Langen, Elizabeth S. MD; Shahine, Lora K. MD; Lamb, Julie D. MD; Lathi, Ruth B. MD; Milki, Amin A. MD; Fujimoto, Victor Y. MD; Westphal, Lynn M. MD
An emerging body of literature has begun to expose racial differences in risks of medical diagnoses and responses to medical therapy.1,2 Several studies have shown that Asian women have fewer pregnancies after in vitro fertilization (IVF) treatments when compared with white women.3–7 A recent study by Purcell et al6 used both national data and specific clinic data to demonstrate a significantly lower pregnancy rate among Asian women undergoing day 3 embryo transfers when compared with white women. No study has identified the underlying cause of this disparity.
Poor embryo quality is often identified as a reason for IVF failure. Data from the Purcell et al study revealed a lower embryo fragmentation rate in the transferred embryos of Asian women compared with white women undergoing IVF embryo transfers suggesting against embryo quality as an etiology for lower pregnancy rates in Asian women. There was, however, no difference in embryo cell cleavage rates.6 Thus, it remains unclear whether the reduced pregnancy rates seen in Asian women are due to differences in embryo quality.
Therefore, we conducted a study to compare IVF outcomes after blastocyst transfer in Asian and white women. In our institution, blastocyst transfer is offered to women with at least three, and typically four, eight-cell embryos at day 3 assessment. We limited our analyses to blastocyst transfer cases to minimize the impact of embryo quality on live birthrate. We hypothesized that if Asian and white women have similar outcomes after blastocyst transfer cycles, perhaps an underlying difference in embryo quality could explain the poor outcomes previously reported for Asian women. Our goal was to evaluate the IVF outcomes among Asian and white women undergoing blastocyst transfer to help understand the underlying cause of the poor outcomes for Asian women after IVF therapy.
MATERIALS AND METHODS
We conducted a chart review of all IVF cycles at our university center from January 1, 2005, to December 31, 2006. Only fresh, nondonor, nongestational carrier cycles with blastocyst transfer were included. If a patient had multiple blastocyst transfer cycles, only her first cycle was included. Ethnicity data were collected from patient self-reporting questionnaires provided to all patients seen in our center. Only women who identified themselves as white/Caucasian or Asian were included. Women who identified themselves as Indian or mixed ethnicity were excluded. Cycles involving preimplantation genetic diagnosis were also excluded. Approval was obtained from the Stanford University Institutional Review Board to conduct this study.
Data collected included demographic information, obstetric history, body mass index, cycle day 3 follicle-stimulating hormone level, infertility diagnosis, number of previous IVF cycles, ovarian stimulation protocol, total dose of gonadotropins used, endometrial lining thickness, number of oocytes retrieved, semen analysis, fertilization method (IVF or intracytoplasmic sperm injection), fertilization rate, total number of eight-cell embryos on day 3, blastocyst formation rate, number of embryos transferred, number of blastocysts frozen, and pregnancy outcomes. With regard to infertility diagnosis, diminished ovarian reserve was defined as women expected to have a poor response by the attending physician based on baseline characteristics such as age, antral follicle count, and follicle-stimulating hormone level. Those women without any identifiable cause of their infertility were categorized with a diagnosis of “unexplained” infertility.
Ovarian stimulation was achieved with one of three standard protocols: luteal down regulation (long protocol), microdose leuprolide (Lupron; Abbott Laboratories, Abbott Park, IL) (flare protocol), or antagonist protocol. Details of these protocols have been described previously.8 Ultrasound monitoring of follicular growth was performed after 4 days of gonadotropin stimulation and approximately every other day as indicated. Serum estradiol (E2) levels were measured as needed. A dose of 10,000 units of human chorionic gonadotropin (hCG) was administered when at least two follicles reached an average diameter of 17 mm. Oocyte retrieval was performed by transvaginal ultrasound guidance 35 hours after hCG administration.
