Effect of low-molecular-weight heparin in women undergoing frozen-thawed embryo transfer cycles: a retrospective cohort study.

In total, 1881 of patients were enrolled in this study. Among them, 107 underwent PGD cycles, which were analyzed individually. All the data were from Clinical Reproductive Medicine Management System/Electronic Medical Record Cohort Database (CCRM/EMRCD). The inclusion criteria were as follows: (1) underwent FET cycles; (2) had at least one failure of embryo transfer (including fresh embryo transfer or FET cycles). The exclusion criteria were as follows: (1) endometriosis and/or adenomyosis; (2) uterine malformation, including congenital uterine dysplasia, uterine fibroids, endometrial polyps, and intrauterine adhesions; (3) tubal factors, including hydrosalpinx; (4) LMWH contraindications, such as active bleeding; and (5) other autoimmune diseases, such as thyroid disorders. The patients were split into two groups depending on the use or nonuse of LMWH. The LMWH group received injections of 4100 IU/d LMWH from the day of transfer until 14 ± 2 days posttransplant. The control group was the comparison group (without LMWH use). Human β-chorionic gonadotropin (HCG) levels were measured at 14 ± 2 days posttransplant in all groups. Laboratory data included routine blood, liver function, and blood coagulation test data in the LMWH group at 14 days posttransplant. If serum HCG was positive, the injection of LMWH was continued until 35 ± 2 days posttransplant. Ultrasonography to determine clinical pregnancy 35 days after transplantation. Laboratory data, including routine blood, liver function, and blood coagulation test data, were also obtained in the LMWH group at 35 ± 2 days posttransplant to evaluate the safety of LMWH. The detailed endometrial preparation protocol for freeze-thaw cycles has been described in previous article, including the classification of endometrial types and thickness measurement methods [ 19 ]. For estrogen-progesterone (EP) cycles, oral estradiol ([Progynova]; Bayer, Germany) administration began on day 2–3 of the target cycle and lasted about two weeks. When the thickness of the endometrium reaches 8 mm and above, the patient is asked to add oil-based progesterone (60 mg), at the same day, the thickness of endometrial was recorded using transvaginal ultrasound examination. To avoid cavity fluid and other unfavorable conditions, patients were hospitalized and re-measurement of endometrial thickness on the morning of the transplantation day. Luteal supplement was altered to vaginal progesterone gel (90 mg, Crinone 8%; Merck Serono) and oral dydrogesterone (20 mg Duphaston; Abbott) after embryo implantation. IBM SPSS, 21.0 (IBM Corp., Armonk, N.Y., USA) was employed. Numerical data were shown as the mean ± standard deviation (SD), while categorical variables were shown as % (n/N). The Man-Whitney test and chi-square test were utilized for continuous and categorical variables, respectively. Two-tailed P < 0.05 was considered as statistical significance.

