Expert consensus on fertility preservation in hematopoietic stem cell transplantation in girls in China

Abstract

Aims: Preconditioning before hematopoietic stem cell transplantation (HSCT) seriously damages the ovarian function and causes female infertility. This consensus focuses on the fertility preservation(FP) for girls needing HSCT, aim to make doctors in different disciplines aware of the importance, necessity and technique of ovarian protection.

Materials and methods: Summarizing relevant literature and organizing multidisciplinary experts, including obstetrics and gynecology, reproductive medicine, oncology, pediatrics and hematology for full discussion.

Results: Individuals exposed to HSCT in childhood are at higher risk of loss of fertility. Considering the high risk of premature ovarian insufficiency (POI) after conditioning and negative impact of POI on fertility, physical and mental health, it is absolutely necessary to protect fertility before HSCT conditioning. Ovarian tissue cryopreservation is the main fertility preservation option for these population.

Conclusions: Fertility preservation before HSCT conditioning is crucial. Ovarian tissue cryopreservation is often the only option for these population.

Keywords:

• Hematopoietic stem cell transplantation
• fertility preservation
• ovarian tissue cryopreservation
• expert consensus
• ovary
• infertility
• hormone replacement therapy

Hematopoietic stem cell transplantation (HSCT) is a medical procedure that infuses autologous or other’s (allogeneic) hematopoietic stem cells to reconstruct normal hematopoietic and immune functions after a high-dose chemotherapy and/or radiotherapy, which intended to clear tumor and abnormally cloned cells, destroy the recipient’s immune system, then reduce or eliminate rejection of donor’s hematopoietic stem cells.

Status of hematopoietic stem cell transplantation

The most common indications of HSCT are hematological diseases, including malignant hematological diseases, such as leukemia, lymphoma, multiple myeloma and nonmalignant hematological diseases, such as aplastic anemia, thalassemia and fanconi anemia. In addition, some congenital immunodeficiency diseases, inherited bone marrow failure syndrome and inherited metabolic diseases can also be cured by HSCT. With the continuous expansion of HSCT indications, the number of HSCT has increased year by year, more than 50,000 HSCTs per year worldwide [1]. Between 2008 and 2019, 58,914 HSCTs were reported to the Chinese Blood and Marrow Transplantation Registry Group (CBMTRG) throughout China [2]. In 2019, a total of 12,323 cases of HSCT were reported, 31% of HSCT recipients were under 18 years of age, the most common diseases for which were acute lymphoblastic leukemia (23%), acute myeloid leukemia (22%), aplastic anemia (18%), thalassemia (15%) and non-Hodgkin lymphoma (5%). At the same time, the 5-year survival rate after HSCT is improved, with some diseases reaching 90% [3,4]

Damage of hematopoietic stem cell transplantation on fertility

There are more and more HSCT children survivors. Due to the development of HSCT technology, the long-term survival rate after HSCT has increased, making quality of life more important, and focusing the fertility of girls. Individuals exposed to HSCT in childhood are at higher risk of loss of fertility in adulthood due to conditioning regimens before HSCT. In a large research, 62,988 pediatric patients received first HSCT in European Society for Blood and Marrow Transplantation(EBMT) centers between 1995 and 2016 among which pregnancy was reported in 406 patients. Although the age, at last follow-up or death was <17 years for 75% of the patients which should not be included in the assessment and the proportion of married patients with a desire to have a child in reproductive period is unknown, the proportion of pregnant patients or their spouses is very low after HSCT [5]. Previous report shows that the pregnancy rate after HSCT is less than 5% [6].

Premature ovarian insufficiency (POI) is defined as the cessation of ovarian function before the age of 40 which is characterized by menstrual disorders (such as amenorrhea or oligomenorrhea), high gonadotropins and low estrogens [7]. It is important to note that menstrual recovery after HSCT does not equate to normal fertility. Immediately after HSCT, ovarian function may not be completely decreased. HSCT itself doesn’t directly damage ovarian function. However, high-dose radiotherapy and chemotherapy before HSCT will seriously damage the ovary, reduce ovarian reserve and accelerate the depletion of follicular pool, leading to early menopause. Briefly, 70%–100% of the women experienced HSCT will have POI [8].

