Patients and Clinical Data
This research was conducted in a single IVF center at Peking University First Hospital from January 2017 to December 2020. The inclusion criteria of patients were as follows: age < 38 years, body mass index (BMI) < 28, history of failure for at least two implantation cycles (either at least 4 high-quality cleavage-stage embryos transferred or at least 2 blastocysts transferred ), and normal uterine cavity (assessed by hysteroscopy or hysterosalpingography). The exclusion criteria included the following: hormonal disorders, hydrosalpinx, a lack of high-quality cleavage stage embryos or high-quality blastocysts for transfer, positive autoantibodies (including antinuclear antibodies, anticardiolipin antibody, lupus anticoagulant, anti-β2 glycoprotein-1 IgG, and anti-β2 glycoprotein-1 IgM), inherited coagulation disorders (including mutations of factor V Leiden, prothrombin gene, methylenetetrahydrofolate reductase gene, and antithrombin III, protein C, and protein S) and abnormal karyotyping. Couples were recruited for one FET cycle only.
The patient demographic characteristics collected included age, body mass index (BMI) (kg/m2), infertility type, infertility duration, the number of previous failed embryo transfer cycles, endometrial thickness (mm) on the day of embryo transfer, the number of embryos transferred, the number of high-quality embryos transferred and the type of embryos transferred (cleavage or blastocyst embryos).
The FET cycle protocol was based on patient characteristics. All patients underwent either a natural cycle or a substituted cycle.
(1) Natural cycle with luteal progestin (P) supplementation: This protocol was used on patients with regular menses (21–37 days). After spontaneous menses, a vaginal ultrasound examination was performed on the 10th–12th day of the cycle to detect the leading follicle. Based on the ultrasound examination, patients had their plasma levels of luteinizing hormone (LH), estradiol (E2) and P checked. ET was scheduled between 3 and 6 days after ovulation upon ultrasound examination. Luteal phase support was commenced on the day of ET using dydrogesterone (Abbott Healthcare Products B.V., Netherlands) at a dose of 10 mg twice daily for 14 days. If the pregnancy test was positive, luteal phase support was continued until the 10th gestational week.
(2) This protocol was used in patients with irregular menses (> 37 days). Progynova (Delpharm Lille S.A.S., France) was orally administered on the 1st-5th day of the cycle at a dose of 2–3 mg, twic a day, to support endometrial proliferation and suppress follicle growth. After 14 days, a vaginal ultrasound examination was performed to confirm whether a dominant follicle had emerged and to measure the endometrial thickness. When the endometrial thickness reached more than 8 mm, 10 mg dydrogesterone (Abbott Biologicals B.V., Holland), twice daily, along with 90 mg vaginal micronized progesterone (Fleet Laboratories Ltd, US), once daily, was used, and ET was planned. If the endometrium remained at less than 8 mm, the FET cycle was canceled. The supplementation was continued until a pregnancy test was performed. In the case of a positive test, the patients were advised to maintain luteal support until the 10th gestational week.
From the day of FET, all patients were experimentally administered low molecular weight heparin (LMWH) (Nadroparin Calcium Injection, 4100 anti-Xa IU, Glaxo Wellcome Production, Britain) subcutaneously (SC) at a standard dose of 0.4 mL per day. The patients self-administered the LMWH. If the pregnancy test 12 to 14 days after ET was positive, LMWH was continued until the 10th week of pregnancy. Otherwise, it was stopped.
The plasma D-dimer and other coagulation indictors of the patients on the day of embryo transferred were monitored. The levels of D-dimer and other coagulation indictors were detected using an ACL-Advance automatic coagulation analyzer (ACL-TOP-700, Werfen Inc, Spain). Total platelet count in blood samples was measured using a Sysmex XN-9100 Hematology Analyzer (Sysmex, Kobe, Japan)
Study Period And Groups
The study was divided into two parts. First, enrolled patients were collected from January 2017 to January 2020 and divided into two groups according to clinical pregnancy outcomes. Patients with at least one live birth (LB) were classified as LB (+), while the others were classified as LB (-). We compared clinical data, especially plasma D-dimer, between these groups. Additionally, the simultaneous relationship of multiple prognostic factors for pregnancy outcome was assessed using binary logistic regression analysis. Moreover, we adopted receiver operating characteristic (ROC) curve analysis to determine the cutoff value of plasma D-dimer for the live birth rate (LBR).
In the second part, we prospectively included patients from January 2020 to December 2020. We used the cutoff value of plasma D-dimer from the retrospective study to classify the included patients into two groups, i.e., a low D-dimer group and a high D-dimer group, and compared their clinical outcomes.
Definition Of Outcomes
The LBR and clinical pregnancy rate (CPR) were the major outcomes of this study. A clinical pregnancy is defined as a pregnancy diagnosed by ultrasonographic visualization of one or more gestational sacs. The CPR is defined as the percentage of clinical pregnancies occurring in every 100 ET cycles. The LBR is defined as the percentage of deliveries that resulted in at least one live birth, expressed per 100 ET cycles. The following clinical outcome was also included: implantation rate, which was defined as the number of gestational sacs detected divided by the number of embryos transferred.
All analyses were performed using SPSS version 11.0 (SPSS Inc., Chicago, IL). The significance of differences in proportions was tested using chi-square and Fisher’s exact statistics. Differences in continuous variables were analyzed using Student’s t-test and the Mann-Whitney U test. Odds ratios (ORs) and 95% confidence intervals (CIs) were calculated.
Binary logistic regression analysis was used to assess whether the pregnancy outcomes were affected by maternal age, BMI (kg/m2), infertility type (primary infertility or secondary infertility), endometrial thickness, the number of embryos transferred, and the type of embryos transferred (cleavage stage embryo or blastocyst). An ROC curve was drawn to obtain the plasma D-dimer threshold, and the area under the curve (AUC) was calculated. Values of P < 0.05 were considered statistically significant unless otherwise noted.