Of the 1881 patients who began FET treatment between 2020 and 2021, 107 women underwent PGD cycles. First, we analyzed 1774 women with normal FET cycles. There were 882 (49.7%) and 892 (50.3%) patients in LMWH and control groups, respectively (Table  1 ). The results were comparable between two groups in age (32.28 ± 0.17 vs. 32.37 ± 0.16), years of infertility (4.54 ± 0.12 vs. 4.65 ± 0.12), body mass index (BMI; 23.74 ± 0.19 vs. 23.56 ± 0.12), basal serum FSH (6.65 ± 0.10 vs. 6.60 ± 0.08), basal serum LH (7.46 ± 0.33 vs. 8.06 ± 0.38), basal serum E2 (242.40 ± 27.51 vs. 263.92 ± 33.65), AMH (4.01 ± 0.12 vs. 4.13 ± 0.12), and AFC (14.65 ± 0.23 vs. 15.21 ± 0.23) between the two groups. Also, no great differences were identified in the number of embryos implanted (1.31 ± 0.02 vs. 1.28 ± 0.02), embryo implantation rate (44.9% [519/1157] vs. 45.5% [522/1146]), biochemical pregnancy rate (52.3% [461/882] vs. 51.6% [460/892]), clinical pregnancy rate (48.4% [427/882] vs. 49.2% [439/892]), live birth rate (37.9% [334/882] vs. 39.9% [356/892]), late abortion rate (2.6% [11/427] vs. 5.0% [22/439]), and ectopic pregnancy rate (1.4% [6/427] vs. 1.4% [6/439]) between the two groups. Compared to the control group, the LMWH group had a higher early abortion rate (17.8% [76/427] vs. 12.5% [55/439], p = 0.030). Table 1 Baseline characteristics and pregnant outcome of patients undergoing FET cycles LMWH CONTROL P Value Cycle number 882 892 Female age 32.28 ± 0.17 32.37 ± 0.16 0.644 Type of infertility 0.128 Primary infertility 315 288 Secondary infertility 567 604 Years of infertility 4.54 ± 0.12 4.65 ± 0.12 0.753 BMI 23.74 ± 0.19 23.56 ± 0.12 0.615 Baseline hormone levels FSH (mIU/mL) 6.65 ± 0.10 6.60 ± 0.08 0.573 E2 (pg/mL) 242.40 ± 27.51 263.92 ± 33.65 0.813 LH (mIU/mL) 7.46 ± 0.33 8.06 ± 0.38 0.081 AMH (ng/mL) 4.01 ± 0.12 4.13 ± 0.12 0.336 AFC 14.65 ± 0.23 15.21 ± 0.23 0.088 No. of embryo implanted 1.31 ± 0.02 1.28 ± 0.02 0.213 Embryo stage 0.285 D3 339 365 D5 543 527 Embryo implantation rate 44.9%(519/1157) 45.5%(522/1146) 0.739 Biochemical pregnancy rate 52.3%(461/882) 51.6%(460/892) 0.769 Clinical pregnancy rate 48.4%(427/882) 49.2%(439/892) 0.735 Live birth rate 37.9%(334/882) 39.9%(356/892) 0.378 Early abortion rate 17.8%(76/427) 12.5%(55/439) 0.030 Late abortion rate 2.6%(11/427) 5.0%(22/439) 0.061 Ectopic pregnancy rate 1.4%(6/427) 1.4%(6/439) 0.961 Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count Baseline characteristics and pregnant outcome of patients undergoing FET cycles Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count To assess the effect of LMWH in different numbers of transfer cycles, we grouped the 1774 women into four groups (Table  2 ). There were 233 (41.7%) and 326 (58.3%) patients who underwent one transfer in the LMWH and control groups, respectively. No statistical differences were found in age, years of infertility, BMI, basal serum FSH, basal serum E2, AMH, and AFC between the two groups. Also, the data were comparable between the two group in the number of embryos implanted, embryo implantation rate, biochemical pregnancy rate, clinical pregnancy rate, live birth rate, late abortion rate, or ectopic pregnancy rates. The LMWH group had lower basal serum LH levels (6.29 ± 0.59 vs. 7.77 ± 0.64, p = 0.017) and a higher early abortion rate (17.5% [18/103] vs. 8.2% [12/147], p = 0.026) than the control group. There were 328 (51.6%) and 308 (48.4%) patients who underwent two transfers in the LMWH and control groups, respectively. Further, 181 (52.9%) in the LMWH group and 161 (47.1%) patients in the control group underwent three transfers. All baseline characteristics and pregnancy outcomes between