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Maternal and perinatal outcomes after fresh versus frozen embryo transfer—what is the risk-benefit ratio?
Maternal and perinatal outcomes after fresh versus frozen embryo transfer—what is the risk-benefit ratio? Siladitya Bhattacharya, M.D. Aberdeen Fertility Centre, Institute of Applied Health Sciences, University of Aberdeen, Aberdeen, United Kingdom
Fresh ET has been the conventional strategy in IVF, but there is a growing opinion suggesting that its maternal and perinatal outcomes can be enhanced by a policy of elective freezing of embryos, followed by transfer at a later date. Available studies suggest a number of improved maternal and perinatal outcomes after frozen ET, although there is also a suggestion of large for gestational age babies associated with this strategy. The observational nature of the available data limit our confidence in the results of available studies. A genuinely unbiased estimate of the advantages of a policy of elective ET can only be confirmed by a definitive randomized controlled trial with an adequate length of follow-up of the offspring. (Fertil SterilÒ 2016;106:241–3. Ó2016 by American Society for Reproductive Medicine.) Key Words: Frozen embryo transfer, perinatal, maternal, outcomes Discuss: You can discuss this article with its authors and with other ASRM members at https://www.fertstertdialog.com/posts/ 10586-maternal-and-perinatal-outcomes-after-fresh-versus-frozen-embryo-transfer-what-is-the-risk-benefit-ratio
S
ince its inception, the conventional strategy in IVF has been to place one or more highquality fresh embryos in the uterus. The advent of cryopreservation technology allowed the possibility of freezing any remaining embryos for subsequent use (1). Further advances in cryobiology have improved outcomes (2), and although fresh ET still remains the default strategy, there is a growing view that elective frozen ET could yield better maternal and perinatal outcomes.
POTENTIAL BENEFITS OF FROZEN ET A policy of elective single ET provides a means of minimizing the risk of multiple pregnancies without compromising
cumulative live-birth rates (3). By ensuring that any surplus embryos are available for future use, it reduces pressure on patients and clinicians to transfer more than one embryo at a time (4, 5). This in turn safeguards an ensuing pregnancy from the added maternal and perinatal risks associated with twins or higher order multiples. A more direct benefit of a policy of elective embryo freezing followed by delayed transfer is reduction of the risk of ovarian hyperstimulation syndrome (OHSS) (6). Exposure to the rising serum bhCG levels that accompany an early pregnancy can aggravate the risk of OHSS in women, particularly in those with a brisk response to gonadotropin stimulation. Avoiding exposure to bhCG by triggering ovulation by means of GnRH agonist (7) can be a
Received May 3, 2016; revised and accepted June 24, 2016. S.B. has nothing to disclose. Reprint requests: Siladitya Bhattacharya, M.D., Aberdeen Fertility Centre, Institute of Applied Health Sciences, University of Aberdeen, Aberdeen AB25 2ZL, United Kingdom (E-mail: s.bhattacharya@ abdn.ac.uk). Fertility and Sterility® Vol. 106, No. 2, August 2016 0015-0282/$36.00 Copyright ©2016 American Society for Reproductive Medicine, Published by Elsevier Inc. http://dx.doi.org/10.1016/j.fertnstert.2016.06.038 VOL. 106 NO. 2 / AUGUST 2016
safer option, but this has been shown to affect the endometrial receptivity and the chances of implantation. This is where elective freezing of embryos with a view to replacement in a future natural or hormonally mediated nonstimulation cycle can reduce the risk of OHSS without compromising pregnancy rates (6). A number of groups have studied maternal and perinatal outcomes in IVF pregnancies. In the absence of any suitable randomized trials with an adequate duration of follow-up, a systematic review of observational studies (8) suggests that singleton pregnancies resulting from frozen ET are associated with lower obstetric and perinatal morbidity. This meta-analysis (8) showed that the combined relative risks (95% confidence intervals [CIs]) of antepartum hemorrhage, preterm birth, small for gestational age, low birth weight, and perinatal mortality were 0.67 (0.55, 0.81); 0.84 (0.78, 0.90); 0.45 (0.30, 0.66); 0.69 (0.62, 0.76); and 0.68 (0.48, 0.96), respectively. A subsequent study based on analysis of a large Nordic data set (9) has confirmed many 241
VIEWS AND REVIEWS of these findings. Singletons conceived from the transfer of frozen embryos had reduced odds of low birth weight, preterm birth, and small for gestational age (adjusted odds ratio (ORs) [95% CI] ¼ 0.81 [0.71, 0.91]; 0.84 [0.76, 0.92]; 0.72 [0.62, 0.83], respectively).
