ORIGINAL ARTICLE: ASSISTED REPRODUCTION
Obstetric and perinatal outcomes after either fresh or thawed frozen embryo transfer: an analysis of 112,432 singleton pregnancies recorded in the Human Fertilisation and Embryology Authority anonymized dataset Abha Maheshwari, M.D.,a Edwin Amalraj Raja, Ph.D.,b and Siladitya Bhattacharya, M.D.b a National Health Service Grampian; and United Kingdom
Institute of Applied Health Sciences, University of Aberdeen, Aberdeen,
Objective: To explore obstetric and perinatal outcomes in singleton pregnancies occurring as a result of fresh and thawed frozen embryo transfer using anonymized Human Fertilisation and Embryology Authority data. Design: Retrospective cohort study. Setting: Not applicable. Patient(s): Singleton births after IVF/intracytoplasmic sperm injection cycles in the United Kingdom (1991–2011). Intervention(s): A total of 112,432 cycles (95,911 fresh and 16,521 frozen) were analyzed using multivariate logistic regression to explore associations between type of embryo transferred (frozen vs. fresh) and obstetric and perinatal outcomes. Relative risks (RRs) and their 99.5% conﬁdence intervals (CIs) were calculated using Stata 14 MP, adjusting for potential confounders. Main Outcome Measure(s): Birth weight. Result(s): The adjusted RR (99.5% CI) of low birth weight [0.73 (0.66–0.80)] and very low birth weight [0.78 (0.63–0.96)] were all lower after frozen embryo transfer; however, RR of having a high birth weight baby was higher [1.64 (1.53–1.76)]. There was no difference in RR of preterm birth [0.96 (0.88–1.03)], very preterm birth [0.86 (0.70–1.05)], and congenital anomalies [0.86 (0.73–1.01)]. Conclusion(s): The ﬁndings of low birth and very low birth weight after thawed frozen embryo transfer are consistent with the literature and provide reassurance regarding the outcome of pregnancies after frozen embryo transfers. However, they highlight the possibility of high birth weight in these babies. Because these results are based on observational data, further evidence from randomized, controlled trials is needed before elective cryopreservation of all embryos is practiced in preference to the current practice of transfer of fresh embryos. (Fertil SterilÒ 2016;-:-–-. Ó2016 by American Society for Reproductive Medicine.) Key Words: Frozen replacement cycles, ICSI, IVF, obstetric outcomes, perinatal outcomes Discuss: You can discuss this article with its authors and with other ASRM members at
Received May 26, 2016; revised and accepted August 24, 2016. A.M. reports funding from the National Institute for Health Research (NIHR); speaker fees for educational meetings; travel and accommodation reimbursement as an invited speaker at national and international conferences; and royalties from a textbook on infertility. E.A.R. reports funding from government agencies; and travel and meeting expenses supported by the institution. S.B. reports funding from government agencies (NIHR, Scottish Chief Scientist Ofﬁce); royalties from a textbook on infertility; travel and accommodation as invited speaker at national and international conferences; and ﬁnancial support to department/institution for seminars and to colleagues for travel, accommodation and attendance at conferences. Reprint requests: Abha Maheshwari, M.D., Aberdeen Maternity Hospital, Consultant Reproductive Medicine, Aberdeen Fertility Centre, Foresterhill, Aberdeen 25 2Zl, United Kingdom (E-mail: [email protected]
). Fertility and Sterility® Vol. -, No. -, - 2016 0015-0282/$36.00 Crown Copyright ©2016 Published by Elsevier Inc. on behalf of the American Society for Reproductive Medicine http://dx.doi.org/10.1016/j.fertnstert.2016.08.047 VOL. - NO. - / - 2016
ORIGINAL ARTICLE: ASSISTED REPRODUCTION