Chemotherapy

Alkylating agents such as cyclophosphamide, busulfan and melphalan have proven to be the most toxic to the ovaries, which are used most commonly for myeloablative conditioning before HSCT. In myeloablative conditioning, the dose of cyclophosphamide reaches 3.6 g/m² [9], much higher than induction regimens (about 750 mg/m²). Alkylating agents damage actively dividing cells, such as mature follicles and granulosa cells. Primordial follicles are recruited out of their inactive state into more vulnerable growth follicles during chemotherapy, rendering them susceptible to cytotoxic damage [3].

Radiation

Less than 2 Gy radiation dose can deplete 50% of the non-growing follicle pool [10]. In myeloablative conditioning regimen, the dose of total body irradiation (TBI) was as high as 12 Gy, the incidence of POI after TBI is as high as 72%–100% [11]. Currently, TBI is generally not recommended for conditioning regimen.

Once POI occurs, ovarian function is difficult to recover. POI not only leads to a significant reduction or loss of female fertility, but also seriously impact on women’s mental health, quality of life and sexual function. Various chronic diseases have occurred earlier, and the risk of premature death has significantly increased [7]. Only 5% to 10% of POI patient can spontaneously become pregnant. POI is an independent risk factor for ischemic heart disease and coronary vascular disease. With 1 year decrease in age at menopause the risk of cardiovascular disease is increased by 3% [12]. Due to insufficient peak bone mass accrual and increased bone resorption associated with estrogen deficiency, bone mineral density in girls under 18 years after HSCT is almost lower than normal [13]. HSCT survivors are prone to anxiety, depression, post-traumatic stress disorder and other psychological problems. Amenorrhea and absence of pubertal development (delayed puberty) are also adverse to mental health.

Considering the high risk of POI after conditioning and negative impact of POI on fertility, physical and mental health, it is absolutely necessary to protect fertility before conditioning HSCT.

Fertility protection technology for girls needs hematopoietic stem cell transplantation

Embryo cryopreservation, mature-oocyte cryopreservation and ovarian tissue cryopreservation (OTC) are valid and effective method of fertility preservation (FP). However, for girls, the only effective fertility protection technique is OTC. Embryo cryopreservation and oocyte cryopreservation are applicable to married and unmarried women of childbearing age, respectively.

Ovarian tissue cryopreservation and transplantation

If gonadotoxicity therapy is needed, ovarian tissue is surgically removed and cryopreserved. When primary disease is cured and POI occurs, the cryopreserved ovarian tissue will be thawed and transplanted to restore ovarian function and fertility after multidisciplinary consultation and evaluation [14]. Oocyte and embryo cryopreservation only preserves few gametes or embryos, ovarian endocrine function cannot be restored and is difficult to get live births. While a piece of ovarian cortex containing hundreds or even thousands of follicles, OTC can preserve great reproductive and endocrine potential for patients [15]. In addition, ovulation induction is not required before OTC, so ovary biopsy can be performed at any time before gonadotoxicity therapy or at the same time as the surgery for primary disease, without delaying the treatment of primary disease. OTC is the only option for FP in prepubertal girls and women who cannot delay the start of chemotherapy [16]. The average age of girls under the age of 14 who have cryopreserved ovarian tissue in Beijing Obstetrics and Gynecology Hospital affiliated to Capital Medical University is 7 years old, and the youngest is only 1 year old [17]. Xiangyan Ruan’s team reported the first live birth after cryopreserved ovarian tissue transplantation in China in 2021 [18]. From the current data, the risk of congenital malformation in infants born through OTC does not increase [19,20]. In Practice Committee of the American Society for Reproductive Medicine highlights that ovarian tissue banking is an acceptable fertility-preservation technique and is no longer considered experimental [21].

Evaluation of ovarian function before ovarian tissue cryopreservation

Multidisciplinary consultation should be conducted before ovarian biopsy. If the primary disease has a good prognosis, fertility protection specialist further evaluate ovarian reserve. Multiple criteria needed to be considered in ovarian reserve assessments, including follicle stimulating hormone (FSH), ovarian volume and antral follicle by gynecological ultrasound and anti-mullerian hormone (AMH). AMH is the most reliable marker of ovarian reserve in adult women and an important reference in girls [22]. However, none of these ovarian reserve indicators can directly reflect the total number of primordial follicles and follicle density. If basic FSH > 25 IU/L in two different test in an interval of more than 4 weeks, it indicates that ovarian function is decreased, OTC is not recommended. However, it cannot be concluded that there is no cryopreservation value only based on high FSH and low AMH. Patients should be fully informed that cryopreservation of ovarian tissue depends on the individual condition.