RISKS OF A ROUTINE ‘‘FREEZE-ALL’’ POLICY A fundamental concern of clinicians and couples is that adoption of a policy of elective embryo freezing on grounds of safety could have an impact on the success of IVF. National registry data from both sides of the Atlantic have shown a steady rise in live-birth rates from frozen ET cycles to a point where these are comparable to those after fresh transfer (10, 11). Any comparisons need to acknowledge the usual limitations associated with observational cycle–based data with significant risks of bias and confounding. Using the ET episode as the denominator can be misleading and lead to spurious results in some cases. For example, frozen ET cycles seem to be particularly successful in older women who undergo several episodes of ovarian stimulation before attempting an ET of thawed pooled embryos. There are few robust data on clinical effectiveness or safety from large randomized trials of frozen versus fresh ET. Although an initial meta-analysis of data from three small trials (12) has reported higher pregnancy rates after frozen ET (OR ¼ 1.32; 95% CI, 1.10–1.59), the methodological quality of these trials is suboptimal and their generalizability is limited. One of them (13) has since been retracted from the literature. The population included in the remaining trials represents women with normal or excessively brisk response to ovarian stimulation whose outcomes cannot be extrapolated to a wider population of women, including those with relatively few eggs and embryos. Women with a relatively poor prognosis have been excluded from the randomized trials published so far. As the decision to freeze is driven by the presence of adequate numbers of good-quality embryos, there is a systematic bias against these women in observational data on frozen ET cycles. Although most perinatal outcomes appear to be improved after frozen ET, there are some that remain a source of concern. For example, the Nordic study (9) has reported increased odds of large for gestational age (adjusted OR [95% CI] ¼ 1.45 [1.27, 1.64]) and macrosomic (adjusted OR [95% CI] ¼ 1.58 [1.39, 1.80]) offspring after frozen ET. These risks persisted after adjusting for birth order in a sibling cohort, suggesting that it is likely that they may be caused by embryo freezing and thawing (13). The Nordic data also suggest that offspring conceived through frozen ET are at higher risk of perinatal mortality (9). There are other reasons why an elective frozen ET strategy may appear to be a risky option. The concept of elective embryo freezing is counterintuitive for many clinicians and patients, who are aware that not all frozen embryos survive the freeze-thaw process. In addition, future achievement of a positive health status (in this case pregnancy) is often valued less than at the present time (14). Finally, the technical skill of the embryology laboratory is a key factor in shaping future pol242
icy, and IVF centers with suboptimal freezing outcomes may, rightly, question the need to change their practice.