the two groups were comparable. There were 140 (59.1%) and 97 (40.9%) patients who underwent more than four transfers in the LMWH and control groups, respectively. Patients using LMWH had fewer years of infertility (6.36 ± 0.32 vs. 7.22 ± 0.37, p = 0.041) and lower embryo implantation (40.6% [73/180] vs. 55.1% [65/118], p = 0.014) and late abortion rates (1.7% [1/60] vs. 13.2% [7/53], p = 0.043). Table 2 Baseline characteristics and pregnant outcome of patients in different number of transfer cycles No. of cycle 1 2 3 ≥ 4 LMWH CONTROL P Value LMWH CONTROL P Value LMWH CONTROL P Value LMWH CONTROL P Value Cycle number 233 326 328 308 181 161 140 97 Female age 32.07 ± 0.35 32.43 ± 0.28 0.392 31.43 ± 0.28 31.98 ± 0.27 0.097 32.89 ± 0.35 32.42 ± 0.39 0.427 33.79 ± 0.39 33.32 ± 0.49 0.345 Type of infertility 0.813 0.014 0.032 0.107 Primary infertility 107 153 134 97 49 28 25 10 Secondary infertility 126 173 194 211 132 133 115 87 Years of infertility 3.60 ± 0.19 3.93 ± 0.19 0.683 4.12 ± 0.18 4.06 ± 0.17 0.880 5.13 ± 0.27 5.73 ± 0.28 0.056 6.36 ± 0.32 7.22 ± 0.37 0.041 BMI 23.49 ± 0.22 23.15 ± 0.22 0.251 23.68 ± 0.44 23.72 ± 0.20 0.167 24.08 ± 0.25 23.79 ± 0.27 0.359 23.88 ± 0.27 24.03 ± 0.32 0.686 Baseline hormone levels FSH (mIU/mL) 6.78 ± 0.20 6.71 ± 0.13 0.591 6.56 ± 0.14 6.56 ± 0.12 0.613 6.56 ± 0.22 6.47 ± 0.20 0.708 6.75 ± 0.27 6.55 ± 0.27 0.854 E2 (pg/mL) 296.30 ± 47.87 286.06 ± 46.85 0.903 258.97 ± 51.71 262.11 ± 69.42 0.498 130.76 ± 26.51 264.07 ± 77.31 0.928 258.17 ± 86.04 195.07 ± 78.42 0.390 LH (mIU/mL) 6.29 ± 0.59 7.77 ± 0.64 0.017 8.65 ± 0.66 8.16 ± 0.58 0.499 6.23 ± 0.46 8.96 ± 1.09 0.072 8.21 ± 0.84 7.25 ± 0.98 0.181 AMH (ng/mL) 3.37 ± 0.20 3.60 ± 0.16 0.152 4.34 ± 0.20 4.56 ± 0.23 0.587 4.05 ± 0.26 4.41 ± 0.27 0.149 4.26 ± 0.31 4.06 ± 0.37 0.362 AFC 13.37 ± 0.43 14.17 ± 0.38 0.207 15.16 ± 0.39 15.81 ± 0.39 0.191 14.90 ± 0.51 16.11 ± 0.53 0.101 15.23 ± 0.60 15.30 ± 0.66 0.922 No. of embryo implanted 1.29 ± 0.03 1.29 ± 0.03 0.984 1.32 ± 0.03 1.30 ± 0.03 0.621 1.35 ± 0.04 1.29 ± 0.04 0.217 1.29 ± 0.04 1.22 ± 0.04 0.232 Embryo implantation rate 40.7%(122/300) 41.2%(173/420) 0.888 48.5%(210/433) 47.1%(189/401) 0.693 46.7%(114/244) 45.9%(95/207) 0.861 40.6%(73/180) 55.1%(65/118) 0.014 Biochemical pregnancy rate 46.8%(109/233) 46.9%(153/326) 0.972 56.7%(186/328) 53.2%(164/308) 0.381 55.2%(100/181) 53.4%(86/161) 0.734 47.1%(66/140) 58.8%(57/97) 0.078 Clinical pregnancy rate 44.2%(103/233) 45.1%(147/326) 0.835 52.7%(173/328) 51.3%(158/308) 0.715 50.3%(91/181) 50.3%(81/161) 0.995 42.9%(60/140) 54.6%(53/97) 0.074 Live birth rate 35.2%(82/233) 39.0%(127/326) 0.364 41.2%(135/328) 41.6%(128/308) 0.918 38.3%(70/183) 40.2%(66/164) 0.704 33.6%(47/140) 38.1%(37/97) 0.469 Early abortion rate 17.5%(18/103) 8.2%(12/147) 0.026 16.2%(28/173) 14.6%(23/158) 0.682 19.8%(18/91) 13.6%(11/81) 0.278 20.0%(12/60) 17.0%(9/53) 0.681 Late abortion rate 1.0%(1/103) 4.8%(7/147) 0.190 4.0%(7/173) 1.9%(3/158) 0.413 2.2%(2/91) 6.2%(5/81) 0.352 1.7%(1/60) 13.2%(7/53) 0.043 Ectopic pregnancy rate 1.9%(2/103) 0.7%(1/147) 0.755 1.7%(3/173) 2.5%(4/158) 0.903 1.1%(1/91) 1.2%(1/81) 1.000 \ \ Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count Baseline characteristics and pregnant outcome of patients in different number of transfer cycles Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count To assess the effect of LMWH at different