All FET cycles (n=23726) Single cleavage-stage embryo transfer group (C-1) (n=763) Double cleavage-stage embryo transfer group (C-2) (n=13004) Single blastocyst-stage embryo transfer group (B-1) (n=7913) Double blastocyst-stage embryo transfer group (B-2) (n=2046)
X2
P
Maternal Characteristics
Maternal age (years) 32 (29, 35) 34 (31, 39) 32 (29, 35) 32 (29, 35) 32 (29, 35)
20-29 7269 (30.6) 129 (16.9) 4107 (31.6) 2417 (30.5) 616 (30.1) 520.688 <0.001
30-34 9640 (40.6) 257 (33.7) 5289 (40.7) 3217 (40.7) 877 (42.9)
35-37 3790 (16.0) 132 (17.3) 1956 (15.0) 1367 (17.3) 335 (16.4)
38-39 1297 (5.5) 75 (9.8) 644 (5.0) 464 (5.9) 114 (5.6)
40-42 1084 (4.6) 71 (9.3) 603 (4.6) 329 (4.2) 81 (4.0)
>42 646 (2.7) 99 (13.0) 405 (3.1) 119 (1.5) 23 (1.1)
Body Mass Index (BMI) 21.6 (19.8, 24.0) 21.5 (19.6, 23.9) 21.5 (19.6, 23.7) 22.0 (20.0, 24.4) 21.5 (19.7, 23.8)
Underweight (BMI<18.5) 1914 (8.1) 62 (8.1) 1128 (8.7) 554 (7.0) 170 (8.3) 87.861 <0.001
Fitting (18.5≤BMI<24) 11474 (48.4) 338 (44.3) 6259 (48.1) 3862 (48.8) 1015 (49.6)
Overweight (24≤BMI<27) 2940 (12.4) 84 (11.0) 1480 (11.4) 1132 (14.3) 244 (11.9)
Obesity (BMI≥27) 1571 (6.6) 39 (5.1) 744 (5.7) 654 (8.3) 134 (6.5)
Missing value 5827 (24.6) 240 (31.5) 3393 (26.1) 1711 (21.6) 483 (23.6)
Infertility type
Primary infertility 14463 (61.0) 402 (52.7) 8034 (61.8) 4731 (59.8) 1296 (63.3) 33.535 <0.001
Secondary infertility 9026 (38.0) 343 (45.0) 4828 (37.1) 3125 (39.5) 730 (35.7)
Missing value 237 (1.0) 18 (2.4) 142 (1.1) 57 (0.7) 20 (1.0)
Cause of infertility
Tubal 8730 (36.8) 259 (33.9) 5075 (39.0) 2539 (32.1) 857 (41.9) 128.751 <0.001
Ovulatory 5575 (23.5) 201 (26.3) 3019 (23.2) 1801 (22.8) 554 (27.1) 20.990 <0.001
Endometriosis 1455 (6.1) 65 (8.5) 835 (6.4) 440 (5.6) 115 (5.6) 14.860 0.002
Male factor 9023 (38.0) 261 (34.2) 5025 (38.6) 3018 (38.1) 719 (35.1) 14.081 0.003
Unexplained 5136 (21.6) 183 (24.0) 2456 (18.9) 2138 (27.0) 359 (17.5) 215.792 <0.001
Duration of infertility (years) 4 (2, 6) 4 (2, 6) 4 (2, 6) 4 (2, 6) 4 (2, 6)
≤4 14142 (59.6) 4234 (55.6) 7964 (61.2) 4584 (57.9) 1170 (57.2) 29.042 <0.001
>4 8957 (37.8) 308 (40.4) 4726 (36.3) 3104 (39.2) 819 (40.0)
Missing value 627 (2.6) 31 (4.1) 314 (2.4) 225 (2.8) 57 (2.8)
Type of fertilization in current cycle
IVF 12113 (51.1) 437 (57.3) 7074 (54.4) 3458 (43.7) 1144 (55.9) 296.204 <0.001
ICSI 10574 (44.6) 304 (39.9) 5423 (41.7) 4075 (51.5) 771 (37.7)
IVF+ICSI 1039 (4.4) 21 (2.8) 507 (3.9) 380 (4.8) 131 (6.4)
Previous ART cycles 3 (2, 4) 2 (1, 3) 1 (1, 2) 2 (1, 3) 2 (2, 4)
0-1 cycles 10710 (45.1) 217 (28.5) 6307 (48.5) 3727 (47.1) 458 (22.4) 768.355 <0.001
2-3 cycles 8590 (36.2) 328 (43.0) 4538 (34.9) 2754 (34.8) 970 (47.4)
4-5 cycles 2185 (9.2) 85 (11.2) 923 (7.1) 831 (10.5) 346 (16.9)
≥6 cycles 804 (3.4) 49 (6.4) 299 (2.3) 317 (4.0) 139 (6.8)
Missing value 1437 (6.1) 83 (10.9) 936 (7.2) 285 (3.6) 133 (6.5)
Maternal & Neonatal Outcomes
Biochemical pregnancy 11427 (48.2) 184 (24.1) 6077 (46.7) 3957 (50.0) 1209 (59.1) 296.037 <0.001
Clinical pregnancy 10237 (43.1) 164 (21.5) 5481 (42.1) 3507 (44.3) 1085 (53.0) 237.023 <0.001
*Monozygotic twins 151 (1.5) 3 (1.8) 47 (0.9) 79 (2.3) 22 (2.0) 31.668@ <0.001@
*Miscarriage 1687 (16.5) 39 (23.8) 786 (14.3) 632 (18.0) 230 (21.2) 48.180 <0.001
*Maternal complication 968 (9.5) 12 (7.1) 515 (9.4) 312 (8.9) 129 (11.9) 9.686 0.021
Live birth 6368 (26.8) 90 (11.8) 3771 (29.0) 1819 (23.0) 688 (33.6) 226.611 <0.001
Twin birth 1116 (17.5) 0 (0.0) 883 (23.4) 34 (1.9) 199 (28.9) 479.969 <0.001
Preterm birth 936 (14.7) 6 (6.7) 609 (16.1) 175 (9.6) 146 (21.2) 71.715 <0.001
Low birth weight 776 (12.2) 3 (3.3) 559 (14.8) 92 (5.1) 122 (17.7) 137.261 <0.001
SGA 362 (5.7) 4 (4.4) 267 (7.1) 55 (3.0) 36 (5.2) 38.245 <0.001