https://orcid.org/0000-0001-9016-4946
Abstract
Introduction
Obesity has been associated with an increased risk of reproductive failure, especially preterm birth. As preimplantation genetic testing for aneuploidies (PGT-A) is increasingly used worldwide, however, it is still unclear whether body mass index (BMI) has an effect on the preterm birth rate in patients undergoing in vitro fertilization (IVF) with PGT-A when transferring a single euploid blastocyst.
Materials and methods
This retrospective, single-center cohort study included 851 women who underwent the first cycle of frozen-thawed single euploid blastocyst transfer with PGT-A between 2015 and 2020. The primary outcome was the preterm birth rate. Secondary outcomes were clinical pregnancy, miscarriage, ectopic pregnancy, pregnancy complications, and live birth.
Results
Patients were grouped by World Health Organization (WHO) BMI class: underweight (<18.5, n = 81), normal weight (18.5–24.9, n = 637), overweight (25–30, n = 108), and obese (≥30, n = 25). There was no difference in the clinical pregnancy, miscarriage, ectopic pregnancy, pregnancy complication, and live birth by BMI category. In multivariate logistic regression analysis, preterm birth rates were significantly higher in women with overweight (adjusted odds ratio [aOR] 3.18; 95% confidence interval [CI], 1.29–7.80, p = .012) and obese (aOR 1.49; 95% CI, 1.03–12.78, p = .027) compared with the normal weight reference group.
Conclusion
Women with obesity experience a higher rate of preterm birth after euploid embryo transfer than women with a normal weight, suggesting that the negative impact of obesity on IVF and clinical outcomes may be related to other mechanisms than aneuploidy.
Obesity has become one of the most important threats to human health, they also contribute to pregnancy complications, including preeclampsia, gestational diabetes mellitus (GDM), preterm delivery, cesarean section, and small as well as large for gestational age neonates [Citation1–3]. Additionally, obesity has been associated with adverse reproductive outcomes [Citation4–7].
An increasing number of obese women are receiving in vitro fertilization (IVF). A recent meta-analysis suggested that IVF singleton pregnancies are at a higher risk of adverse perinatal outcomes compared with those conceived naturally [Citation8]. However, the etiology of impaired clinical outcomes in obesity remains unclear. Obesity might influence clinical outcomes by reducing oocyte quality, maturity, and endometrial receptivity [Citation9]. The risk of maternal and fetal complications arises in obese women, irrespective of the conception mode [Citation10,Citation11]. Aneuploidy is believed to be the most common cause of pregnancy failure. Considering that women with obesity have worse clinical outcomes, it could be assumed that women with obesity may suffer from adverse pregnancy outcomes.
One of the major limitations in the evaluation of BMI is that embryo chromosome testing is often not performed [Citation12]. Most of the studies on the effects of BMI on IVF focused on the impact of adiposity on ovarian responsiveness during ovarian hyperstimulation. PGT-A use has steadily increased worldwide as an embryo selection tool in the hope of decreasing negative clinical outcomes. In contrast, the effects of pre-pregnancy BMI on risks of preterm birth in women undergoing frozen-thawed euploid embryo transfer have not been fully adequately studied.
The objective of this study was to determine whether BMI is associated with preterm birth in women with euploid embryo transfer. This was performed to determine if euploid embryos from women with different BMI have an altered potential.
Material and methods
Study design and population
This retrospective cohort study includes 851 women with euploid embryos who underwent frozen-thawed embryo transfer between 2015 and 2020 in the Center for Assisted Reproductive Technology of Northwest Women’s and Children’s Hospital, China. All the patients were followed-up for one year after embryo transfer. This study was approved by the ethics committee of the hospital (number 2019013). Because of the retrospective character of the study, informed consent was waived. Patient data were anonymized. Data were extracted from the electronic medical record system. Inclusion criteria were: first cycle transfer of a single frozen-thawed, euploid blastocyst embryo after IVF with preimplantation genetic testing for aneuploidies (PGT-A). The exclusion criteria were multiple cycles of embryo transfer. Patients were included a single time in the study. BMI was calculated as weight divided by squared height. Women were considered underweight if their BMI was <18.5 kg/m2, normal weight if 18.5 kg/m2 ≤ BMI <25 kg/m2, overweight if 25 kg/m2 ≤ BMI <30 kg/m2, and obese if BMI was ≥30 kg/m2 according to the most recent BMI classification by the World Health Organization (WHO) [Citation13].