ages, we grouped the 1774 women into four groups: ages < 30, 30–35, 35–40, and ≥ 40 years (Table  3 ). In the LMWH and control groups respectively, there were 260 (49.9%) and 261 (50.1%) patients aged < 30 years, 379 (51.3%) and 360 (48.7%) patients aged 30–35 years, 157 (45%) and 192 (55%) patients aged 35–40 years, and 86 (52.1%) and 79 (47.9%) patients aged ≥ 40 years. All baseline characteristics and pregnancy outcomes were comparable among the four age groups. To further specify the effect of LMWH at different ages and numbers of transfer cycles, we grouped the patients according to the number of transfer cycles in the four age groups (Supplementary 1 ). The LMWH group had a lower biochemical pregnancy rate (50% [13/26] vs. 78.3% [18/23], p = 0.041) and clinical pregnancy rate (38.5% [10/26] vs. 69.6% [16/23], p = 0.029) than the control group among patients aged < 30 years who underwent more than four transfers (Supplementary 1 ). The LMWH group had a lower late abortion rate (0.0% [0/32] vs. 19.0% [4/21], p = 0.042) among 30–35-year-old patients who underwent more than four transfers (Supplementary 1 ). Table 3 Baseline characteristics and pregnant outcome of patients in different age Age < 30 years old 30–35 years old 35–40 years old ≥ 40 years old LMWH CONTROL P Value LMWH CONTROL P Value LMWH CONTROL P Value LMWH CONTROL P Value Cycle number 260 261 379 360 157 192 86 79 Female age 26.68 ± 0.14 26.85 ± 0.13 0.432 31.97 ± 0.07 31.87 ± 0.07 0.311 36.90 ± 0.11 36.97 ± 0.10 0.644 42.09 ± 0.21 41.71 ± 0.19 0.250 Type of infertility 0.039 0.308 0.499 0.038 Primary infertility 151 128 141 121 19 28 4 11 Secondary infertility 109 133 238 239 138 164 82 68 Years of infertility 3.28 ± 0.13 3.40 ± 0.14 0.770 4.51 ± 0.14 4.51 ± 0.17 0.400 5.57 ± 0.34 6.28 ± 0.31 0.097 6.62 ± 0.60 5.49 ± 0.49 0.366 BMI 23.27 ± 0.21 23.58 ± 0.21 0.371 23.50 ± 0.17 23.32 ± 0.18 0.441 23.87 ± 0.24 23.60 ± 0.21 0.416 26.03 ± 1.55 24.49 ± 0.54 0.231 Baseline hormone levels FSH (mIU/mL) 6.11 ± 0.11 6.03 ± 0.11 0.407 6.27 ± 0.12 6.50 ± 0.12 0.031 7.00 ± 0.23 6.92 ± 0.19 0.468 9.29 ± 0.58 8.10 ± 0.31 0.355 E2 (pg/mL) 310.14 ± 63.17 327.14 ± 72.27 0.866 200.06 ± 37.82 256.15 ± 57.09 0.872 234.60 ± 57.45 233.46 ± 53.64 0.557 238.39 ± 65.75 164.54 ± 52.20 0.541 LH (mIU/mL) 8.10 ± 0.68 7.91 ± 0.67 0.549 7.20 ± 0.50 8.75 ± 0.67 0.008 6.69 ± 0.63 6.75 ± 0.60 0.808 8.11 ± 1.20 8.90 ± 1.46 0.939 AMH (ng/mL) 4.85 ± 0.23 5.24 ± 0.24 0.136 4.39 ± 0.19 4.56 ± 0.20 0.315 2.93 ± 0.20 2.72 ± 0.14 0.416 1.79 ± 0.18 1.91 ± 0.18 0.525 AFC 17.13 ± 0.40 18.07 ± 0.37 0.054 15.55 ± 0.33 16.24 ± 0.33 0.158 12.11 ± 0.54 11.76 ± 0.46 0.674 7.80 ± 0.59 9.46 ± 0.65 0.042 No. of embryo implanted 1.35 ± 0.03 1.32 ± 0.03 0.482 1.32 ± 0.02 1.29 ± 0.02 0.367 1.24 ± 0.03 1.23 ± 0.03 0.778 1.29 ± 0.05 1.29 ± 0.05 0.995 Embryo implantation rate 50.3%(177/352) 49.9%(172/345) 0.910 46.9%(234/499) 48.2%(223/463) 0.693 45.6%(89/195) 42.4%(100/236) 0.496 17.1%(19/111) 26.5%(27/102) 0.097 Biochemical pregnancy rate 58.8%(153/260) 42.9%(112/261) 0.684 51.2%(194/379) 51.9%(187/360) 0.837 51.6%(81/157) 48.4%(93/192) 0.558 22.1%(19/86) 30.4%(24/79) 0.266 Clinical pregnancy rate 54.6%(142/260) 55.6%(145/261) 0.829 51.2%(194/379) 51.9%(187/360) 0.837 47.8%(75/157) 44.8%(86/192) 0.579 18.6%(16/86) 26.6%(21/79) 0.220 Live birth rate 42.7%(111/260) 47.1%(123/261) 0.309 40.6%(154/379) 