In our study of women with infertility and normal ovarian reserves who underwent IVF/ICSI, those with supraphysiological E2 levels on the day of hCG trigger had significantly higher rates of SGA, LBW and full-term LBW than did women with lower E2 levels. Multivariate analysis revealed associations between E2 levels of more than 4000 pg/ml on the trigger day and SGA, LBW and full-term LBW.
Moreover, we found that lower BMI was an independent risk factor for the SGA, LBW and full-term LBW rates.
Comparisons with other reports
Singleton births via ART are associated with significantly increased risks of SGA, LBW, PTB, perinatal mortality, and maternal complications [17–23] compared with spontaneous singleton births. Biological traits that may be associated with this phenomenon include intrinsic characteristics of the infertile couple, in vitro culture methods/media, the supraphysiological hormonal environment associated with COH, or a combination of these factors; however, the exact mechanisms leading to increased risks of these conditions remain unknown. In recent years, observational studies have demonstrated that singletons from FET have a reduced risk of LBW[4, 5, 24]. The most important physiological difference between FET and fresh ET is that FET involves a hormonal milieu closer to that of the physiological environment. Therefore, studies regarding how supraphysiological E2 levels during COH are associated with LBW or SGA have begun to emerge[9, 11–14]. However, no unified standard definition exists for E2 levels. Supraphysiological serum E2 levels have been defined in various ways, such as in terms of the 50th[14, 25], 75th[26], 90th[15], and 95th[11] percentiles. Therefore, no universal standard exists for E2 levels for different populations, ovulation protocols or laboratory standards. Many studies have suggested that a supraphysiological hormonal milieu may contribute to LBW, albeit with variable confounding factors. Recently, Pereira et al.[13], using ROC analysis and multivariable logistic regression analysis to account for potential confounding factors, highlighted that supraphysiological E2, defined as E2 ≥ 2500 pg/ml, independently predicted LBW for full-term singletons born from fresh ET, with an AOR of 10.8 (95% CI: 9.2–12.5). However, this study included cycles with pregnancy complications, which are also associated with neonatal outcomes, including LBW, SGA, macrosomia and large size for gestational age (LGA). For example, Yen et al. [27] found that preeclampsia increased the risk of VLBW. Additionally, a population-based study showed that chronic hypertension increased the probability of SGA after adjusting for important confounders. Therefore, the effects of high E2 levels on SGA occurrence, accounting for neonatal sex and gestational weeks, should be further explored. SGA is an important cause of neonatal death; it affects physical and mental development during childhood and adolescence and is associated with significantly increased arterial stiffness and metabolic dysfunction during adulthood[28].
Plausible biological mechanisms
The results of our study suggest that supraphysiological E2 levels on the day of hCG trigger may contribute to SGA pathogenesis; however, the exact biological mechanism underlying this phenomenon remains unknown. The main mechanisms addressed in the research are as follows. First, Valbuena et al.[29] reported that high E2 levels were deleterious to embryo adhesion in vitro, mainly because they exert direct toxic effects on embryos. Moreover, Ertzeid G et al.[30] performed a study using an embryo donation model in mice and concluded that ovarian stimulation appeared to impair embryo quality and uterine milieu. Additionally, Bittner et al.[31] analyzed the effects of ovulation on embryonic development in mice and showed that a supraphysiological hormonal milieu had toxic effects on fertility and embryonic development during ART treatment. Second, at least in animal models, elevated E2 levels also affect normal trophoblastic vascular invasion. Albrecht et al.[32] reported that E2 during early baboon pregnancy suppressed the extravillous trophoblast invasion of uterine spiral arteries. In addition, Bonagura et al.[33] found that prematurely elevating E2 during the early stages of baboon pregnancies diminished uterine artery remodeling and affected the expression of extravillous placental vascular endothelial growth factors, thereby potentially contributing to neonatal SGA and LBW pathogeneses. Consistent with these animal model studies, human research has shown that elevated E2 levels can affect endometrial receptivity[34] by altering endometrial gene expression profiles[35, 36], leading to abnormal implantation and placentation. Ng et al.[37] used 3D ultrasound examination to reconfirm that ovarian stimulated cycles were associated with lower endometrial and subendometrial blood flow than natural cycles. Kolibianakis et al.[38], using aspirational endometrial biopsies, observed more advanced endometrial maturation in fresh embryo cycles. Given this evidence from both humans and animals, supraphysiological levels of E2 after COH expose embryos to an abnormal uterine and endometrial environment, which negatively affects embryo quality, extravillous trophoblast invasion, endometrial receptivity and placental development, thus disrupting fetal growth.
Regarding maternal weight, we found that lower BMI was associated with higher risks of SGA, LBW and full-term LBW, although our study did not explore specific values. Consistent with the findings from a prior prospective multicenter cohort study, low maternal birth weight was an independent risk factor for SGA[39]. Spada et al.[40] reported that maternal height and BMI should be considered when evaluating birth weight. In further research, we will explore the effects of maternal BMI on offspring birth weights to provide suggestions for clinics to reduce the occurrence of adverse pregnancy outcomes. Such research will require a large prospective data sample for the examined population.
Strengths and limitations
The strengths of this study include its large dataset (n = 3131). To our knowledge, our study, for which selection and statistical bias were minimized, is the largest investigation (n = 3131) to explore the effect of serum E2 levels on the day of hCG trigger on SGA for fresh ET cycles. Moreover, to reduce potential bias, our study accounted for variables that could potentially impact neonatal birthweight, including maternal age, BMI, parity, infertility type, infertility diagnosis, gonadotropin dosage, endometrial thickness on the hCG trigger day, ART method, embryo stage, number of embryos transferred, delivery mode, neonatal sex and E2 levels on the day of hCG trigger. Another strength of our study is that our findings are applicable to clinical practice: when E2 ≥ 4000 pg/ml, embryos should be frozen and ET performed later in the subsequent cycle.
Our study was limited by its retrospective design; therefore, a prospective cohort study is needed. Furthermore, between-group differences were found in certain baseline characteristics, including BMI (P = 0.00), infertility diagnosis (P = 0.01), basal FSH (P = 0.01), AMH (P = 0.00), antral follicle count (P = 0.00), gonadotropin dosage (P = 0.00), number of embryos retrieved (P = 0.00), endometrial thickness on the hCG trigger day (P = 0.03) and embryo stage (P = 0.00); however, multivariable regression analysis was performed to minimize sources of bias.