Ovarian biopsy

For children, ovarian biopsy can be done by laparotomy or laparoscopically. Laparoscopic surgery has smaller trauma and faster postoperative recovery. Generally, the patient is discharged within 1 to 2 days postoperatively which will not delay the subsequent therapy. As single-port laparoscopic surgery has many advantages, including esthetic incision and less trauma. It is generally recommended that the follow-up treatment can be done 3 days after the surgery. In recent years, one center reported that 62% of patients started HSCT conditioning within 24 h after surgery and no postoperative complications occurred [23]. Due to the small size of girl’s ovary, unilateral ovariectomy is recommended for cryopreservation. Menopause occurred 1 to 2 years earlier after unilateral ovariectomy compared to women with two intact ovaries [24,25]. An animal study demonstrated that even up to 75% of total ovarian tissue can be removed without impact on estradiol (E2) and progesterone (P) production in rats [26]. The removal of one ovary generally does not affect E2 and P secretion.

Try to use a ‘no-touch’ technique with minimal manipulation of the ovarian cortex, since 95% primordial follicle is located just deep to the cortex. To prevent thermal damage to follicles, the resection of ovarian tissue should use cold knife or scissors instead of energy equipment [27]. After the ovarian blood supply is cut off, ovarian tissue is removed as soon as possible. Then, the excised ovarian tissue is immediately placed in sterile fluid container (4–8 °C) provided by the cryopreservation center.

Considering the gonadal toxicity of chemotherapy, it is recommended to cryopreserve before gonadal toxicity treatment for nonmalignant hematological diseases. In general, OTC is an efficacious technique for restoration of fertility for girls who have received relatively low-dose chemotherapy that has not yet completely destroyed the ovary before HSCT conditioning. To minimize the risk of carrying malignant cells in ovary, ovarian biopsy should be performed when primary disease get complete remission after chemotherapy for girls with hematological malignancies [28,29].

Except alkylating agents, chemotherapy used for induction regimen have relatively low gonadal toxicity, and have no significant adverse effects on follicle density [17] and the outcome of ovarian tissue cryopreservation and transplantation [30–33]. Time required for ovarian function recovery (OFR) and the cumulative incidence of OFR and pregnancy have no difference related to previous chemotherapy. Therefore, prior exposure to chemotherapy should not be a contraindication of OTC [33].

In European Society of Human Reproduction and Embryology (ESHRE) guideline, ‘Patients who have already received low gonadotoxic treatment or a previous course of chemotherapy, can be offered OTC as FP option’ [29].

For ovarian tissue cryopreservation, the freezing method is divided into slow freezing and vitrification. The slow-freezing protocol is considered standard [33].