PGT-a procedure
All the patients received a standardized ovarian stimulation regimen in the PGT-A cycles according to their ovarian function. In brief, recombinant FSH was used for ovarian stimulation in gonadotropin-releasing hormone agonists, gonadotropin-releasing hormone antagonist protocols, and others. Oocyte retrieval was performed 36 h after triggering with hCG.
Insemination of retrieved oocytes was done by intracytoplasmic sperm injection (ICSI). Laser-assisted breaching of the zona pellucida was performed on day 3. The embryos were assessed on day 5 and 6, and a trophectoderm (TE) biopsy was performed. Biopsied TE cells were then stored at −20 °C for future whole genome amplification (WGA) and next-generation sequencing (NGS) [Citation14]. After the biopsy, blastocysts were vitrified to be replaced in the subsequent frozen-thawed embryo transfer cycle. Only cycles where at least one euploid blastocyst was available for transfer were included in this study.
Endometrial preparation
Natural cycle. Transvaginal ultrasound was performed from day 8 of the menstrual cycle. Frozen-thawed embryo transfer was scheduled 5 days after ovulation confirmed by transvaginal ultrasound.
Hormone replacement treatment (HRT). Oral estradiol valerate at 6 mg daily was given from day 5 of the menstrual cycle for 10 to 12 days. When the endometrial thickness reached 7 mm and the progesterone level did not exceed 1.5 ng/ml, vaginal progesterone at 600 mg daily was commenced.
GnRHa + HRT. GnRHa was injected on day 2 of the menstrual cycle. Oral estradiol valerate was administered 30 days after GnRHa and HRT regimen was performed as previously described.
Luteal phase support
600 mg daily of vaginal progesterone and 30 mg of oral progesterone were administered from the day of embryo transfer. Pregnancy was confirmed by serum β-hCG assessment 14 days after embryo transfer. Luteal phase support continued if serum β-hCG was positive. Ultrasound was performed 5 weeks after embryo transfer.
Definition of clinical outcomes
Clinical pregnancy was defined as the presence of a gestational sac at the 6–8 weeks transvaginal ultrasound. Ectopic pregnancy was defined as a gestational sac observed by ultrasound outside the uterine cavity. The miscarriage rate was calculated as the number of miscarriages up to the 24th week of pregnancy divided by the number of patients with clinical pregnancies. The preterm birth was defined as a birth that takes place after 24 weeks and before 37 completed weeks of gestational age, and further categorized based on gestational age as <28 weeks (extremely preterm), 28–32 weeks (very preterm), and 32–37 weeks (moderate to late preterm). Preterm birth rate was calculated as the number of preterm live births divided by the number of patients with clinical pregnancy. Live birth was defined as the delivery of at least one live-born baby beyond 24 weeks of gestational age. The live birth rate was defined as live deliveries (at least one live birth) per woman after embryo transfer.
Sample size calculation
According to the previous study, the preterm birth rate in IVF was 10.9% [Citation8]. Based on other studies within fertility care as well as the discussion by gynecologists and epidemiologists, we assumed that the minimal clinically important difference would be 10%. To demonstrate this difference with a two-sided test, 5.0% alpha-error, and 80% statistical power, the lowest number of participants we need to enroll is 418.