In most cases, fertilization of the oocytes was achieved with IVF, but intracytoplasmic sperm injection was used if there were poor semen parameters or other indications. The oocytes were examined for fertilization status 16–18 hours after fertilization. The zygotes with two pronuclei were cultured for 48 hours in Sage Cleavage Medium with 10% Serum Protein Substitute (SPS; Irvine Scientific, Santa Ana, CA). The embryos were examined at cleavage stage and were culture extended to blastocyst if there were three or more eight-cell embryos (in Quinn's Advantage Blastocyst medium; Cooper Surgical, Trumbull, CT). Blastocysts were classified with the classification system as described previously.9 Only blastocysts of 3BB quality or better were cryopreserved for potential future use. Ultrasonography-guided embryo transfer was performed using a Tefcat or Echotip Softpass catheter (Cook Ob/GYN, IN). Progesterone vaginal suppositories, 200 mg three times daily, were used for luteal support.
A positive pregnancy test result was considered a β-hCG greater than 5 milli–international units/mL. Clinical intrauterine pregnancy was defined as a gestational sac seen between 6 and 7 weeks of gestation on transvaginal sonogram. Spontaneous pregnancy loss was defined as loss of pregnancy after clinically seen intrauterine pregnancy, and biochemical pregnancy was defined as positive β-hCG with no gestational sac seen on transvaginal sonogram. Live birth data were collected by contacting patients.
Statistical analyses were performed using the two-tailed Student t test, χ2, and analysis of variance as appropriate. Each cycle was considered an independent event. All statistical tests used an α of .05. Univariable analyses were performed to compare baseline characteristic and cycle parameter differences between the Asian and white populations. To estimate the independent contribution of Asian ethnicity on treatment outcomes, a multivariable logistic regression analysis was performed. Data were analyzed using Stata 9 (StataCorp LP, College Station, TX), and SAS 8.2 (SAS Institute Inc., Cary, NC).
From January 1, 2005, through December 31, 2006, a total of 1,964 IVF embryo transfers were performed at our center, including 409 blastocyst transfer cycles. Of these, 180 met our inclusion criteria for analysis with women identifying themselves as white (62%) or Asian (38%). The groups were similar in most baseline characteristics, including age, cycle day 3 follicle-stimulating hormone level, antral follicle count, and parity (Table 1). Asian women, however, had a statistically significant lower body mass index (22.6 compared with 24.2, P=.02), were more likely to be nulligravid (53% compared with 35%, P=.03), and were more likely to have had at least one prior IVF cycle (37% compared with 20%, P=.02).
The groups had similar stimulation protocols and response to treatment. A similar percentage of women in each group used each particular stimulation protocol. The dose of gonadotropins used for stimulation was also similar between the two groups. The groups had similar numbers of oocytes retrieved, use of intracytoplasmic sperm injection, and fertilization rates (Table 2). The groups had a similar number of embryos transferred (Table 2). Asian women, however, had a statistically significant thicker endometrial lining (10.9 compared with 10.2 mm, P=.02).
The blastocyst quality was similar in both groups as well. A similar percentage of patients in each group had at least one 4AA or 4AB or better blastocyst transferred (Table 2). In addition, among those women with such high-quality blasts, a similar percentage had only one or two embryos transferred (87% for white women and 90% for Asian women, P=.86).
Despite similarities in baseline characteristics, ovarian stimulation, response to treatment, and number of blastocysts transferred, Asian women had significantly lower implantation, clinical pregnancy, and live birthrates (Table 3). Of note, white and Asian women had a similar number of blastocysts frozen after fresh IVF cycles (Table 2).
Univariable logistic regression analysis showed that Asian women were significantly less likely to have a clinical pregnancy (odds ratio 0.52, 95% confidence interval 0.28–0.95, P=.04) or live birth (odds ratio 0.48, 95% confidence interval 0.25–0.90, P=.02) after blastocyst transfer than white women. In multivariable analysis adjusting for nulligravidity, body mass index, history of prior IVF cycles, and endometrial lining thickness, the decreased likelihood of an Asian woman achieving a live birth persisted (Table 4).
A growing body of literature has identified race as having a significant effect on response to medical treatments. In the infertility literature, Asian ethnicity has emerged as a risk factor for poor outcomes after IVF treatment without a clear understanding of the biologic mechanism for this finding.3–7 In the current study, we investigated whether a difference in embryo quality might be responsible for the poor outcomes seen in Asian women. We controlled for embryo quality by limiting our cohort to only those women selected to undergo blastocyst transfer. In addition, a study by della Ragione et al10 showed that 4AA, 4AB, or better blastocysts have better implantation potential and pregnancy outcome compared with poorer-quality blastocysts after a single blastocyst transfer. To further demonstrate similarity in embryo quality in our study population, we found that a similar percentage of patients in each group had at least one 4AA, 4AB, or better blastocyst transferred. Despite similar embryo quality and baseline and treatment characteristics, we found Asian women to have significantly lower pregnancy and live birthrates when compared with white women.