Ovarian tissue transplantation

The first pregnancy after ovarian tissue transplantation (OTT) was reported in 2004 [34] and the number of live births exceeded 200 up to now [35]. Reports on OTT in children are limited. In 2012, induction of puberty by autograft of cryopreserved ovarian tissue was reported. A 10-year-old girl with severe homozygous sickle-cell anemia underwent right oophorectomy by laparoscopy and ovarian tissue was cryopreserved by slow freezing, 27 months after the HSCT, she had autotransplantation of the ovarian tissue to induce puberty. Hormonal assays showed FSH 71.7 IU/L, luteinising hormone (LH) 24.1 IU/L, E2 less than 73 pmol/L, inhibin B less than 15 ng/L, and AMH 0.71 pmol/L. Bilateral breast growth started 2 months after the graft, pubic and axillary hair appeared 4 months after the graft. Her first menstruation was 8 months after transplantation [36]. In 2013, Ernst et al. described induction of puberty by heterotopic transplantation of previously cryopreserved ovarian tissue. A 9-year-old girl with Ewing sarcoma did OTC prior to chemo- and radiotherapy. Functional activity of the remaining ovary was destroyed during treatment. Four and a half years later the girl remained pre-pubertal with postmenopausal levels of FSH. Four months after OTT, FSH returned to low levels. During the following year, puberty gradually progressed to Tanner stage B4 and P3 and regular menstrual cycles started [37]. In 2015, the first live birth after the graft of ovarian tissue cryopreserved before menarche offers reassuring evidence for the feasibility of OTC when performed during childhood. A girl with homozygous sickle-cell anemia underwent OTC at 13 years of age. A right oophorectomy was carried out by laparoscopy. As expected, following myeloablative conditioning, the patient developed POI, with elevated gonadotropins. Menarche was induced at the age of 15.5 years with the use of a third-generation estrogen–progesterone preparation. Ten years after OTC, patients wanted to be pregnant and stopped hormone replacement therapy, then menstruation was stopped, FSH reached postmenopausal level. OTT was performed and 4 months later hormone levels reached childbearing status (FSH 5 IU/L, LH 6 IU/L, E2 166 pg/mL). Menstruation first occurred 5 months post-OTT and was followed by regular menstrual cycles. After more than two years post-transplantation, the patient had a spontaneous pregnancy and naturally delivered a healthy boy (birthweight 3140 g, Apgar score of 9 and 10 at 1 and 5 min, respectively) [38]. Subsequently, the recovery of ovarian function after OTT has been achieved in multi-diseases. Ovarian cortex frozen before 18 years of age can also respond to gonadotropin stimulation after transplantation. The increasing clinical evidence indicates that the interaction between the ovary and the hypothalamic pituitary in girls who had OTC before the age of 18 is similar to those after the age of 18. In girls who cryopreserve their ovarian tissue before the age of 18, the rate of recovery of ovarian function, pregnancy and giving birth to at least one child are not lower than those who cryopreserve after the age of 18 [35]. OTT can be performed at puberty for puberty induction or in adulthood for fertility. It should be individualized according to the number of cryopreserved ovarian cortex, follicle density and patient’s willingness. For patient with a large number of cryopreserved ovarian cortex, high follicle density and wishing to induce puberty by OTT, transplantation can be performed in adolescence. For patient with a small number of cryopreserved cortex and a low follicle density, considering the long-span reproductive age in the future, it is recommended to induce puberty through hormone replacement therapy and then OTT can be performed in adulthood when there is a need for fertility to ensure that there is enough tissue to obtain live birth.

The recovery of ovarian function usually takes 3–6 months. After reimplantation of ovarian tissue in the pelvic cavity, ovarian function can be restored in more than 95% of the pateint. The mean duration of ovarian function after reimplantation is 4 to 5 years. The function can be maintained even for 7 years [16].

Safety issues of cryopreserved ovarian tissue transplantation

Although there is a risk of carrying malignant cells in the ovaries of patients with hematological malignancies, it should not be listed as a absolute contraindicantion for OTC. Among all OTC patients in Europe, the largest proportion is hematological diseases (35%), the most common of which is Hodgkin lymphoma (57%), followed by non-Hodgkin lymphoma (24%), chronic myeloid leukemia, acute myeloid leukemia, acute lymphoblastic leukemia, myelodysplastic syndrome and myelodysplasia account for 6% in total [20]. In Beijing Obstetrics and Gynecology Hospital, Capital Medical University, 63.2% girls under the age of 14 who have cryopreserved their ovarian tissue have hematologic diseases among which 16.3% are malignant and 46.9% are nonmalignant [17].

In leukemia, ovary may be invaded. The risk of retransplantation of malignant cells needs to be considered [14]. Since the first report of an patient who underwent OTC in complete remission of acute myeloid leukemia and obtained a live birth in 2017 through OTT [39], four patients with leukemia also had a child after OTT [40–42]. Time from OTT to reporting the cases varied from 24 to 61 months, no recurrence of primary disease occurred. After application of maximal safety measures, restrictive criteria may be relaxed to make OTT possible for patients with hematologic malignancy [43]. Obtaining mature and safe gametes from frozen ovarian cortex may be possible in the future with the development of in vitro activation, growth and maturation of primordial follicle and artificial ovary.

Ovarian tissue transplantation is divided into orthotopic graft (intrapelvic) and heterotopic graft (outside pelvic cavity). Orthotopic transplantation into original ovarian site or in the corresponding parts of the peritoneum is more commonly used in the clinic [14].

This consensus mainly focuses on the fertility protection of girls before HSCT. For the indications of OTC, and the standard for transportation, processing, cryopreservation and transplantation, please refer to the ‘Chinese Society of Gynecological Endocrinology affiliated to the International Society of Gynecological Endocrinology Guideline for Ovarian Tissue Cryopreservation and Transplantation’ [14].