ince the birth of Louise Brown in 1978, more than 6 million babies have been born as a result of IVF and intracytoplasmic sperm injection (ICSI). Conventionally, the aim has been to transfer the best-quality embryo or embryos in a fresh treatment cycle; any spare embryos are frozen for subsequent use. With reﬁnement of technology in recent years, the numbers of thawed frozen embryo transfers have increased (1), as have pregnancy rates associated with them (2). Follow-up data from children conceived through thawed frozen embryo transfers have been reassuring (3). A metaanalysis (4) based on data extracted from 11 published observational studies based on 37,703 singleton pregnancies (10,017 and 27,686 singleton pregnancies after thawed frozen embryo transfer and fresh, respectively) showed that in women carrying singleton pregnancies after IVF/ICSI, the relative risks (RRs) (95% conﬁdence intervals [CIs]) of small for gestational age [0.45 (0.30–0.66)], preterm birth [0.84 (0.78–0.90)], low birth weight [0.69 (0.62–0.76)], perinatal mortality [0.68 (0.48–0.96)], and antepartum hemorrhage [0.67 (0.55–0.81)] were lower in pregnancies following thawed frozen as opposed to fresh embryos. However, this meta-analysis was unable to adjust for confounders, such as age, parity, and duration of infertility. There was signiﬁcant heterogeneity in terms of the population sampled, design of studies, and regimens used for freezing, thawing, and replacement of embryos. Since then, a number of large studies have (5, 6) reported an increased risk of preterm labor, whereas others have found no difference in gestational age at delivery (7). Recent studies have also raised concerns about a higher proportion of macrosomic babies being born after thawed frozen embryo transfers (5–7). Thus the controversy around risks vs. beneﬁts of pregnancies following thawed frozen embryo transfer still continues. The Human Fertilisation and Embryology Authority (HFEA) database is the oldest and one of the largest national IVF registries. Availability of a number of covariates also provides an opportunity to adjust for confounders in a way that was impossible in meta-analysis of pooled data from other individual studies. In this study we analyzed anonymized HFEA data from 1991 to 2011 to explore obstetric and perinatal outcomes in singleton pregnancies occurring as a result of fresh and thawed frozen embryo transfer.
was undertaken by the Data Management Team at the University of Aberdeen. Descriptive analysis was used to compare the characteristics of women undergoing IVF cycles using fresh and frozen embryo transfer, including age, cause of infertility, previous pregnancy, cycle number, type of insemination (IVF/ICSI), number of embryos transferred, and year of delivery. Poisson regression was used to investigate the association between type of ET (frozen vs. fresh) and congenital abnormalities in IVF offspring, with a robust sandwich variance estimator. Multinomial logistic regression was used to explore associations between type of embryo transferred (frozen vs. fresh) and obstetric and perinatal outcomes, such as gestation at birth (preterm) and birth weight (low birth weight). The RRs and 99.5% CIs were calculated as a measure of strength of association. Statistical analyses were carried out using Stata 14 MP (StataCorp). Analyses were adjusted for potential confounders, such as maternal age, type of infertility, previous live birth, duration of infertility, and year of delivery (to account for improvements in embryo freezing over the years). Statistical signiﬁcance was set at .005.
RESULTS A total of 1,071,040 cycles were recorded in the HFEA dataset from 1991 to 2012. Data from 2012 were excluded because live birth outcomes were not available in the anonymized dataset. A total of 112,432 cycles (95,911 fresh and 16,521 frozen) were suitable for analysis, as detailed in Figure 1. Table 1 details the baseline characteristics between the two groups. There was a statistically signiﬁcant difference in the two groups in terms of age at which treatment was commenced, number of embryos transferred, cause of infertility, and gender of the baby. The proportion of women who had undergone previous treatment was signiﬁcantly higher in the thawed frozen embryo transfer group (56.6% vs. 2.7%). This is not surprising: thawed frozen embryos are generally transferred either after fresh embryo transfer is unsuccessful or for when a couple wishes to have a second baby using their pool of frozen embryos.
