Human chorionic gonadotropin (hCG) is synthesized in trophoblast cells, and its serum level associates with the status of the trophoblastic mass. After embryo implantation, the hCG concentration dynamically increases, and hCG could be detected from the maternal circulation at day 8 [1]. hCG is important for the corpus luteum maintainence during early gestation. In addition, hCG is involved in trophoblast propagation, angiogenesis, placental development, and immunotolerance [2]. In general, the serum hCG level is an indicator of pregnancy progression, and the increase of its concentration is an indicator of ongoing intrauterine gestation. However, a low maternal serum hCG concentration in early gestation is linked to the prevalence of severe pre-eclampsia [3]. Furthermore, the serum hCG concentration is significantly higher in women with multiple pregnancies achieved by IVF-ET than in those with singleton pregnancies [4]. These findings prompted us to ask whether other factors can affect the maternal serum hCG concentration in early pregnancy.

In vitro maturation (IVM), an assisted reproductive technology (ART), refers to the maturation of cumulus oocyte complexes (COCs) in vitro [5]. Since the first IVM baby delivered and reported in 1991, this technology has been wildly applied. Since no ovarian stimulation in IVM, it has a great advantage in safety, convenience and reducing the OHSS risk, and attracts more and more attention [6, 7]. Therefore, IVM is applicable for PCOS patients, fertility preservation and cost-sensitive people for its unique advantage. Benefit by the biphasic IVM system, the application of IVM has been largely expanded. Additionally, Liu et al. [8] reported that during routine gynecological surgery oocytes could be retrieved for in vitro maturation, which can be considered as a “fertility insurance” for fertility preservation or in vitro fertilization.

Tanbo TG et al. [9] reported a negative association between the oocyte retrieved number and the serum hCG concentration when a GnRH agonist or antagonist protocol was used during IVF [9], and Almog B et al. [10] reported that the hCG concentration in early pregnancy is notably higher after IVM than after traditional IVF. As we all know, IVM does not involve in ovarian hyperstimulation [11]. Therefore, we wondere whether the number of oocyte retrieved influence the maternal serum hCG concentration in early pregnancy achieved by IVM. To reconcile the issue, we studied the association between the number of oocyte retrieved and the serum hCG concentration on day 14 and 21 after IVM.

51 women had delivered singletons throug IVM and the data were collected and analyzed in the present study. The clinical characteristics were shown in Table 1. The mean female age was 29.94 $\pm$ 3.73. The mean number of oocytes retrieved was 16.78 $\pm$ 11.23, and the hCG concentration was 614.47 $\pm$ 445.50 IU/L on Day 14 and 10930.20 $\pm$ 7223.17 IU/L on day 21 after embryo transfer, respectively.

After confounding factors adjusted, the number of oocyte retrieved was positively associated with the concentration of the maternal serum hCG on days 14 (p = 0.046, Table 2) and 21 (p = 0.022, Table 3). Furthermore, crude linear regression analysis showed a positive relationship between the number of oocytes retrieved and the serum concentration of hCG on days 14 (Fig. 1A) and 21 (Fig. 1B).

Fig. 1.

Crude linear regression analysis of the relationship between the number of oocytes retrieved and the maternal serum concentration of hCG on days 14 (A) and 21 (B). hCG: IU/mL.

As we all know, hCG is critical for the corpus luteumm, trophobast proliferation and placental development in early gestation, and the serum hCG is an important indicator for early pregnancy. The factors which would influence the serum hCG level were concerned and studied. In the present study, we analyzed the data of 51 women who had successfully delivered singletons after IVM, and the results showed that there was a positive association between the number of oocyte retrieved and the serum concentration of hCG at days 14 and 21 after embryo transplantation.