INTERPRETATION OF THE AVAILABLE DATA There are serious limitations associated with aggregation of nonrandomized data, and therefore the results of the meta-analysis (8) should be treated with caution. First, there is heterogeneity in terms of the research methods used as well as the definition and ascertainment of key outcomes. As most studies have reported outcomes per fresh or frozen cycle (rather than per woman), there is a risk of clustering of women within cycles that can amplify the imprecision of any results. It is also difficult to rule out a degree of bias in terms of the populations who undergo either fresh or frozen ET. Women in the latter category are, to a great extent, a self-selecting group who tend to be younger, respond better to ovarian stimulation, produce more embryos of quality, and have a better prognosis. There are also major differences among the included studies in the techniques used for cryopreservation (slow freezing or vitrification), thawing, the stage at which embryos were frozen (cleavage stage or blastocyst), and the choice of natural versus hormonally modulated cycles for frozen ET. The last factor is unlikely to be particularly influential, as a Cochrane review has failed to reveal any difference in success rates between the two strategies (15). Finally, it is not possible, in this type of meta-analysis, to adjust for confounding factors such as age, smoking, parity, and preexisting health conditions—all of which have an influential role in determining perinatal outcomes. Wennerholm et al. did adjust for a number of confounders but was unable to do so for maternal smoking and body mass index (9). Interpretation of the above data should also take into account the biological plausibility of the results. Replacement of embryos in a superovulated IVF cycle runs the risk of exposing them to a hostile uterine uterine environment associated with high levels of estrogen and P (16), which can be avoided by transfer in a subsequent natural or hormonally manipulated cycle. A more natural endometrial environment may be more conducive to placentation—thus resulting in lower risk of antepartum hemorrhage (17). A systematic review has shown that elevated P levels during the initial ovarian stimulation phase of IVF is associated with lower pregnancy rates, which can be avoided by elective freezing of the embryos and delayed transfer transfers (18). Although it is very unlikely that freezing and thawing improves embryo quality, these additional procedures could lead to better pregnancy outcomes by filtering out those with poor growth potential.
CONCLUSION Elective freezing of all embryos currently offers benefits that outweigh disadvantages in some women undergoing IVF at risk of OHSS. Our confidence in the results of existing studies suggesting improved maternal and perinatal outcomes after frozen ET is limited by the observational nature of the available data. A genuinely unbiased estimate of the advantages of VOL. 106 NO. 2 / AUGUST 2016
Fertility and Sterility® a policy of elective ET can only be confirmed by a definitive randomized controlled trial with an adequate length of follow-up of the offspring.
REFERENCES 1. 2.
3.
4. 5.
6.
7. 8.
Edwards RG. In vitro fertilization: past and future. Ann Biol Clin 1987;45: 321–9. Shi Y, Wei D, Liang X, Sun Y, Liu J, Cao Y, et al. Live birth after fresh embryo transfer vs elective embryo cryopreservation/frozen embryo transfer in women with polycystic ovary syndrome undergoing IVF (FreFro-PCOS): study protocol for a multicenter, prospective, randomized controlled clinical trial. Trials 2014;15:154. McLernon DJ, Harrild K, Bergh C, Davies MJ, de Neubourg D, Dumoulin JC, et al. Clinical effectiveness of elective single versus double embryo transfer: meta-analysis of individual patient data from randomised trials. Br Med J 2010;341:c6945. Gerris J. Single-embryo transfer versus multiple-embryo transfer. Reprod Biomed Online 2009;18(Suppl 2):63–70. Fauque P, Jouannet P, Davy C, Guibert J, Viallon V, Epelboin S, et al. Cumulative results including obstetrical and neonatal outcome of fresh and frozen-thawed cycles in elective single versus double fresh embryo transfers. Fertil Steril 2009;101:1618–23. Fatemi HM, Popovic-Todorovic B, Humaidan P, Kol S, Banker M, Devroey P, et al. Severe ovarian hyperstimulation syndrome after gonadotropinreleasing hormone (GnRH) agonist trigger and ‘‘freeze-all’’ approach in GnRH antagonist protocol. Fertil Steril 2014;101:1008–11. Devroey P, Polyzos NP, Blockeel C. An OHSS-free clinic by segmentation of IVF treatment. Hum Reprod 2011;26:2593–7. Maheshwari A, Pandey S, Shetty A, Hamilton M, Bhattacharya S. Obstetric and perinatal outcomes in singleton pregnancies resulting from the transfer of frozen thawed versus fresh embryos generated through in vitro fertilization treatment: a systematic review and meta-analysis. Fertil Steril 2012;98. 368–77.e1–9.
VOL. 106 NO. 2 / AUGUST 2016
9.
10.
11.
12.
13.
14. 15. 16.
17.
18.