MATERIALS AND METHODS As part of its role as the statutory regulator of assisted conception treatment in the United Kingdom, the HFEA has collected data on all IVF treatment cycles performed in the country since 1991. Because anonymized data are freely available on their website, ethics approval was not required for this study. We extracted data from the HFEA dataset on all singleton live births following IVF/ICSI (1991–2012). Cycles involving, egg donation, egg sharing, embryo donation, preimplantation genetic diagnosis, surrogacy, oocyte cryopreservation, or for which the source of embryos was not speciﬁed were excluded. Cycles in which no ET was done and for which pregnancies have been lost to follow-up (Fig. 1) were excluded as well. Data cleaning and recoding 2
Preterm delivery was deﬁned as the delivery before 37 completed weeks of gestation. There was no statistical difference in the risk of preterm delivery in pregnancies occurring as a consequence of thawed frozen embryo transfer in comparison with those following fresh embryo transfer (8.5% vs. 9.4%, adjusted RR 0.96, 99.5% CI 0.88–1.03) (Table 2) after adjusting for maternal age, duration of infertility, previous pregnancy, and year of treatment. There was no difference in the risk of very preterm delivery (deﬁned as delivery before 32 completed weeks of gestation) in pregnancies conceived after thawed frozen embryo transfer when compared with fresh embryo transfer (1.4% vs. 1.8%, adjusted RR 0.86, 99.5% CI 0.70–1.05) (Table 2). Data on spontaneous and iatrogentic preterm delivery cannot be separated in this database. VOL. - NO. - / - 2016
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FIGURE 1 Total cycle 1,071,040
Excluded--Non singleton births (n=622,384) --loss to fu (n=3,341) --uncertain whether fresh/frozen ( n=1,149) --Treatment in 2012 (n=57,244) --non IVF treatment ( n=179) --unsmulated cycles ( n=167) --donated eggs/embryos/surrogacy ( n=39,576) --Egg cryopreserved / produced for donaon/ research( n= 11,909) --Donor inseminaon (n=219,559)- PDG (n=2,974) - Embryo transfer =0 (n=126)
Total cycles for analysis
n=112,432 Following fresh embryo transfer
Following frozen embryo transfer
Data selection process for analysis of obstetric and perinatal outcomes in singleton live births as a result of frozen vs. fresh embryo transfer. Maheshwari. Frozen vs. fresh embryos. Fertil Steril 2016.
Low Birth Weight
The risk of having a low birth weight baby (deﬁned as birth weight of <2,500 g) was lower (6.2% vs. 9.4%) in pregnancies occurring as a result of frozen embryo transfer when compared with those after fresh embryo transfer (adjusted RR 0.73, 99.5% CI 0.66–0.80) (Table 2). Very low birth weight was deﬁned as birth weight of <1,500 g. The risk of a very low birth weight baby was lower (1.3% vs. 1.8%) in pregnancies as a result of frozen embryo transfer when compared with fresh embryo transfer (adjusted RR 0.78, 99.5% CI 0.63–0.96) (Table 2).
After adjusting for maternal age, duration of infertility, previous pregnancy, and year of treatment, there was no difference in the risk of any congenital anomaly (2.2% vs. 2.3%) in offspring conceived after either type of ET (adjusted RR 0.86, 99.5% CI 0.73–1.01). Congenital anomalies are selfreported by couples to the treating IVF centers, who include these as part of the mandatory birth outcome data required by the HFEA. These are only known anomalies at birth and will not include those that get picked up later.
High Birth Weight
High birth weight was deﬁned as weight of >4 kg at birth. The risk of a high birth weight baby was higher (15.1% vs. 8.9%) in pregnancies as a result of thawed frozen embryo transfer when compared with fresh embryo transfer (adjusted RR 1.64, 99.5% CI 1.53–1.76) (Table 2). Very high birth weight was deﬁned as weight of >4.5 kg. The risk of a very high birth weight baby weight was higher (3.1% vs. 5.1%) in pregnancies as a result of thawed frozen embryo transfer when compared with fresh embryo transfer (adjusted RR 1.95, 99.5% CI 1.67–2.29) (Table 2).
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Singleton pregnancies following frozen embryo transfer are associated with lower risk of low birth weight babies and very low birth weight babies and a higher risk of high birth weight babies when compared with those from fresh embryo transfer. The chance of having a healthy baby was higher among women who had thawed frozen embryo transfers than in women who had fresh embryos replaced. There was no difference in the risk of preterm delivery, very preterm delivery, and congenital anomalies in both groups when 3
ORIGINAL ARTICLE: ASSISTED REPRODUCTION
TABLE 1 Baseline characteristics of IVF/ICSI treatment cycles.