Statistical analysis
Data are presented as mean and standard deviation (continuous variables) or counts and proportions (categorical variables). One-way analysis of variance was used for continuous variables, and Pearson’s χ2 test was used for categorical variables with Fisher’s exact test when necessary. We performed logistic regression to explore the effects of BMI on preterm birth and live birth after accounting for the following potential confounders: infertility duration, endometrial thickness, infertility type (primary infertility vs secondary infertility), protocol in the fresh cycle (agonist, antagonist, other), biopsied blastocysts, no result embryos. We calculated crude odds ratios (OR) and adjusted ORs (aOR) with a 95% confidence interval (CI). Data were analyzed with the use of the statistical packages R (The R Foundation; http://www.r-project.org;version 3.4.3) and Empower (R) (http://www.empowerstats.net/en/, X&Y solutions, inc. Boston, Massachusetts). The level of significance was set at p < .05 if the Bonferroni correction is not applied.
Results
Among 851 cycles that were started between 2013 and 2019 and in which a single frozen-thawed, euploid blastocyst embryo transfer strategy was applied, 637 women were of normal weight (BMI, 18.5–25 kg/m2). A total of 81 of the cycles involved underweight women with a low BMI (BMI < 18.5 kg/m2), 108 of the cycles involved overweight women (BMI, 25–30 kg/m2), and 25 of the cycles involved obese women (BMI ≥ 30 kg/m2).
The baseline characteristics and IVF treatment of participants are presented in . Infertility duration differed significantly across the four groups, with obese women more prevalent with longer infertility duration, although less than 1 year. Infertility type differed significantly across the four groups, with obese women more prevalent with primary infertility. There was a significant difference in the protocol in the fresh cycle between the four groups, with obese women performed agonist protocol more often. These findings were clinically irrelevant because they did not influence the live birth rate, as shown in . There was a significant difference in endometrial thickness between groups, with obese women more likely to have thicker endometrium.
Table 1. Baseline characteristics and IVF treatment.
Table 3. Logistic regression analysis of live birth rate.
Clinical outcomes of different BMI categories are shown in . Women with underweight have more biopsied blastocysts, while obese women have more embryos with no results. Although there was a trend increase in clinical pregnancy, miscarriage, and live birth, they did not reach statistical significance among the four BMI groups (). The logistic regression showed endometrial thickness as a factor influencing the live birth rate. No other factor (BMI, infertility duration, infertility type, protocol in fresh cycle, biopsied blastocysts, no result embryos) was found to influence the live birth rate ().
Figure 1. Clinical outcomes in different BMI groups (A) Percentage of clinical pregnancy rate, live birth rate, and miscarriage rate per transfer after PGT-A. (B) Percentage of preterm birth rate after euploid embryo transfer. Statistically significant for total preterm birth rate.
Table 2. Clinical outcomes of different BMI categories of WHO category.
The preterm birth rate was significantly higher in the overweight group (), however, different preterm birth categories showed no significant difference among the four BMI groups (). After adjusting for covariates, BMI was still statistically significantly associated with preterm birth (), with overweight (aOR 3.18; 95% confidence interval [CI], 1.29–7.80, p = .012) and obese (aOR 1.49; 95% CI, 1.03–12.78, p = .027) women have a higher risk of preterm birth compared with normal weight women.
Table 4. Relationship between BMI and preterm birth in different models.
As this study was performed in Chinese women, a subgroup analysis of BMI of Asian classification was conducted (). Women were considered underweight if BMI was <18.5 kg/m2, normal weight if 18.5 kg/m2 ≤ BMI ≤ 22.9 kg/m2, overweight if 23 kg/m2 ≤ BMI ≤ 24.9 kg/m2, and obese if BMI was ≥ 25 kg/m2. There was no significant difference among BMI groups in clinical pregnancy, miscarriage, and live birth rate. Women with obese had a higher rate of preterm birth compared with the other groups.
Table 5. Clinical outcomes of different BMI categories of Asian classification.
Discussion
In this retrospective cohort study of 851 women undergoing IVF with euploid blastocyst embryo transfer, we showed that preterm birth was higher in women with overweight and obesity than in women with normal weight.
