Reproductive Medicine Unit, Department of Obstetrics and Gynaecology, University of Adelaide, The Queen Elizabeth Hospital, 28 Woodville Road, Woodville, South Adelaide, Australia 5011
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Abstract |
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Key words: ART/early pregnancy loss/risk factors
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Introduction |
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High rates of EPL, ranging from 12 to 48%, have been reported in ART (Barlow et al., 1988; Acosta et al., 1990
; Levy et al., 1991
; Schmidt et al., 1994
; Simon et al., 1999
; Fedorcsak et al., 2000
; Sugantha et al., 2000
). EPL significantly reduces the initial success of ART treatment, decreases the efficiency of treatment and increases the psychological burden on patients. Risk factors for EPL in pregnancies after ART, however, have not been comprehensively studied, despite its apparent significance. With a view to altering practice, an important conceptual issue relates to the mutability of risk factors for EPL, as potentially modifiable factors can form the basis for clinical or preventive interventions. For example, maternal age is not `reversible'; the age at which women have children is, however, amenable to modification within populations, as indicated by secular trend of increasing maternal age at the birth of the first child. A recent report has linked obesity with greater risk of EPL (Fedorcsak et al., 2000
), and obesity has been shown to be a detrimental factor for pregnancy rate (Wang et al., 2000
) and spontaneous abortion (Hamilton-Fairley et al., 1992
) in ART programmes. Obesity is also potentially modifiable, possibly amenable to low cost, non-invasive, self-management by patients. The objective of this retrospective study is to investigate the effect of some potential risk factors on EPL in a large cohort of pregnant women treated by ART.
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Materials and methods |
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Treatment addressed various infertility aetiologies, including tubal blockage (23%), endometriosis (9%), male factor (50%), unexplained infertility (12%) and other factors (15%). A total of 27% of all patients had multiple infertility factors. The main stimulation protocol used during the study period has been described previously (Norman et al., 1991), including both down-regulation (>90%) with leuprolide and gonadotrophin and <10% of the cycles under a flare regime. The treatments were IVF, ICSI, and gamete intra-Fallopian transfer (GIFT).
While hCG measurement was routinely obtained in the morning of day 16 after oocyte retrieval (85% of the study population), a small proportion of patients had their hCG measured on day 15 or day 17. In 1994, hCG was measured with an automated fluorometric enzyme immunoassay using two monoclonal antibodies (Stratus II; Baxter, Miami, FL, USA). Sensitivity of the assay was 2 IU/l with coefficients of variation consistently <5% across a wide dynamic range (2500 IU/l). Since 1995, the assay has been performed with an automated system, using a two-site chemiluminometric sandwich immunoassay [automated chemiluminescence (ACS) 180; Chiron Diagnostics Corporation, East Walpole, MA, USA]. Sensitivity was 2 IU/l, and coefficients of variation were 58% over the dynamic range of the assay (21000 IU/l). Pregnancy was defined as a serum hCG concentration of 10 IU/l on day 16 after ovulation (equivalent to 30 days after last menstrual period in the general population). After initial confirmation of pregnancy following hCG rise, EPL was ascertained by either a self-reported miscarriage before 6 weeks gestation, or by an absence of embryonic sac or blank embryonic sac(s) as detected by ultrasound around 67 weeks gestation. Pregnancy loss after that was not considered in this study.
The risk factors for EPL investigated in this study were: maternal age, body mass index (BMI, kg/m2), smoking and polycystic ovary syndrome (PCOS) status, infertility aetiology, response to stimulation (both maximum estradiol concentration and the number of oocytes recovered), a quasi measurement of embryo quality (refer to Table I for details), and treatment type. Patient age and BMI were categorized for the clarity of data analysis and presentation of results. Four age subgroups were formed: age
30 years, 30.135 years, 35.140 years and >40 years. BMI subgroups were: <18.5 kg/m2 (lean), 18.525 kg/m2 (normal), 25.130 kg/m2 (overweight), 30.135 kg/m2(obese) and
35.1 kg/m2 (very obese). In 73 cases (6%), BMI was not recorded and was treated as missing in the analysis. PCOS was determined by both the ultrasound scan result and hormonal measurement as defined previous (Norman et al., 1995
).