Fresh embryo transfer (n [ 95,911)
Age at treatment (y) 18–34 53,573 (55.9) 35–37 24,590 (25.6) 38–39 11,285 (11.8) 40–42 5,868 (6.1) >42 595 (0.6) Previous IVF cycle Yes 41,639 (43.4) No 54,272 (56.6) Previous pregnancy Yes 11,089 (11.6) No 84,822 (88.4) Previous live birth Yes 7,822 (8.2) No 88,089 (91.8) No. of embryos transferred 1 13,319 (13.9) 2 66,349 (69.1) R3 16,243 (16.9) Duration of infertility (y) 0–4 22,385 (23.3) 5–9 15,268 (15.9) 10–14 3,500 (3.7) 15–20 1,266 (1.3) Missing 53,492 (55.8) Cause of infertility Tubal 21,385 (22.3) Ovulatory 11,560 (12.1) Male factor 9,287 (9.7) Unexplained 29,625 (30.9) Endometriosis 6,763 (7.1) Gender of baby Female 46,740 (48.7) Male 49,171 (51.3)
Frozen embryo transfer (n [ 16,521)
8,580 (51.9) 4,250 (25.8) 2,138 (12.9) 1,316 (8.0) 237 (1.4)
16,071 (97.3) 450 (2.7)
5,731 (34.7) 10,790 (65.3)
4,607 (27.9) 11,914 (72.1)
2,918 (17.6) 10,881 (65.7) 2,722 (16.4)
3,033 (18.4) 4,020 (24.3) 1,030 (6.2) 307 (1.9) 8,131 (49.2)
4,018 (24.3) 2,304 (14.0) 1,340 (8.1) 4,752 (28.8) 1,053 (6.4)
< .001 < .001 < .001 < .001 < .001
8,124 (49.2) 8,397 (50.8)
Note: Values are n (%). Maheshwari. Frozen vs. fresh embryos. Fertil Steril 2016.
adjusted for age, parity, year of treatment, and duration of infertility (Fig. 2).
Strengths These results are based on analysis of one of the largest national datasets. The higher numbers provide greater precision than in previous individual studies, and more conﬁdence in the outcomes owing to the ability to adjust for age, parity, duration of infertility, and year of treatment. We have been able to comment on the newly observed risk of macrosomia which has not been reported on by older studies.
Limitations A major limitation is the cycle-based nature of data logged in the HFEA database. The HFEA data are anonymized and report on outcomes of cycles of IVF rather than women. We have therefore not been able to identify women who have been included more than once within our dataset. Our inability to adjust for the clustering of women within cycles may have caused the SE of the estimates and CIs to seem narrower than they actually are. We have elected to address this 4
by reporting 99.5% CIs instead of the more traditional value of 95%. For similar reasons, statistical signiﬁcance was set at .005 rather than the more conventional value of .05. As an observational study, the results could be confounded by differences in the characteristics of women undergoing fresh or frozen embryo transfer (Table 1). Although we are able to adjust for maternal age, parity, and duration of infertility, we are limited by the number of variables available from the dataset and were unable to adjust for smoking, body mass index, number of goodquality embryos, total dose of gonadotropins, and the method used to freeze or thaw embryos, because these are not recorded by the HFEA. Because the anonymized HFEA data present gestational age and weight in bands rather than actual values, we were unable to adjust birth weight according to gestational age and calculate parameters such as Z scores. We are also limited by the fact that, like most IVF registries (6), the dataset does not contain records of pregnancy complications, such as pre-eclampsia and antepartum hemorrhage. In addition, perinatal mortality is not available in the anonymized HFEA database. Although we are not able to distinguish between natural vs. hormone replacement cycles for frozen embryo transfers, recent data do not show any difference in pregnancy rates (8, 9).
Comparison with Existing Studies Our ﬁndings, based on analysis of data from the United Kingdom, are consistent with the published data from other registries (5, 6) as well as an existing meta-analysis of data published up until 2012 (4) for outcomes of low and very low birth weight. However, it differs for preterm and very preterm delivery from meta-analysis (4). This could be explained by the large number from a single dataset and the ability to adjust for important confounding factors. No difference in the risk of preterm delivery between two groups was also noted by Japanese data that reported on a large number of singleton births after thawing of vitriﬁed embryos (7). Data on high birth weight are consistent with data from other large IVF registries (6).
