The number of oocytes recovered in the fresh treatment cycle and the number of fresh cycle attempts was also analysed but failed to show significant impact on the implantation rate.
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Discussion |
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Similar to previous findings following the transfer of fresh embryos, a reduced pregnancy rate following frozen embryo transfer was recorded with increasing female age, with the major reduction occurring after 40 years of age (P < 0.01). The effect of age on implantation rate seemed to be independent of the other two major factors, i.e. infertility aetiology and the outcome of the fresh cycle. A relatively consistently high implantation rate in women aged <40 years implies that this group of patients will benefit most from frozen embryo transfer. However, women >40 years had a poor prognosis of a successful pregnancy from frozen embryo transfer and even when the number of embryos transferred was increased from two to three, the chance of pregnancy was not improved. As previous noted (Toner et al., 1991b), the chance of implantation and pregnancy following frozen embryo transfer was higher if conception occurred in the fresh cycle. Increasing the number of embryos transferred in these subjects was more likely to increase the chance of multiple conception rather than their chance of pregnancy. The third major factor found to affect the implantation rate in our frozen embryo transfer cycles was the aetiology of the infertility. Patients with tubal blockage had a significantly lower implantation rate than average. This finding is consistent with earlier reports of lower general fecundity in this group of patients (Templeton and Morris, 1998
). The lack of an effect of the time of embryo storage was reassuring.
The expectation of pregnancy and the risk of multiple conception are important issues for both the clinician and the couple. Indeed, efforts to reduce multiple conception have demanded a great deal of recent attention (Bronson et al., 1997; Coetsier and Dhont, 1998
; Roest, 1998
). Ideally, the approach should be individualized; however, there has been a lack of useful up-to-date data associated with frozen embryo transfer. The use of a prognostic table to show the risk of an adverse event has been documented in a number of medical fields (Anderson et al., 1991
), including cardiology (Jackson, 1999
). However, to date, this approach has not been applied in reproductive medicine. Using the current data, a similar table was constructed to document the expectation of pregnancy and importantly the risk of multiple conception.
Using the factors that were shown to influence the implantation rate in frozen embryo transfer, women could be stratified into five groups depending on their estimated chance of pregnancy per frozen embryo transfer cycle.
In Table III, the chance of pregnancy following one-, two- or three-embryo transfer is shown. The risk of multiple implantation was documented as well as the chance of pregnancy. If a single frozenthawed embryo was transferred, the pregnancy rate would be between 10 and 14% in younger patients (<40 years old) who had conceived on their fresh cycle. No multiple implantation was recorded. For other subjects with a single embryo transferred the expectation of pregnancy would be much lower, <10%. If two embryos were transferred the expectation of pregnancy could be >20% in younger women who had achieved pregnancy in the fresh cycle and their risk of multiple conception was 1529%. With three embryos transferred, the pregnancy chance would not be higher than two-embryo transfer in the majority of subjects but the risk of multiple conception would be increased. Using a prognostic table similar to that described here several factors could be considered before frozen embryo transfer regarding the chance of pregnancy and risk of multiple conception. Such easier-to-understand summarized information would allow improved patient decision making.
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In conclusion, female age, the aetiology of infertility and the outcome of the fresh cycle are the most important factors so far analysed that influence the implantation rate following frozen embryo transfer. Based on our data, it has been possible to construct a prognostic table that may assist in determining the optimal number of embryos to replace in frozen embryo transfer to provide better individualized counselling and secure an optimal expectation of pregnancy while reducing the risk of multiple conception.
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Notes |
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References |
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Submitted on March 28, 2001; accepted on June 28, 2001.