The prevalence of obesity continues to rise worldwide. The negative consequences of obesity are identifiable in every organ system. Obesity is a known risk factor for subfertility, miscarriage, feto-maternal complications, and long-term risks in adult life [Citation15]. Obese women are at particularly high risk for adverse pregnancy outcomes. Up till now, the relationship between obesity and negative reproductive outcomes is clear, but the mechanism by which obesity affects fertility remains unclear. Data from large trials have demonstrated fewer normally fertilized oocytes, and lower pregnancy and live birth rates in obese women [Citation16]. A large cohort study of 152,500 cycles similarly reported significantly higher odds of cycle cancelation and pregnancy failures with overweight women [Citation17]. A systematic review and meta-analysis found decreased pregnancy rates, increased gonadotropins, and a higher miscarriage rate in obese and overweight women. These differences are evident even at a BMI≥ 25 kg/m2 [Citation18].
Our study differs from previous studies on obesity and clinical outcomes in three ways. First, most studies dealt with embryo transfer without PGT-A. Theorizing that oocyte from obese patients are subject to alterations in normal mitotic checkpoints leading to aneuploidy [Citation19]. Therefore, they cannot accurately elucidate the effects of BMI on clinical outcomes in obese women. Second, many studies dealt with obesity as a categorical variable. In contrast, in our study, BMI was also analyzed as a continuous variable that allowed a subtle increase in a live birth. Third, some studies compared clinical outcomes in fresh cycles [Citation20,Citation21]. One possible mechanism suggested that endometrial advancement is induced by ovarian hyperstimulation in a fresh embryo transfer cycle, resulting in embryo-endometrium asynchrony [Citation22]. Therefore, our study focused on frozen embryo transfer cycles, which eliminate the negative effect of controlled ovarian stimulation.
We found an association between obesity and preterm birth when transferring a single euploid blastocyst embryo. Our results suggest that the negative impact of being overweight or obese may be related to factors other than embryonic aneuploidy. A prior meta-analysis showed that a high BMI did not affect the IVF clinical outcomes in donor oocyte recipients, which suggested that oocyte quality may be the most important factor in obese women [Citation23]. Previous studies suggested metabolic alterations in the serum are reflected in the follicular fluid and that some of these alterations may affect oocyte quality with higher BMI both in vitro and in vivo [Citation24–26]. In addition, one prospective cohort study showed that elevated follicular free fatty acid (FFA) was associated with poor cumulus-oocyte complex (COC) morphology [Citation27]. Moreover, severe obesity is also associated with a greater prevalence of spindle anomalies and nonaligned chromosomes in failed fertilized oocytes [Citation28]. Although there is no unifying mechanism responsible for the increased risk of preterm birth, maternal insulin resistance, hyperinsulinemia, and oxidative stress may be contributing factors to placental dysfunction [Citation29].
Embryo aneuploidy, which is widely known to be the most important factor for negative outcomes, was not given enough attention in previous studies when comparing the live birth rate between obese and normal-weight women [Citation30]. Obesity has been reported to alter the early embryo metabolomic signature [Citation31], raising the possibility of epigenetic-mediated impairment of clinical outcomes [Citation32]. Frozen-thawed embryo transfer was associated with decreased risk of preterm birth and other perinatal complications like small for gestational age compared with fresh embryo transfer [Citation33,Citation34], however, this difference was still significant in women with obesity.
There are some limitations to this study, such as its retrospective nature, which reduces its direct application to clinical practice. The study question would best be addressed by a multicenter, prospective trial. Our study is also limited by the possible unknown confounders that might affect the results. The number of women with a BMI ≥30 was small and may not be representative of the general population, and the results need to be confirmed in a larger cohort.
Conclusion
Our results demonstrated a relationship between BMI and preterm birth in women undergoing single euploid blastocyst embryo with PGT-A, suggesting that the detrimental effect of an elevated BMI on pregnancy and IVF outcomes may be the result other than genomic, or an unspecified alternative etiology. While the findings of this study are valuable in contributing to the current understanding of the relationship between BMI and pregnancy outcome, further research needs to be undertaken before the results can be extrapolated to the general population.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Additional information
Funding
References
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