Wennerholm UB, Henningsen AK, Romundstad LB, Bergh C, Pinborg A, Skjaerven R, et al. Perinatal outcomes of children born after frozenthawed embryo transfer: a Nordic cohort study from the CoNARTaS group. Hum Reprod 2013;28:2545–53. Sunderam S, Kissin DM, Crawford SB, Folger SG, Jamieson DJ, Warner L, et al. Assisted reproductive technology surveillance—United States, 2013. Morb Mortal Wkly Rep Surveill Summ 2015;64:1–25. European IVF-Monitoring Consortium (EIM), European Society of Human Reproduction and Embryology (ESHRE)Kupka MS, D’Hooghe T, Ferraretti AP, de Mouzon J, Erb K, Castilla JA, et al. Assisted reproductive technology in Europe, 2011: results generated from European registers by ESHRE. Hum Reprod 2016;31:233–48. Roque M, Lattes K, Serra S, Sola I, Geber S, Carreras R, et al. Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: a systematic review and meta-analysis. Fertil Steril 2013;99:156–62. Pinborg A, Henningsen AA, Loft A, Malchau SS, Forman J, Andersen AN. Large baby syndrome in singletons born after frozen embryo transfer (FET): is it due to maternal factors or the cryotechnique? Hum Reprod 2014;29:618–27. Ortendahl M, Fries JF. Discounting and risk characteristics in clinical decisionmaking. Med Sci Monitor 2006;12:RA41–5. Ghobara T, Vandekerckhove P. Cycle regimens for frozen-thawed embryo transfer. Cochrane Database Syst Rev 2008:CD003414. Evans J, Hannan NJ, Edgell TA, Vollenhoven BJ, Lutjen PJ, Osianlis T, et al. Fresh versus frozen embryo transfer: backing clinical decisions with scientific and clinical evidence. Hum Reprod Update 2014;20:808–21. Healy DL, Breheny S, Halliday J, Jaques A, Rushford D, Garrett C, et al. Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria Australia. Hum Reprod 2010;25:265–74. Venetis CA, Kolibianakis EM, Bosdou JK, Tarlatzis BC. Progesterone elevation and probability of pregnancy after IVF: a systematic review and meta-analysis of over 60,000 cycles. Hum Reprod Update 2013; 19:433–57.
243
Fresh ET has been the conventional strategy in IVF, but there is a growing opinion suggesting that its maternal and perinatal outcomes can be enhanced by a policy of elective freezing of embryos, followed by transfer at a later date. Available studies suggest a number of improved maternal and perinatal outcomes after frozen ET, although there is also a suggestion of large for gestational age babies associated with this strategy. The observational nature of the available data limit our confidence in the results of available studies. A genuinely unbiased estimate of the advantages of a policy of elective ET can only be confirmed by a definitive randomized controlled trial with an adequate length of follow-up of the offspring. (Fertil SterilÒ 2016;106:241–3. Ó2016 by American Society for Reproductive Medicine.) Key Words: Frozen embryo transfer, perinatal, maternal, outcomes Discuss: You can discuss this article with its authors and with other ASRM members at https://www.fertstertdialog.com/posts/ 10586-maternal-and-perinatal-outcomes-after-fresh-versus-frozen-embryo-transfer-what-is-the-risk-benefit-ratio
S
ince its inception, the conventional strategy in IVF has been to place one or more highquality fresh embryos in the uterus. The advent of cryopreservation technology allowed the possibility of freezing any remaining embryos for subsequent use (1). Further advances in cryobiology have improved outcomes (2), and although fresh ET still remains the default strategy, there is a growing view that elective frozen ET could yield better maternal and perinatal outcomes.