41.4%(149/360) 0.835 36.3%(57/157) 35.9%(69/192) 0.943 14.0%(12/86) 19.0%(15/79) 0.383 Early abortion rate 17.6%(25/142) 11.0%(16/145) 0.112 16.0%(31/194) 12.8%(24/187) 0.383 21.3%(16/75) 10.5%(9/86) 0.058 25.0%(4/16) 28.6%(6/21) 1.000 Late abortion rate 3.5%(5/142) 2.8%(4/145) 0.975 2.6%(5/194) 5.9%(11/187) 0.108 1.3%(1/75) 8.1%(7/86) 0.105 \ \ Ectopic pregnancy rate 0.7%(1/142) 1.4%(2/145) 1.000 2.1%(4/194) 1.6%(3/187) 1.000 1.3%(1/75) 1.2%(1/86) 1.000 \ \ Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count Baseline characteristics and pregnant outcome of patients in different age Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count To assess the effect of LMWH in different numbers of transferred embryos, we grouped the 1774 women into four groups (first group, one transferred embryo; second group, one transferred blastocyst; third group, two transferred embryos; fourth group, two transferred blastocysts) (Table  4 ). In the LMWH and control groups respectively, 119 (44.9%) and 146 (55.1%) patients had one transferred embryo, 488 (49.8%) and 492 (50.2%) patients had one transferred blastocyst, 220 (50.1%) and 219 (49.9%) patients had two transferred embryos, and 55 (61.1%) and 35 (38.9%) patients had two transferred blastocysts. To further specify the effect of LMWH at different ages and numbers of transferred embryos, we grouped the patients according to the number of transferred embryos at different ages (Supplementary 2 ). The LMWH group had a higher early abortion rate [13.0% [6/46] vs. 0.0% [0/45], p = 0.037) among patients aged < 30 years who had two transferred embryos (Supplementary 2 ). Table 4 Baseline characteristics and pregnant outcome of patients transferred different number of embryos No.of embryo transferred one embryo one blastocyst two embryo two blastocyst LMWH CONTROL P Value LMWH CONTROL P Value LMWH CONTROL P Value LMWH CONTROL P Value Cycle number 119 146 488 492 220 219 55 35 Female age 35.24 ± 0.52 34.23 ± 0.40 0.070 31.80 ± 0.21 32.08 ± 0.21 0.513 31.87 ± 0.35 31.97 ± 0.35 0.777 31.71 ± 0.49 31.20 ± 0.77 0.108 Type of infertility 0.841 0.299 0.662 0.611 Primary infertility 19 22 171 157 100 95 25 14 Secondary infertility 100 124 317 335 120 124 30 21 Years of infertility 5.67 ± 0.40 5.75 ± 0.33 0.617 4.35 ± 0.14 4.61 ± 0.16 0.692 4.42 ± 0.23 4.11 ± 0.20 0.431 4.35 ± 0.44 4.14 ± 0.48 0.990 BMI 23.99 ± 0.27 23.22 ± 0.23 0.073 23.58 ± 0.15 23.84 ± 0.17 0.450 23.39 ± 0.24 23.14 ± 0.22 0.554 26.01 ± 2.44 23.61 ± 0.59 0.741 Baseline hormone levels FSH (mIU/mL) 7.93 ± 0.44 7.22 ± 0.25 0.195 6.35 ± 0.10 6.36 ± 0.09 0.770 6.78 ± 0.19 6.74 ± 0.15 0.567 5.95 ± 0.23 6.46 ± 0.32 0.199 E2 (pg/mL) 217.41 ± 57.64 125.20 ± 36.38 0.477 282.16 ± 44.40 280.19 ± 51.22 0.922 197.46 ± 36.81 317.23 ± 61.79 0.819 123.42 ± 42.89 280.42 ± 209.65 0.898 LH (mIU/mL) 8.41 ± 1.03 6.87 ± 0.78 0.649 7.56 ± 0.48 8.14 ± 0.54 0.079 6.72 ± 0.54 8.19 ± 0.72 0.153 7.49 ± 0.84 11.15 ± 2.22 0.309 AMH (ng/mL) 2.53 ± 0.24 2.62 ± 0.19 0.257 4.15 ± 0.15 4.32 ± 0.16 0.513 4.22 ± 0.25 4.38 ± 0.25 0.264 5.14 ± 0.61 6.16 ± 0.94 0.599 AFC 10.91 ± 0.63 11.50 ± 0.55 0.378 15.27 ± 0.30 15.93 ± 0.29 0.124 14.92 ± 0.47 15.56 ± 0.46 0.296 16.07 ± 0.87 18.31 ± 1.29 0.169 Embryo implantation rate 30.3%(36/119) 29.5%(43/146) 0.887 54.1%(264/488) 53.9%(265/492) 0.941 39.3%(173/440) 39.0%(171/438) 0.933 41.8%(46/110) 54.3%(38/70) 