Study population
This is a retrospective study of 913 women who underwent fresh ET first and/or subsequent frozen-thawed cycles following one IVF/ICSI cycle. The study was carried out in the Reproductive Centre of the 901th Hospital of the Joint Logistics Support Force of the People’s Liberation Army (PLA) between January 2014 and June 2019. Women ≥ 38 years at the time of treatment and transferred all the viable embryos available and/or achieved a live birth were included. Cycles with oocyte donation, embryo biopsy, or high risk of ovarian hyperstimulation syndrome (OHSS) were excluded. The study was approved by the Reproductive Medicine Ethics Committee of the 901th Hospital of the Joint Logistics Support Force of the PLA. All the patients provided written informed consent. Patients were divided into three groups based on ovarian response: poor ovarian response (POR) with 1-3 oocytes retrieved; normal ovarian response (NOR) with 4-15 oocytes retrieved; and high ovarian response (HOR) with more than 15 oocytes retrieved. Demographic information including patients’ age, infertility duration and type, body mass index (BMI), the basic serum concentrations of follicle-stimulating hormone (FSH), luteinizing hormone (LH), oestradiol (E2), prolactin, and the number of AFC were extracted from medical records.
Ovarian stimulation and embryo transfer
All the patients received a controlled ovarian stimulation beginning from day 2 or day 3 of menses with a starting dose of 150-300 IU recombinant FSH (r-FSH, Merck Serono, Italy) and/or urinary gonadotrophins (HMG, Lizhu, China) based on the patient’s age, hormone profile, AFC, and BMI. The dose of gonadotrophin was adjusted according to the ovarian response through monitoring a serial of ultrasonography and serum levels of FSH, LH, E2, and progesterone. 250 µg recombinant human chorionic gonadotropin was used to trigger ovulation (rHCG, Merck Serono, Germany) when one or two leading follicles reached 18 mm or more in diameter.
All the oocytes ≥ 12 in diameter were retrieved transvaginally 36 hours later. Conventional IVF or ICSI was performed 2-4 hours after oocyte retrieval according to the sperm quality. Embryo quality was graded according to Cummins et al. [8].
Fresh ET was performed under ultrasound guidance three or five days after oocyte retrieval. For luteal phase support, intramuscular injection progesterone at a dose of 40 mg per day was started the day of the oocyte retrieval and maintained until a negative β-hCG or 10 weeks of gestation. Supernumerary available embryos were cryopreserved on day 3, day 5 or day 6 post oocyte retrieval following vitrification standard protocols. For FET cycles, hormone replacement treatment was carried out for endometrial preparation. The protocol has been described in detail elsewhere [9]. The frozen embryos were thawed with commercial warming kit (Kitazato Biopharma Co., Shizuoka, Japan) and cultured for 2 hours before transferred into the uterine cavity.
Definition of outcomes
The primary outcome in this study was CLBR, calculated as the proportion of cycles that achieved at least one cumulative live birth per oocyte retrieval (the delivery of at least one live-born infant in the fresh ET or in the subsequent frozen-thawed cycles until all frozen embryos were used). Live birth was defined as birth of at least one newborn that exhibits any sign of life. Cumulative pregnancy rate (CPR) was calculated as the proportion of cycles that achieved at least one clinical pregnancy per oocyte retrieval. Clinical pregnancy was defined as the presence of a gestational sac on ultrasound 30-35 days after ET. Miscarriage was defined as the loss of clinical pregnancy less than 24 weeks’ gestation. The premature infant was defined as preterm birth less than 37 weeks’ gestation. Multiple pregnancy was defined as at least two detectable gestational sacs or heartbeats on ultrasound. Low birth weight was defined as birth weight less than 2,500g at birth, and the birth weight 4,000g or heavier was defined as macrosomia.
Statistical Analysis
Mean with standard deviation (SD) [or median with interquartile range (IQR)] was reported for continuous variables, and proportions with numbers for categorical variables. Comparisons among POR, NOR, and HOR groups were compared using the one-way ANOVA test for continuous variables and chi-square test for categorical variables.
Univariate and multivariable logistic regression analyses were used to derive the odds ratio (OR) to identify the relationship of CLBRs and CPRs with different ovarian response. In the adjusted model, the results of fresh ET were adjusted for age, BMI, endometrial thickness, the number of embryos transferred, and the number of good embryos transferred. Furthermore, the cumulative results were adjusted for age and BMI.
All analyses were performed with STATA 16.0 (StataCorp, College Station, Texas, USA), a two-tailed P value of less than 0.05 was considered statistically significant.