POTENTIAL BENEFITS OF FROZEN ET A policy of elective single ET provides a means of minimizing the risk of multiple pregnancies without compromising
cumulative live-birth rates (3). By ensuring that any surplus embryos are available for future use, it reduces pressure on patients and clinicians to transfer more than one embryo at a time (4, 5). This in turn safeguards an ensuing pregnancy from the added maternal and perinatal risks associated with twins or higher order multiples. A more direct benefit of a policy of elective embryo freezing followed by delayed transfer is reduction of the risk of ovarian hyperstimulation syndrome (OHSS) (6). Exposure to the rising serum bhCG levels that accompany an early pregnancy can aggravate the risk of OHSS in women, particularly in those with a brisk response to gonadotropin stimulation. Avoiding exposure to bhCG by triggering ovulation by means of GnRH agonist (7) can be a
Received May 3, 2016; revised and accepted June 24, 2016. S.B. has nothing to disclose. Reprint requests: Siladitya Bhattacharya, M.D., Aberdeen Fertility Centre, Institute of Applied Health Sciences, University of Aberdeen, Aberdeen AB25 2ZL, United Kingdom (E-mail: s.bhattacharya@ abdn.ac.uk). Fertility and Sterility® Vol. 106, No. 2, August 2016 0015-0282/$36.00 Copyright ©2016 American Society for Reproductive Medicine, Published by Elsevier Inc. http://dx.doi.org/10.1016/j.fertnstert.2016.06.038 VOL. 106 NO. 2 / AUGUST 2016
safer option, but this has been shown to affect the endometrial receptivity and the chances of implantation. This is where elective freezing of embryos with a view to replacement in a future natural or hormonally mediated nonstimulation cycle can reduce the risk of OHSS without compromising pregnancy rates (6). A number of groups have studied maternal and perinatal outcomes in IVF pregnancies. In the absence of any suitable randomized trials with an adequate duration of follow-up, a systematic review of observational studies (8) suggests that singleton pregnancies resulting from frozen ET are associated with lower obstetric and perinatal morbidity. This meta-analysis (8) showed that the combined relative risks (95% confidence intervals [CIs]) of antepartum hemorrhage, preterm birth, small for gestational age, low birth weight, and perinatal mortality were 0.67 (0.55, 0.81); 0.84 (0.78, 0.90); 0.45 (0.30, 0.66); 0.69 (0.62, 0.76); and 0.68 (0.48, 0.96), respectively. A subsequent study based on analysis of a large Nordic data set (9) has confirmed many 241
VIEWS AND REVIEWS of these findings. Singletons conceived from the transfer of frozen embryos had reduced odds of low birth weight, preterm birth, and small for gestational age (adjusted odds ratio (ORs) [95% CI] ¼ 0.81 [0.71, 0.91]; 0.84 [0.76, 0.92]; 0.72 [0.62, 0.83], respectively).
RISKS OF A ROUTINE ‘‘FREEZE-ALL’’ POLICY A fundamental concern of clinicians and couples is that adoption of a policy of elective embryo freezing on grounds of safety could have an impact on the success of IVF. National registry data from both sides of the Atlantic have shown a steady rise in live-birth rates from frozen ET cycles to a point where these are comparable to those after fresh transfer (10, 11). Any comparisons need to acknowledge the usual limitations associated with observational cycle–based data with significant risks of bias and confounding. Using the ET episode as the denominator can be misleading and lead to spurious results in some cases. For example, frozen ET cycles seem to be particularly successful in older women who undergo several episodes of ovarian stimulation before attempting an ET of thawed pooled embryos. There are few robust data on clinical effectiveness or safety from large randomized trials of frozen versus fresh ET. Although an initial meta-analysis of data from three small trials (12) has reported higher pregnancy rates after frozen ET (OR ¼ 1.32; 95% CI, 1.10–1.59), the methodological quality of these trials is suboptimal and their generalizability is limited. One of them (13) has since been retracted from the literature. The population included in the remaining trials represents women with normal or excessively brisk response to ovarian stimulation whose outcomes cannot be extrapolated to a wider population of women, including those with relatively few eggs and embryos. Women with a relatively poor prognosis have been excluded from the randomized trials published so far. As the decision to freeze is driven by the presence of adequate numbers of good-quality embryos, there is a systematic bias against these women in observational data on frozen ET cycles. Although most perinatal outcomes appear to be improved after frozen ET, there are some that remain a source of concern. For example, the Nordic study (9) has reported increased odds of large for gestational age (adjusted OR [95% CI] ¼ 1.45 [1.27, 1.64]) and macrosomic (adjusted OR [95% CI] ¼ 1.58 [1.39, 1.80]) offspring after frozen ET. These risks persisted after adjusting for birth order in a sibling cohort, suggesting that it is likely that they may be caused by embryo freezing and thawing (13). The Nordic data also suggest that offspring conceived through frozen ET are at higher risk of perinatal mortality (9). There are other reasons why an elective frozen ET strategy may appear to be a risky option. The concept of elective embryo freezing is counterintuitive for many clinicians and patients, who are aware that not all frozen embryos survive the freeze-thaw process. In addition, future achievement of a positive health status (in this case pregnancy) is often valued less than at the present time (14). Finally, the technical skill of the embryology laboratory is a key factor in shaping future pol242
icy, and IVF centers with suboptimal freezing outcomes may, rightly, question the need to change their practice.
