The main purpose of our study was to explore the influence of EMT before embryo transfer on the outcome of singleton live births resulting from freeze–thaw cycles. Mean birth weight of newborns was greater for mothers with EMT > 12 mm before embryo transfer compared with mothers with lower EMT. In addition, birth weight was affected by several confounding factors. Therefore, we established a multivariate linear regression model comprising all the clinical factors that may affect birth weight. After adjustment for confounding variables in the multivariate linear regression, the effect of EMT on newborn birth weight remained significant. At this point, we believed that a thicker endometrium before embryo transfer during freeze–thaw cycles was clinically significant with regard to increasing newborn birth weight and improving other newborn outcomes.
Few studies have focused on the influence of EMT before embryo transfer on pregnancy complications and neonatal outcomes after IVF–FET. Guo et al. studied EMT on the day of hCG detection in fresh-embryo transfer cycles. They found insufficient EMT to be a risk factor for neonatal SGA . Thus, in accordance with our results, newborns with a thinner endometrium had a lower weight and an increased risk of SGA. After identification of a relationship between EMT and newborn birth weight, we assessed the relationship between EMT and prevalence of low birth weight or macrosomia among full-term newborns: there were no significant differences in the prevalence of low birth weight or macrosomia among the three groups. This finding may have been because our study involved FET because Ginod et al. found that fetal growth after FET was faster than that after fresh-embryo transfer cycles . Hwang et al. also showed that, compared with fresh-embryo transfer cycles, FET led to a higher birth weight and lower risk of newborns with low birth weight (adjusted odds ratio: 0.72; 95% confidence interval: 0.59–0.88) . Therefore, FET can improve newborn outcomes. As a result, in our study, we identified a relationship only between EMT and newborn birth weight but not with the prevalence of low birth weight or macrosomia.
A study by Moffat et al. on EMT as well as maternal and neonatal complications showed that increased EMT was not a predictor of birth weight in a normal pregnancy but, for those with pregnancy complications, EMT before embryo transfer was proportional to birth weight . Therefore, birth weight is affected by multiple factors, and pregnancy complications, such as pregnancy-induced hypertension, pre-eclampsia, eclampsia, GDM, placenta previa, and premature rupture of fetal membranes, may all affect the growth and development of the fetus in the uterus, which is ultimately reflected in the difference in birth weight. Therefore, in the present study, pregnancy complications were also investigated. GDM prevalence was highest in the > 12 mm group and was significant. Saito and colleagues showed that hormone-replacement cycles can reduce GDM risk . In our study, the number of hormone-replacement cycles was lower in patients with EMT > 12 mm than that in the other two groups. Thus, the > 12 mm group had a higher proportion of GDM. He et al. explored the relationship between EMT and perinatal outcomes. They found that preimplantation EMT < 8 mm increased the risk of premature rupture of membranes significantly : that finding is consistent with our results. In the present study, the prevalence of premature rupture of membranes was higher in the EMT > 12 mm group than that of patients with EMT 8–12 mm. This difference may be related to the higher prevalence of hypertension during pregnancy in this group. Oron et al. set an EMT cutoff of 7.5 mm, and found that the prevalence of obstetric complications in patients with EMT < 7.5 mm was increased significantly, including premature delivery, hypertension in pregnancy, placenta previa, and premature rupture of membranes . In the present study, the EMT was divided into more detailed (i.e., three) groups, and the difference in pregnancy duration among the three groups was significant. Patients with a thin endometrium were more likely to have a preterm birth, a finding that is consistent with previous studies.
Newborn birth weight is affected by several confounding factors. In the present study, univariate regression was used to identify the factors that may affect birth weight. All significant variables were used in the multivariate linear regression to establish the following regression equation for predicting birth weight:
Y (birth weight) = 25.942×(EMT of 8–12 mm) + 85.107×(EMT > 12 mm) + 123.483×(hypertension during pregnancy) + 148.859×(premature rupture of membranes) + 182.342×(placental position) − 126.242×(newborn sex) + 23.837×(number of days of pregnancy) + 130.487×(delivery mode) − 55.023×(number of implanted embryos) − 6.215×FSH level − 1.124×E2 level + 22.218×BMI − 4468.101.