Ovarian tissue cryopreservation combined with in vitro maturation

In vitro maturation (IVM) refers to the maturation of immature cumulus–oocyte complexes (COCs) from prophase I through meiosis I to reach metaphase II (MII) in vitro [44]. In 1991, the world’s first live birth by IVM was reported. First pregnancy and live birth resulting from cryopreserved embryos obtained from in vitro matured oocytes after oophorectomy was achieved in 2014 [45], proving the feasibility of OTC combined with IVM.

In children undergoing OTC, immature follicles can be isolated from antral follicles visible on the ovarian surface [46], culture fluid used in the preparation of ovarian cortex [47] and ovarian medulla [48]. Cryopreservation of the mature oocyte can be obtained from IVM and in vitro fertilization–embryo transfer (IVF-ET) can be performed when fertility is required, which can further increase the fertility potential especially for patients with malignant hematological diseases. Oocyte retrieval can be performed during the follicular or luteal phase without ovulation induction. The time of oocyte retrieval does not affect follicular maturation rate. Therefore, OTC combined with IVM can preserve the girl’s reproductive potential to the maximum extent without delaying the subsequent treatment. However, there are still some problems with the use of IVM for fertility protection of girls. First of all, oocytes retrieved by IVM is very limited and IVM rate (IVMR) is not desirable. IVMR in postmenarche women is about 28%, only 10% to 15% in premenarche children and only 4.6% in girls under 5 years old [47–49]. Second, vitrification-thawing may impair the reproductive potential of IVM oocytes. Fertilization and embryo cleavage rates, clinical pregnancy and live birth rates were significantly lower in the vitrified-freezing oocytes compared to fresh IVM oocytes [50]. Based on aforementioned reasons, IVM alone is not effective enough in preserving girl’s fertility. IVM should combine with OTC to protect the fertility.

Embryo and oocyte cryopreservation

Embryo and oocyte cryopreservation are not suitable for FP in HSCT girls. In prepubertal girls, ovary cannot respond to gonadotropins and obtain mature gametes. For postpubertal girls, although ovary is responsive to gonadotropins, most of the children has experienced chemotherapy exposure, resulting in the loss of growing follicles, and the residual primordial follicles have poor response to gonadotropins. Oocyte harvesting during or shortly after chemotherapy increases the risk of chromosomal aberrations [6]. In addition, in unmarried girls with intact hymen, oocyte pick-up requires vaginal operation. Due to above factors, embryo and oocyte cryopreservation are not suitable for girls.

Gonadotropin-releasing hormone agonists

Gonadotropin-releasing hormone agonist (GnRHa) is controversial for FP. ESHRE guideline states that in malignancies other than breast cancer, GnRHa should not be routinely offered as an option for ovarian function protection and FP without discussion of the uncertainty about its benefit [29]. The theory that GnRHa can protect ovarian function is that GnRHa can inhibit follicle development, keep the follicle in a quiescent state, and improve the tolerance to gonadal toxicity. However, in prepubertal girls, the hypothalamic–pituitary–ovarian axis is not established, GnRHa cannot improve tolerance to gonadal toxicity. There is no report of GnRHa for FP in prepubertal girls. GnRHa should not be offered as routine FP method for girls.

In vitro activation of primordial follicles

In vitro activation (IVA) of primordial follicles refers to in vitro activation of dormant primordial follicles to gonadotropins-responsive phase, combined with ovulation induction to obtain mature follicles and finally achieve pregnancy through IVF-ET. IVA is mainly used for POI and low ovarian response who still have residual primordial follicles. Although the world’s first IVA baby was born in 2013 [51], pregnancy rate and live birth rate of IVA are low, less than 20 live births reported up to now, so it cannot be used as a routine fertility protection technique.

In vitro growth and maturation of primordial follicles

In vitro growth and maturation (IVGM) of primordial follicles refers to in vitro activation of primordial follicles and culture to MII. Live birth has been achieved in mice through IVGM. Human primordial follicles can also develop to preantral and antral follicles by IVGM, and obtain MII oocytes, but live birth has not yet been achieved [52].

Artificial ovary

The isolated follicles and ovarian stromal cells are planted in biomaterials to construct an artificial ovary which can not only restore fertility but also endocrine function. Live birth has been achieved in mice through artificial ovary and further research will be expected to be applied in the clinic [53].