INTERPRETATION OF THE AVAILABLE DATA There are serious limitations associated with aggregation of nonrandomized data, and therefore the results of the meta-analysis (8) should be treated with caution. First, there is heterogeneity in terms of the research methods used as well as the definition and ascertainment of key outcomes. As most studies have reported outcomes per fresh or frozen cycle (rather than per woman), there is a risk of clustering of women within cycles that can amplify the imprecision of any results. It is also difficult to rule out a degree of bias in terms of the populations who undergo either fresh or frozen ET. Women in the latter category are, to a great extent, a self-selecting group who tend to be younger, respond better to ovarian stimulation, produce more embryos of quality, and have a better prognosis. There are also major differences among the included studies in the techniques used for cryopreservation (slow freezing or vitrification), thawing, the stage at which embryos were frozen (cleavage stage or blastocyst), and the choice of natural versus hormonally modulated cycles for frozen ET. The last factor is unlikely to be particularly influential, as a Cochrane review has failed to reveal any difference in success rates between the two strategies (15). Finally, it is not possible, in this type of meta-analysis, to adjust for confounding factors such as age, smoking, parity, and preexisting health conditions—all of which have an influential role in determining perinatal outcomes. Wennerholm et al. did adjust for a number of confounders but was unable to do so for maternal smoking and body mass index (9). Interpretation of the above data should also take into account the biological plausibility of the results. Replacement of embryos in a superovulated IVF cycle runs the risk of exposing them to a hostile uterine uterine environment associated with high levels of estrogen and P (16), which can be avoided by transfer in a subsequent natural or hormonally manipulated cycle. A more natural endometrial environment may be more conducive to placentation—thus resulting in lower risk of antepartum hemorrhage (17). A systematic review has shown that elevated P levels during the initial ovarian stimulation phase of IVF is associated with lower pregnancy rates, which can be avoided by elective freezing of the embryos and delayed transfer transfers (18). Although it is very unlikely that freezing and thawing improves embryo quality, these additional procedures could lead to better pregnancy outcomes by filtering out those with poor growth potential.
CONCLUSION Elective freezing of all embryos currently offers benefits that outweigh disadvantages in some women undergoing IVF at risk of OHSS. Our confidence in the results of existing studies suggesting improved maternal and perinatal outcomes after frozen ET is limited by the observational nature of the available data. A genuinely unbiased estimate of the advantages of VOL. 106 NO. 2 / AUGUST 2016
Fertility and Sterility® a policy of elective ET can only be confirmed by a definitive randomized controlled trial with an adequate length of follow-up of the offspring.
REFERENCES 1. 2.
3.
4. 5.
6.
7. 8.