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Results |
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Univariate analysis
A total of 195 (16%) pregnancies ended as EPL in the study population. No clear linear trend in the risk of EPL in relation to either age or BMI was observed (Table I). In women up to 40 years old, the risk of EPL was very similar at around 1517%, while women >40 years old had a significantly increased risk (23%) compared with the rest (P < 0.05). There was some variation between the BMI groups in their risk of EPL, with the lowest risk of EPL (12%) occurring in the overweight women (2530 kg/m2), while lean women (<18.5 kg/m2) had a much greater risk, 35% (P < 0.01) and very obese women (>35 kg/m2) had a non-significant increased risk (20%, P = 0.16). The normal weight (18.525 kg/m2) women also had a significantly higher risk of EPL than the overweight group (18 versus 12%, P < 0.05). There was a significant increase of risk (P < 0.05) in women who received IVF treatment compared with those who received ICSI or GIFT treatment (combined). Smoking was linked with a significant increase of risk (P < 0.001) in EPL. There seemed to be little effect of the level of response to ovarian stimulation defined by either the maximum estradiol concentration or the number of oocytes recovered. Women who received embryo(s) of poorest `quality' as characterized by non-elective transfer of a single embryo had the greatest risk (P < 0.01) in contrast to that following multiple embryo transfer, particularly elective transfer of two or three embryos, where the women probably received the best quality embryos. The PCOS status of patients was marginally (P = 0.05) associated with an increased risk, while aetiology of infertility was not associated with any significantly increased risk of EPL.
Multivariate analysis
After adjusting for possible confounding effects from these risk factors with multivariate logistic regression analysis, only smoking and poorest embryo quality remained as significant risk factors for EPL (Table I). Smoking doubled the risk of EPL, while transferring the poorest quality embryos more than tripled the risk of EPL compared with the group receiving the best quality embryos.
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Discussion |
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There is a high wastage of pregnancy as EPL in both general population and ART population. The risk of EPL, 16%, reported in this study is lower than the estimates given in the general population. However, direct comparison of the two populations can be problematic since generally speaking, more sensitive assays, earlier hCG measurement and lower criterion of hCG for pregnancy detection have been utilized in general population studies (Wilcox et al., 1988, for example). Another possible factor that may complicate the early detection of pregnancy in ART is the common use of large doses of hCG for inducing ovulation or luteal support. Early work, however, has suggested that the residual hCG from either ovulation induction or luteal support would have been reduced to an undetectable level at the time of early pregnancy detection (Lenton et al., 1988
). The practice of multiple embryo transfer in ART also means that some EPL may be `hidden' by the continuing growth of companion embryo(s). It is likely that the real risk of EPL would be higher in the ART population than in the general population, although direct evidence to support this assertion is yet to be obtained. In fact, many methodological differences also exist between the reports of EPL in an ART population, which may account for the varying risks of EPL reported. The risk of EPL of 16% reported in this study, by and large, is within the range of a few other reports of ART population using similar criteria (Simon et al., 1999
; Fedorcsak et al., 2000
). The level of hCG used here for pregnancy detection was well within the sensitivity of hCG assay, although it was lower than the common clinical criterion for positive pregnancy (>3050 IU/l), which is usually for informing patients and is considered conservative. On the other hand, since the criterion used in this study, hCG >10 IU/l at day 16, was much higher than that used in general population studies (Wilcox et al., 1988
), it can be anticipated that some very early pregnancy loss would not have been counted.