0.102 Biochemical pregnancy rate 30.3%(36/119) 32.2%(47/146) 0.735 53.5%(261/488) 52.4%(258/492) 0.743 57.8%(127/220) 58.0%(127/219) 0.955 67.3%(37/55) 80%(28/35) 0.189 Clinical pregnancy rate 29.4%(35/119) 28.1%(41/146) 0.812 48.8%(238/488) 50.6%(249/492) 0.565 54.5%(120/220) 55.7%(122/219) 0.807 61.8%(34/55) 77.1%(27/35) 0.129 Live birth rate 23.5%(28/119) 20.5%(30/146) 0.559 37.1%(181/488) 40.2%(198/492) 0.311 45.5%(100/220) 48.9%(107/219) 0.475 45.5%(25/55) 60%(21/35) 0.178 Early abortion rate 20.0%(7/35) 12.2%(5/41) 0.352 20.2%(48/238) 14.5%(36/249) 0.095 11.7%(14/120) 6.6%(8/122) 0.167 20.6%(7/34) 22.2%(6/27) 0.877 Late abortion rate 0.0%(0/35) 14.6%(6/41) 0.053 2.5%(6/238) 4.4%(11/249) 0.254 3.3%(4/120) 4.1%(5/122) 1.000 2.9%(1/34) 0.0%(0/27) 1.000 Ectopic pregnancy rate \ \ 1.3%(3/238) 1.6%(4/249) 1.000 1.7%(2/120) 1.6%(2/122) 1.000 2.9%(1/34) 0.0%(0/27) 1.000 Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count Baseline characteristics and pregnant outcome of patients transferred different number of embryos Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count Of the 1881 patients who began FET treatment between 2020 and 2021, 107 women who underwent PGD cycles were analyzed separately. There were 50 (46.7%) and 57 (53.3%) patients in the LMWH and control groups, respectively (Table  5 ). The results were comparable between two groups in age (30.00 ± 0.51 vs. 30.46 ± 0.45), years of infertility (2.42 ± 0.27 vs. 2.53 ± 0.25), BMI (23.30 ± 0.32 vs. 22.89 ± 0.33), basal serum FSH (5.98 ± 0.24 vs. 5.96 ± 0.24), basal serum LH (5.89 ± 0.74 vs. 6.98 ± 1.47), basal serum E2 (66.55 ± 10.01 vs. 71.30 ± 12.31), AMH (4.38 ± 0.45 vs. 3.81 ± 0.30), and AFC (16.62 ± 0.80 vs. 16.19 ± 0.67). No statistical differences were found in embryo implantation rate (60% [30/50] vs. 56.1% [32/57]), biochemical pregnancy rate (66% [33/50] vs. 64.9% [37/57]), clinical pregnancy rate (60% [30/50] vs. 56.1% [32/57]), live birth rate (42% [21/50] vs. 38.8% [26/57]), early abortion rate (23.3% [7/30] vs. 18.8% [6/32]), late abortion rate (3.3% [1/30] vs. 0.0% [0/32]), and ectopic pregnancy rate (3.3% [1/30] vs. 0.0% [0/32]) between the two groups. Table 5 Baseline characteristics and pregnant outcome of patients undergoing PGD LMWH CONTROL P Value Number 50 57 Female age 30.00 ± 0.51 30.46 ± 0.45 0.467 Type of infertility 0.288 Primary infertility 12 19 Secondary infertility 38 38 Years of infertility 2.42 ± 0.27 2.53 ± 0.25 0.776 BMI 23.30 ± 0.32 22.89 ± 0.33 0.372 Baseline hormone levels FSH (mIU/mL) 5.98 ± 0.24 5.96 ± 0.24 0.955 E2 (pg/mL) 66.55 ± 10.01 71.30 ± 12.31 0.609 LH (mIU/mL) 5.89 ± 0.74 6.98 ± 1.47 0.750 AMH (ng/mL) 4.38 ± 0.45 3.81 ± 0.30 0.461 AFC 16.62 ± 0.80 16.19 ± 0.67 0.740 embryo stage 0.804 D5 31 34 D6 19 23 Embryo implantation rate 60%(30/50) 56.1%(32/57) 0.687 Biochemical pregnancy rate 66%(33/50) 64.9%(37/57) 0.906 Clinical pregnancy rate 60%(30/50) 56.1%(32/57) 0.687 Live birth rate 42%(21/50) 38.8%(26/57) 0.707 Early abortion rate 23.3%(7/30) 18.8%(6/32) 0.658 Late abortion rate 3.3%(1/30) 0.0%(0/32) 0.484 Ectopic pregnancy rate 3.3%(1/30) 0.0%(0/32) 0.484 Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count Baseline characteristics and pregnant outcome of patients undergoing PGD Note: Numbers are mean ± standard deviation; BMI = body mass index; FSH = follicle-stimulating hormone; E2 = estradiol; LH = luteinizing hormone; AMH = Anti-mullerian hormone; AFC = Antra follicular count