We did observe a higher percentage of women in the Asian group who were nulligravid as well as a higher rate of previous IVF cycles among Asian women. However, the higher gravidity among the white women did not translate into a higher parity. The finding that Asian women had a higher number of previous IVF cycle attempts is not surprising given the findings of this and other studies demonstrating that Asian women are less likely to have a live birth after IVF treatments.3–7 Although both of these observations may have suggested that the Asian women were predisposed to having worse outcomes, the difference in live birthrate persisted even when accounting for these baseline differences in a multivariable regression (Table 4).
The other significant difference in baseline characteristics that we identified was an increased body mass index in white compared with Asian women (24.2 compared with 22.6, P=.02). Both obesity and below-normal body mass index have been associated with poorer IVF outcomes.11 However, both groups had body mass index averages in the normal range. Furthermore, the range and the standard deviations (SDs) of the two groups were overall quite similar (white 17.5–37.8, SD 4.25; Asian 18.3–33.4, SD 4.22). In addition, the overall numbers of women with an abnormal body mass index in the white group were 36 women (33%) with a body mass index over 25 and two women (2%) with a body mass index of less than 18.5. In the Asian group, 12 women (17%) were overweight, and three (4%) had a body mass index less than 18.5. Given the fact that both groups had a normal average body mass index with similar distributions, it is unlikely that this statistically significant difference could have had an important clinical impact.
Several authors have suggested endometriosis as a possible explanation for poor reproductive outcomes in Asian women.12,13 In 1995, a study of fertile patients undergoing laparoscopic tubal ligations found Asian race to be a significant risk factor for endometriosis.13 More recently, however, Missmer et al14 used the Nurses' Health Study II database to examine risk factors for endometriosis. In their study, Asian race was not associated with an increase in endometriosis when compared with white women, regardless of fertility status. Our study was not able to assess the prevalence of endometriosis among Asian and white women because diagnostic laparoscopy was not routinely performed before IVF. We did not, however, find a difference in the rate of diagnosed endometriosis in those patients with prior laparoscopy. Although we cannot rule out occult endometriosis as a contributing factor to the poor outcomes in Asian women, we also cannot attribute the Asian disparity in IVF outcomes to endometriosis.
Although our study has not identified the cause for the poor IVF outcomes observed in Asian women, it does suggest that factors other than ovarian function or embryo quality may be causing this discrepancy. Our results suggest that despite the fact that Asian and white women have similar baseline characteristics, response to therapy, and even embryo quality, they have substantial differences in implantation rates and subsequently clinical pregnancy and live birthrates. In our group of patients, we found a small but significant difference in endometrial lining thickness between Asian and white women. However, the clinical relevance of this difference in endometrial thickness is not known. Basir et al15 demonstrated that women responding with higher E2 levels and greater numbers of follicles had increased glandular stromal dyssynchrony in histologically examined endometrium. It is possible that the hormonal milieu created by gonadotropin stimulation may have a different effect on the endometrium of Asian women that may lead to a less hospitable environment for implantation. Further study is needed to investigate this possibility.
One limitation of our study is that we grouped all East Asian women together even though their backgrounds were from multiple countries of origin, including Japan, China, and the Philippines. We attempted to minimize the effect of grouping all Asian women together by excluding a large subgroup in our population, South Asian/Indian women (results published independently7). To fully understand the racial differences that exist in IVF outcomes, studies are needed that can further stratify women by country of origin. In addition, data are lacking on paternal ethnicity, which may interact with maternal ethnicity or independently influence treatment outcomes. Finally, we are unable to comment on lifestyle differences that may impact health outcomes such as differences in diet, use of herbal supplements, and duration of time living in the United States. Such differences should be included in future prospective research.
Despite these limitations, our study is unique in our comparison of IVF outcomes between Asian and white women after blastocyst transfer. The findings in this study add to the increasing body of evidence that Asian women may have more difficulty conceiving with IVF treatments. The substantial decrease in pregnancy rates seen with blastocyst transfer despite similar blastocyst formation rates suggests that factors other than embryo development influence IVF outcomes in Asian women.
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