Couples who are planning their pregnancies want to know the potential hazards, while those receiving infertility treatment will try to avoid any potential risk factors causing pregnancy loss. Since EPL causes a big reduction of the initial success of ART, it reduces its efficiency and increases the burden of psychological stress to infertile couples. Therefore, establishing the risk factors for EPL has important clinical implications in improving the efficiency of ART treatment, in addition to increasing the understanding of possible mechanisms of early pregnancy failure. The lack of effect of maternal age on EPL until women reached 40 years of age appeared to be consistent with that reported in two previous studies (Wilcox et al., 1990; Dickey et al., 1993
). Alternatively, Balmaceda et al suggested that there is an increased incidence of EPL and chromosomal abnormalities of oocytes in older women (Balmaceda et al., 1994
). It is possible that embryos in older women may have lost their ability to develop even before implantation, although the uterus may have the capacity to conceive with donated embryos from younger women (Sauer et al., 1992
; Balmaceda et al., 1994
; Sauer, 1997
). In this study, there appeared to be a U-shaped effect of body mass. Both lean and very obese women had increased risk, while the optimal group was the slightly overweight women with a BMI between 25 and 30 kg/m2. This is different from the results reported recently (Fedorcsak et al., 2000
). It is possible that other obesity related factors, such as fat distribution or fat type, have confounded the effect of BMI and resulted in inconsistency between different studies. Further investigation is needed to clarify the matter. The association of smoking with a higher risk of EPL is an interesting observation. Detrimental effects of smoking on spontaneous abortion have been reported recently (Ness et al., 1999
; Mishra et al., 2000
). The association between embryo quality and the risk of EPL is another interesting finding which needs to be confirmed. Though the embryo quality defined here is not a direct measurement, it is known that the selection from large number of embryos available leads to a better clinical pregnancy rate. So the high risk of EPL suffered by embryos transferred with little or no selection does suggest that current selection criteria seem to be able to discriminate the embryos' ability to grow after the initial implanting stage. The lack of relationship between the stimulation level and the risk of EPL was supported by one earlier study (Dickey et al., 1993
).
There may be other possible risk factors that should be considered such as the endometrium thickness (Dickey et al., 1993) or bacterial vaginosis, which is associated with adverse pregnancy outcomes across all gestational ages (Ugwumadu, 2002
). Lack of data, due to the retrospective nature of the study, prevented us adjusting for their possible effect and this may have reduced the sensitivity of the study. Another limitation of this study is the relative large proportion of patients (38%) who did not come back for their early pregnancy test, even though they had a luteal period possibly longer than 16 days. Reasons for not coming back for early pregnancy detection were usually personal. Comparisons showed that this group were 0.8 years older and had lower BMI, 0.5 kg/m2 less, both statistically significant. Given the finding that age and BMI were not risk factors for EPL, however, small differences may not be meaningful. On the other hand, there was no difference in their likelihood of being a smoker or of having had poor quality embryos transferred. Finally, the smoking status was based on self-reporting, which can be a source of error, in particular under-reporting bias. However, in the present study, doctors obtained the data during consultation and the population can be characterized as one seeking specialist care over a long period of time. Both are elements known to be compatible with relatively accurate self-reporting of smoking status (Schofield and Hill, 1999
; Donnelly et al., 2000
). Even if under-reporting was a likely scenario in this study, then it would have made the observed smoking effect a conservative estimate instead of changing the direction of the effect.
Although we could not investigate the mechanism of EPL in this retrospective study, it has been suggested that EPL may be due to failure of the maternal support system or through impairment of the fetus itself. Due to the extreme difficulty in obtaining pregnancy material in EPL, the determination of either a pathological or a chromosomal cause is hard. A recent review (Norwitz et al., 2001) has discussed in detail early implantation, implantation failure and the possible mechanism involved.
In summary, this study has investigated many factors for their potential association with the risk of EPL and found that smoking doubled the risk of EPL and poor quality of embryos was also associated with an increased risk of EPL. An effect of age, obesity and several other factors on the risk of EPL was not evident.
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Acknowledgements |
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Notes |
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References |
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Submitted on February 5, 2002; resubmitted on June 5, 2002; accepted on August 7, 2002.