For those infertility couple, in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) is one of the most effective and successful assisted reproductive technologies (ARTs). Infertility affects approximately 15% of couples, and IVF/ICSI contributes to 1–5% of all newborns in developed countries. Embryo implantation, a low-efficiency process in the menstrual cycle and assisted reproductive technologies, is a key step in establishing pregnancy [ 1 , 2 ]. Therefore, it is imperative to identify effective treatments. Meanwhile, recurrent pregnancy loss (RPL) and recurrent implantation failure (RIF) during IVF treatment are still tough issues without effective treatments [ 3 , 4 ]; thus, they are hot research topics. RPL is characterized by the occurrence of two or more pregnancy failures before 20–24 weeks of gestation, which affect approximately 2.5% of couples of childbearing age [ 5 – 7 ]. RPL can be caused by chromosomal abnormality, infection, structural and functional abnormalities of the reproductive system, and autoimmune disorders. Although various therapies have been evolved to prevent pregnancy loss in these patients, effective treatment are still elusive and urgently needed. Current studies have demonstrated that low molecular weight heparin (LMWH) has the effect in improving reproductive outcomes in unexplained RPL; however, the results are conflicting [ 8 – 10 ]. There is no standardized definition of RIF. Nonetheless, RIF is defined as three or more consecutive transfers of at least four high-quality embryos in fresh or frozen cycles without clinical pregnancy in most studies [ 11 , 12 ]. RIF can be caused by chromosomal abnormalities, uterine anatomical abnormalities, and maternal immune dysfunction [ 13 ]. Previous studies have estimated the function of LMWH in RIF, but the conclusions are controversial [ 14 – 16 ]. Heparin was discovered in 1916, late in the 1930s, unfractionated heparin (UFH), the first therapeutic form was introduced. Currently, a variety of different types of heparin are clinically applied, include UFH, LMWH, and synthetic heparins [ 17 ]. LMWH has a longer half-life, more stable dose-response relationship, better safety profile, reduced monitoring requirement, shorter oligosaccharide/monosaccharide chain, and higher anti-Xa/anti-IIa ratios, which make it more attractive than other heparin forms [ 18 ]. Since the anticoagulative and anti-inflammatory function of LMWH, it is now extensively used for the treatment of RPL and RIF, either alone or in combination with other agents. However, it tends to be broadly used in frozen-thawed embryo transfer (FET) cycles in IVF/ICSI treatment. Our study aimed to investigate the effect of LMWH on pregnancy outcomes in women with different numbers of transfer cycles, ages, numbers of transferred embryos, and preimplantation genetic diagnosis (PGD) cycles.

In the general population, women using LMWH had higher early abortion rate compared to the control group, subgroup analysis showed it only presented in patients who had embryo implantation failure once and aged under 30. However, LMWH did not improve the pregnant outcomes in the general population and PGD patients, therefore, the routine use of LMWH is not recommended for early treatment.