After adjustment for confounding factors, the EMT grouping remained meaningful for predicting birth weight, with increased EMT increasing the birth weight. Compared with patients with EMT < 8 mm, patients with EMT > 12 mm had an increase in the mean birth weight of 85.107 g. The regression coefficient was significant. This observation is consistent with the results of a similar study by Zhang et al. on freeze–thaw cycles : they showed that EMT < 8 mm was associated with a lower mean birth weight.
We also found that the E2 level was an independent predictor of birth weight, with a regression coefficient of − 1.124. That is, for each unit change in the E2 level, the predicted weight of the newborn would decrease by 1.124 g. A study by Pereira et al. on fresh-embryo transfer cycles showed that the high estrogen level produced during superovulation can affect the environment of embryo implantation, thereby leading to platelet dysfunction and low birth weight . In addition, experiments conducted by Weinerman et al. showed that the superovulation environment was not conducive for the growth and development of fetal mice . Our study focused on freeze–thaw cycles. The high-estrogen effect of superovulation was absent, but a high E2 level regulated birth weight negatively. This finding is consistent with the low birth weight caused by a high-estrogen environment during fresh-embryo transfer cycles. Whether there is a definite connection requires further exploration in studies with large sample sizes. Spada et al. showed that the BMI of pregnant women in the first trimester was a strong predictor of newborn birth weight . In our study, for each unit increase in BMI of a pregnant woman, the newborn birth weight increased by 22.218 g. This finding is consistent with data from other studies [20, 21].
The mean birth weight of male babies is higher than that of female babies, and IVF technology does not seem to change the sex-dependent differences in birth weight . Our results also reflect those findings. According to our regression equation, the mean birth weight of female babies was reduced by 126.242 g compared with that of male babies. Gestational age is a confounding factor that affects birth weight. In our study, the newborn birth weight increased by 23.837 g for each additional day of pregnancy. Moreover, pregnancy complications and abnormal function of fetal appendages may affect the blood circulation between the mother and child during pregnancy, thereby leading to adverse neonatal outcomes . The regression analysis in our study revealed that hypertension during pregnancy, premature rupture of membranes, and placenta previa were independent risk factors for decreased newborn birth weight. We also found that the delivery mode affected newborn birth weight. The birth weight of newborns delivered by cesarean section was relatively low, which may have been because that women who deliver by cesarean section have common pregnancy complications, and emergency pregnancy termination due to other reasons in the circumstance that the fetus is not yet mature. In addition, the number of implanted embryos was a predictor of newborn birth weight. The greater the number of implanted embryos, the greater was the probability of twins and an increased risk of pregnancy complications. Moreover, the number of embryos implanted can also affect early development of the newborn .
The mechanism underlying how EMT before embryo transfer affects newborn birth weight is thought to mainly involve changes in the intrauterine environment during embryo implantation and establishment of normal placental–fetal circulation. In the early stages of pregnancy, the hypoxic environment of the endometrium is a prerequisite for normal development of an embryo . A thin endometrium increases the concentration of oxygen from the mother, which produces reactive oxygen species; this action affects the intrauterine environment during early embryonic development, resulting in impaired fetal growth . Kelley et al. cultured mouse embryos under different oxygen concentrations, and found that an oxygen concentration of 20% compared with an oxygen concentration of 5% reduced the weight of fetal mice . That observation suggests that excessive reactive oxygen species interfere with early embryonic implantation and development, and reduce newborn birth weight. The most important function of the placenta is the exchange of nutrients and oxygen between the mother and fetus. In the first trimester, establishment of a healthy placenta requires adequate remodeling of spiral arteries. If remodeling of the spiral arteries is impaired, placental function can be impaired, causing pregnancy complications and reducing the blood supply to the fetus in the third trimester, which can lead to fetal hypoxia, malnutrition, and low birth weight . In patients with a thin endometrium, resistance to blood flow of the uterine basilar artery is increased, and blood vessels are often underdeveloped, which leads to poor remodeling of the spiral arteries and affects the placental blood supply .
Our study had four main strengths. First, we used strict inclusion criteria and exclusion criteria. Second, we excluded patients with underlying diseases (e.g., hypertension, DM) that may affect fetal development. Third, the treatment regimens were conducted in accordance with uniform standards to ensure treatment consistency. Fourth, EMT measurement was undertaken by the same experienced sonographer, which reduced measurement variability.
The main limitation of our study was that it was from a single center and retrospective. To obtain more precise evidence on the relationship between EMT and birth weight, a large-scale prospective study with multicenter collaboration may be required. Another limitation was that we did not explore the mechanism by which EMT affects birth weight.