After embryo implantation, placental trophoblasts invade the endometrium until the first trimester [13]. During trophoblast invasion, the placenta secretes hCG, progesterone, erythropoietin, angiotensin II, and adrenomedullin [14]. hCG, which consists of alpha and beta subunits, as well as various carbohydrate moieties, is the first message secreted from the embryo to the mother. Additionally, the hCG mRNA is detected in 2-cell stage embryos [15], and hCG is considered as a good predictor of early gestation outcomes. To induce priming, 10,000 IU of urinary hCG was injected subcutaneously at 36 hours before oocyte retrieval. Since the half-life of hCG is 2.3 days, the injected hCG did not affect our results. Although the level of serum hCG increases dramatically during early pregnancy, its level is subject to large inter-individual variations [16]. During traditional IVF-ET, the estradiol level is high in pre-ovulatory follicles after treatment with a GnRH agonist or antagonist. Estradiol can also inhibit the protein expression of leukemia inhibitory factor (LIF), which is critical for embryo implantation [17]. Therefore, a high estradiol concentration led to slower trophoblast proliferation and lower hCG concentration because of the decreased receptivity of the endometrium. The number of oocytes retrieved by traditional IVF associates negatively with the serum hCG concentration in early gestation [9]. In the present analysis, however, we found a positive association between the high oocytes retrieved number and the serum hCG concentration in early pregnancy after IVM. In general, traditional IVF allows for the retrieval of oocytes that are 20 mm in diameter. However, IVM allows for the retrieval of oocytes that are only 10 mm in diameter. It is possible that the estradiol concentration is low in IVM follicles, which promotes trophoblast proliferation and stimulates hCG secretion via the optimal receptivity of the endometrium. However, further research is needed to understand its underlying mechanism.

Multiple linear regression analysis was applied to assess the association among the maternal serum concentration of hCG and the confounding factors. The number of oocyte retrieved was positively associated with the hCG concentration at day 14 and 21 after embryo transplantation. Crude linear regression analysis also showed that there was a positive association between the number of oocyte retrieved and the concentration of serum hCG. In this study, we transferred 2 cleavage stage embryos for most patients (Table 1). It’s reported that the multiple pregnancy rate of IVF was 20–30 times higher than natural pregnancy [18, 19]. Therefore, single elective embryo transfer was recommended for many centers [20]. Single cleavage embryo transfer, however, was associated with lower on-going pregnancy rate and delivery rate in comparison with single blastocyst transfer [21]. After comprehensive assessment of financial pressure, pregnancy rate and other risks, two cleavage stage embryos were transferred in our center. In fact, previous studies revealed that maternal age, embryo grade and the number of embryo transfer all were factors that would affected hCG levels [22, 23, 24]. The transfer of multiple embryos was associated with higher hCG value compared with the transfer of single embryo [24, 25, 26]. Although in the current study we had only one gestational sac observed after the transplantation of two cleavage stage embryos, it was also possible that the non-implanted embryo affected the hCG level before vanishing. Therefore, more studies with single embryo transfer and larger number of samples are needed in the future. We also checked the relationship between the serum concentration of hCG and the birth weight (3366.4 $\pm$ 93.7 g) of these singletons and the results showed that there was no positive relationship (p $>$ 0.05). This result was consistent with the research performed by Thomas Strowitzki [27]. In addition, we found an association between the male age and the serum hCG concentration on day 14. Although we do not know the reason for this association, future studies will address the association between male age and the serum hCG concentration using more patients.

Pingping Qiu et al. [28] reported that the concentration of serum $\beta$-HCG on day 10 were associated with the outcomes of vitrified-warmed blastocyst transplantation at different ages. Many models have been built to predict the pregnancy outcomes based on the serum hCG values [29, 30, 31, 32]. Our findings provide a new idea for the study and prediction of the pregnancy outcomes after IVM, and can better help us to understand the hCG values after IVM treatment. Our results also suggested that the number of oocytes retrieved should be considered when we predicted the pregnancy outcome of IVM patients through the hCG level, and a new model different from IVF should be established for IVM patients. Our data analysis was limited because of the small number of patients that were available and the retrospective design of this study. Therefore, further research is needed and the underlying mechanisms call for further investigation.

In summary, the present study showed a positive association between the number of oocyte retrieved and the concentration of serum hCG in early pregnancies after IVM. Our results partially explain the transient changes in the concentration of maternal serum hCG after IVM. But for the small number of patients, further researches with larger samples and multiple centers were needed.

Conceived and designed the study—PL; coordinated data collection—YPW, SLL, XYZ, YL and RY; analyzed the data—YPW and SLL; wrote the paper—YPW and SLL; all authors interpreted the data. All authors read and approved the final manuscript.

This study was confirmed by the ethics committee of the Peking University Third Hospital (No. 2008013). Patients received a subcutaneous injection of hCG (Serono, Aubonne, Switzerland; Approval Number: S20110045).

Not applicable.

This study was supported by the National Key Research and Development Program [2018YFC1002106], and the National Natural Science Foundation of China for Young Scholars [81601275].

The authors declare no conflict of interest.