Embryo implantation is a complicated physiological process that includes proliferation and differentiation, adhesion and migration, and extracellular matrix remodeling. It can be influenced by many factors, such as abnormal uterine cavity anatomy, reduced endometrial receptivity, immune disorders, pre-thrombotic state, advanced age, excessive BMI, abnormal thyroid function, and psychological factors. For decades, researchers have investigated effective treatments to improve pregnancy outcomes in IVF cycles. Previous research has shown that impaired placental function may cause arterial thrombosis, which can lead to subsequent abortion. In addition, venous thromboembolism is more prevalent during gestation compared to arterial thrombosis [ 20 ]. To ensure the nutritional supply to the fetus, maternal blood flow is exchanged with the fetus through the placental intervillous space from about 10 weeks of gestation onwards [ 21 ]. In the last century, a relationship between RPL and antiphospholipid antibodies (APAs) was identified. APAs increase the generation of thrombin, leading to thrombotic damage in the placental [ 22 ]. LMWH is commonly used clinically for the treatment of acute VTE; thus, it was used to prevent miscarriage in women with APS by its antithrombotic function [ 23 ]. LMWHs may be useful in controlling endometrial differentiation and receptivity by regulating IGFBP-1, PRL, and IGF-I in assisted reproduction [ 24 ]. By increasing placental production of matrix metalloproteinases (MMPs) and tissue inhibitors metalloproteinases (TIMPs), LMWH might also regulate trophoblast invasiveness [ 25 ]. Therefore, many clinicians have attempted to use it in FET cycles to improve reproductive outcomes in ART treatment, and not just in RPL or RIF. Currently, there is little research regarding the role of LMWH in the entire population undergoing FET cycles [ 26 ]. In our study, LMWH had no obvious advantage in decreasing the risk of abortion or increasing the rate of conception in women with or without PGD. It’s reported that advanced age greatly increase the chance of adverse pregnancy outcomes, which could impair the safety of both mother and baby[ 27 ]. Dmitry et al. showed that patients aged  40 years had a higher chance of a good perinatal outcome by transferring two 3-day embryos [ 28 ]. To exclude the effects of age and the number of transferred cycles and embryos, we performed further subgroup stratification analysis. Among the patients who underwent more than four transfers, the use of LMWH reduced the late abortion rate. While patients aged 30–35 years who underwent more than four transfers had a lower late abortion rate in the LMWH group. In this study, LMWH reduced late abortion when the patients underwent more than four transfers, which is consistent with the findings of studies on RPL and RIF [ 29 ]. However, previous studies have generally been insufficiently subgrouped, have observed a simple outcome indicator, and few have explored the role of LMWH on late abortion rate. Studies have shown that the main causes of late abortion were APAs, cervical incompetence, infections, and placental insufficiency [ 30 ]. All the patients in our study were APAs negative, and LMWH did not show the tendency of reducing late abortion rate in the whole population, therefore, it’s unreasonable to draw the conclusion that LMWH make contribution for the protection of late abortion. A meta-analysis also reported that LMWH could not significantly reduced the chance of abortion in non-thrombophilic patients in fresh cycles [ 31 ]. To further investigate the relationship between LMWH and late abortion, large sample and multi-center studies are needed. Genetic factors are the main causes of early miscarriages [ 32 ]. In our study, we excluded this factor from the PGD. However, LMWH has no obvious advantage in decreasing the risk of abortion or increasing the pregnancy rate. A limitation in the PGD cycles was the insufficient samples to process the subgroup analysis. In contrast to other heparin components, LMWH has a favorable safety profile as an anticoagulant. Many studies reported the effectiveness of LMWH as a therapeutic method for unexplained RPL (URPL). However, due to the mechanism of LMWH, side effects such as allergic reactions and thrombocytopenia are inevitable in pregnant women [ 33 ]. Therefore, To avoid some possible side effects such as bleeding, rash, liver and kidney impairment, patients on LMWH should be strictly monitored [ 34 ]. Moreover, a study reported some maternal and fetal complications after using LMWH for the treatment of URPL [ 9 ]. In our study, LMWH increased early abortion rate in the whole population, further subgroup analysis showed that this happened only in patients who had embryo implantation failure once and aged under 30. However, the limited sample could not support us to draw the conclusion that using LMWH resulted higher early abortion rate. Given the side effects of LMWH and few studies explored its function on early abortion, We proposed that the using of LMWH in these younger patients caused abnormal bleeding and induced pregnancy loss in early stage. In the light of the above findings, we need to balance the use of LMWH. According to our findings, LMWH is not recommended for routine use in patients without confirmed immune disorders in the first two cycles in FET treatment.

Below is the link to the electronic supplementary material. Additional file 1: Supplementary 1 and 2 Additional file 1: Supplementary 1 and 2