Edwards RG. In vitro fertilization: past and future. Ann Biol Clin 1987;45: 321–9. Shi Y, Wei D, Liang X, Sun Y, Liu J, Cao Y, et al. Live birth after fresh embryo transfer vs elective embryo cryopreservation/frozen embryo transfer in women with polycystic ovary syndrome undergoing IVF (FreFro-PCOS): study protocol for a multicenter, prospective, randomized controlled clinical trial. Trials 2014;15:154. McLernon DJ, Harrild K, Bergh C, Davies MJ, de Neubourg D, Dumoulin JC, et al. Clinical effectiveness of elective single versus double embryo transfer: meta-analysis of individual patient data from randomised trials. Br Med J 2010;341:c6945. Gerris J. Single-embryo transfer versus multiple-embryo transfer. Reprod Biomed Online 2009;18(Suppl 2):63–70. Fauque P, Jouannet P, Davy C, Guibert J, Viallon V, Epelboin S, et al. Cumulative results including obstetrical and neonatal outcome of fresh and frozen-thawed cycles in elective single versus double fresh embryo transfers. Fertil Steril 2009;101:1618–23. Fatemi HM, Popovic-Todorovic B, Humaidan P, Kol S, Banker M, Devroey P, et al. Severe ovarian hyperstimulation syndrome after gonadotropinreleasing hormone (GnRH) agonist trigger and ‘‘freeze-all’’ approach in GnRH antagonist protocol. Fertil Steril 2014;101:1008–11. Devroey P, Polyzos NP, Blockeel C. An OHSS-free clinic by segmentation of IVF treatment. Hum Reprod 2011;26:2593–7. Maheshwari A, Pandey S, Shetty A, Hamilton M, Bhattacharya S. Obstetric and perinatal outcomes in singleton pregnancies resulting from the transfer of frozen thawed versus fresh embryos generated through in vitro fertilization treatment: a systematic review and meta-analysis. Fertil Steril 2012;98. 368–77.e1–9.
VOL. 106 NO. 2 / AUGUST 2016
9.
10.
11.
12.
13.
14. 15. 16.
17.
18.
Wennerholm UB, Henningsen AK, Romundstad LB, Bergh C, Pinborg A, Skjaerven R, et al. Perinatal outcomes of children born after frozenthawed embryo transfer: a Nordic cohort study from the CoNARTaS group. Hum Reprod 2013;28:2545–53. Sunderam S, Kissin DM, Crawford SB, Folger SG, Jamieson DJ, Warner L, et al. Assisted reproductive technology surveillance—United States, 2013. Morb Mortal Wkly Rep Surveill Summ 2015;64:1–25. European IVF-Monitoring Consortium (EIM), European Society of Human Reproduction and Embryology (ESHRE)Kupka MS, D’Hooghe T, Ferraretti AP, de Mouzon J, Erb K, Castilla JA, et al. Assisted reproductive technology in Europe, 2011: results generated from European registers by ESHRE. Hum Reprod 2016;31:233–48. Roque M, Lattes K, Serra S, Sola I, Geber S, Carreras R, et al. Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: a systematic review and meta-analysis. Fertil Steril 2013;99:156–62. Pinborg A, Henningsen AA, Loft A, Malchau SS, Forman J, Andersen AN. Large baby syndrome in singletons born after frozen embryo transfer (FET): is it due to maternal factors or the cryotechnique? Hum Reprod 2014;29:618–27. Ortendahl M, Fries JF. Discounting and risk characteristics in clinical decisionmaking. Med Sci Monitor 2006;12:RA41–5. Ghobara T, Vandekerckhove P. Cycle regimens for frozen-thawed embryo transfer. Cochrane Database Syst Rev 2008:CD003414. Evans J, Hannan NJ, Edgell TA, Vollenhoven BJ, Lutjen PJ, Osianlis T, et al. Fresh versus frozen embryo transfer: backing clinical decisions with scientific and clinical evidence. Hum Reprod Update 2014;20:808–21. Healy DL, Breheny S, Halliday J, Jaques A, Rushford D, Garrett C, et al. Prevalence and risk factors for obstetric haemorrhage in 6730 singleton births after assisted reproductive technology in Victoria Australia. Hum Reprod 2010;25:265–74. Venetis CA, Kolibianakis EM, Bosdou JK, Tarlatzis BC. Progesterone elevation and probability of pregnancy after IVF: a systematic review and meta-analysis of over 60,000 cycles. Hum Reprod Update 2013; 19:433–57.
243