Implications for Clinical Practice Our ﬁndings provide a degree of reassurance in terms of reduction in low birth weight babies following thawed frozen embryo transfer. The possible explanations for this are that women with surplus embryos possess good ovarian reserve and generally have a good prognosis after IVF treatment. In addition, cryopreserved embryos tend to be of good quality, with vitality that has been tested by the freezing and thawing procedure (9). The other possible biological explanation for low birth weight in fresh embryo transfers is the possibility of a higher risk of abnormal placentation due to the presence of over-estrogenized endometrium in stimulated cycles (10, 11). However, the ﬁndings also raise concerns about high birth weight babies. The reasons for this are not fully VOL. - NO. - / - 2016
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TABLE 2 Obstetric and perinatal outcomes for singleton pregnancies following frozen vs. fresh embryo transfer. Frozen embryo transfer (n [ 16,521) Outcome Gestation at birth (wk)a Normal, 37þ Preterm, <37 Very preterm, <32 Birth weight (kg)a Very low birth weight, <1.5 Low birth weight, <2.5 Normal weight, 2.5–4.0 High birth weight, 4.0þ High birth weight, 4.5þ Congenital anomaly No Yes
Fresh embryo transfer (n [ 95,911)
Unadjusted RR (95% CI)
Adjusted RR (95% CI)
15,111 1,410 237
91.5 8.5 1.4
86,946 8,965 1,704
90.7 9.4 1.8
1.00 0.91 (0.85–0.99) 0.80 (0.66–0.97)
1.00 0.96 (0.88–1.03) 0.86 (0.70–1.05)
1,022 13,007 2,492 506
6.2 78.7 15.1 3.1
9,056 78,346 8,509 1,421
9.4 81.7 8.9 1.5
0.68 (0.62–0.75) 1.00 1.76 (1.65–1.89) 2.15 (1.85–2.49)
0.73 (0.66–0.80) 1.00 1.64 (1.53–1.76) 1.95 (1.67–2.29)
1.00 0.92 (0.79–1.08)
1.00 0.86 (0.73–1.01)
Note: Owing to overlapping in the categories, numbers will not add up to column total. a Adjusted for age group, previous pregnancy, duration of infertility, and year of treatment. Maheshwari. Frozen vs. fresh embryos. Fertil Steril 2016.
understood, although patient and embryo selection could play a role. There are emerging data on the risk of large babies following thawed frozen embryo transfers, but no satisfactory explanation for this phenomenon is available to date. It is possible that higher implantation potential leads to better placentation and overgrowth of the fetus. Birth order, which
is higher in babies conceived from frozen–thawed embryos, may play a role, but has been challenged by the fact that the difference has persisted after adjustment for parity in our study as well as others (12). It has also been suggested that the freezing and thawing procedures may play an independent role for the growth potential of the fetus, owing to
Normal (37+ weeks) Preterm (<37 weeks) Very Preterm (<32 weeks) Very low BW (< 1.5 kg) Low BW (< 2.5 kg) Normal BW (2.5− 4.0 kg) High BW (4.0+ kg) Very High BW ( 4.5+ kg) Congenital anomaly 1
2 Relative Risk
Singleton pregnancies following frozen vs. fresh embryo transfer. Analysis of HFEA anonymized data. Maheshwari. Frozen vs. fresh embryos. Fertil Steril 2016.
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ORIGINAL ARTICLE: ASSISTED REPRODUCTION epigenetic alterations at the early embryonic stages during freezing/thawing (12). Another point to note is that most embryos are frozen at the blastocyst stage, with widespread use of vitriﬁcation. There are existing data suggesting that singleton pregnancies resulting from transfer of embryos at the blastocyst stage are more likely to be associated with large for gestational age babies than those after cleavage-stage transfer of embryos (13).
cryopreservation of all embryos is practiced in preference to the current practice of transfer of fresh embryos.
Implications for Research Despite its size, an observational study such as this has a number of inherent limitations, and the best way to conﬁrm its ﬁndings will be to compare obstetric and perinatal outcomes in pregnancies following fresh and frozen embryo transfer in an adequately powered randomized, controlled trial. This is essential before contemplating any changes to existing practice in favor of frozen embryo transfer, especially given current concerns about the risk of producing infants at risk of being large for gestational age. We are still uncertain about the effect on preterm delivery. Large randomized, controlled trials take a long time to set up and complete and are only powered for primary outcomes. Although individual patient data meta-analysis of data from randomized, controlled trials provides a means of investigating a number of perinatal outcomes in babies born as a result of frozen vs. fresh embryo transfers, individual patient data meta-analysis of all existing RCTs will not be powered to ﬁnd the differences in rare outcomes, such as congenital anomaly and perinatal mortality. The only way to address this issue is to collate data from all registries across the world and do an individual patient data meta-analysis on registry data. Although over-estrogenized endometrium has been identiﬁed as a possible reason for some of the poorer outcomes in pregnancies subsequent to fresh embryo transfer (14), this does not explain higher risk of macrosomic babies in pregnancies subsequent to frozen embryo transfer. This phenomenon seems to persist in comparisons involving sibling pairs (12), and more studies are needed to identify the exact mechanisms for it. In conclusion, our ﬁndings are consistent with preexisting data for low and very low birth weight babies but show no difference in preterm deliveries. Although they provide some reassurance regarding the outcome of pregnancies following frozen embryo transfers, they highlight the possibility of high birth weight in these babies. Because these results are based on observational data, further evidence from randomized, controlled trials is needed before elective
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