Summary

HSCT conditioning seriously destroy the ovarian function and fertility of girls. Ovarian tissue cryopreservation and transplantation is the most important or only option to preserve fertility for girls. For girls with leukemia, cryopreserving ovarian cortex when primary disease reach complete remission can improve the safety of ovarian tissue transplantation. On the other hand, it is expected to obtain live birth in the future with the development of IVA, IVGM and artificial ovary with preserved germ cells by ovarian tissue cryopreservation.

Experts involved in the formulation of this consensus

Xiangyan Ruan (Corresponding author: Department of Gynecological Endocrinology, Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Jian Zhang (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Rong Liu (Capital Institute of Pediatrics, Beijing, China); Lanping Xu (Peking University Peoples Hospital, Peking University Institute of Hematology); Liangzhi Xu (Department of Obstetrics and Gynecology, West China Second University Hospital, Sichuan University); Yuan Sun (Beijing Jingdu Children’s Hospital); Yurui Wu (Department of Thoracic and Oncology Surgery, Capital Institute of Pediatrics, Beijing, China); Maoquan Qin (Hematology Center, Beijing Children’s Hospital, Capital Medical University, National Center for Children’s Health, Beijing, China); Jun Yang (Department of Hematopoietic stem cell transplantation, Hematology Center, Beijing Children’s Hospital, Capital Medical University, National Center for Children’s Health, Beijing, China); Huanmin Wang (Beijing Children’s Hospital, Capital Medical University, National Center for Children’s Health, Beijing, China); Xiaodong Shi (Capital Institute of Pediatrics, Beijing, China); Min Hao (Second Hospital of Shanxi Medical University); Chunxiu Gong (Department of Endocrine genetics & metabolism Beijing Children’s Hospital, Capital Medical University, National Center for Children’s Health, Beijing, China); Liying Sun (Department of Pediatric Adolescent Gynecology the Children’s Hospital, Zhejiang University School of Medicine, National Clinical Research Center for Child Health); Yinmei Dai (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Zhuoying Hu (The First Affiliated Hospital of Chongqing Medical University); Xiaodong Li (The First Hospital of Hebei Medical University); Jing Liu (Beijing Children’s Hospital, Capital Medical University, National Center for Children’s Health, Beijing, China); Honghua He (Affiliated Hospital of Guangdong Medical University); Lin Zou (Affiliated Hospital of Guangdong Medical University); Yue Wang(Henan Provincial People’s Hospital); Aiping Min(People’s Hospital of Leshan); Tianyuan Zhu (Gansu Maternal and Child Health Care Hospital/Gansu Central Hospital); Youguo Chen (The First Affiliated Hospital of Soochow University);Ruijuan Sun(Heibei Yanda Lu Daopei Hospital); Yifei Cheng(Peking University People’s Hospital, Peking University Institute of Hematology); Fuling Wang (The Affiliated Hospital of Qingdao University); Yanhong Huang (Xi’an International Medical Center Hospital); Xiyang Ye (Department of Gynecology, Shenzhen People’s Hospital, The Second Clinical Medical College, Jinan University; The First Affiliated Hospital, Southern University of Science and Technology, Shenzhen, Guangdong, China); Suiyu Luo(Henan Provincial People’s Hospital); Xiaofan Zhu (State Key Laboratory of Experimental Hematology, National Clinical Research Center for Blood Diseases, Institute of Hematology & Blood Disease Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College); Rui Ju (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Yu Yang (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Muqing Gu(Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Juan Du (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Fengyu Jin (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Jiaojiao Cheng (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Yanqiu Li (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital); Alfred O. Mueck (Beijing Obstetrics and Gynecology Hospital, Capital Medical University. Beijing Maternal and Child Health Care Hospital; University Women’s Hospital and Research Centre for Women’s Health, Department of Women’s Health, University of Tuebingen);

Disclosure statement

The authors report no conflicts of interest.

Additional information

Funding

This work was supported by Beijing Natural Science Foundation (No. 7202047); Capital’s Funds for Health Improvement and Research (No. 2020-2-2112); Beijing Municipal Administration of Hospitals’ Ascent Plan (No. DFL20181401); China Association for Promotion of Health Science and Technology Special Fund project for Scientific research (JKHY2020003); Beijing Municipal Health Commission, demonstration construction project of Clinical Research ward (No: BCRW202109)