Table 1 Baseline characteristics, ovarian stimulation outcomes and cycle features of FETs

Patients

942

1686

FET cycles

1406

2716

Female age at oocyte retrieval (years; mean (SD))

31.0 (4.3)

31.2 (4.6)

0.480

Etiology of infertility

0.542

  Tubal factor

509 (54.0)

923 (54.7)

  Male factor

257 (27.3)

448 (26.6)

  PCOS

126 (13.4)

206 (12.2)

  Endometriosis

50 (5.3)

109 (6.5)

Duration of infertility (years; mean (IQR))

3.5 (2, 4)

3.3 (2, 4)

0.023

Nulliparous

491 (52.1)

885 (52.5)

0.856

Gravidity (median (IQR))

0 (0, 1)

1 (0, 1)

0.808

Parity (median (IQR))

0 (0, 0)

0 (0, 0)

0.261

BMI (kg/m²; mean (SD))

23.9 (3.9)

23.7 (4.0)

0.529

AMH (ng/ml; median (IQR))

3.3 (1.8, 5.5)

3.3 (1.8, 5.5)

0.890

Basic FSH (mIU/ml; mean (SD))

6.7 (2.4)

6.8 (2.3)

0.258

Basic LH (mIU/ml; mean (SD))

6.0 (4.2)

6.1 (4.9)

0.950

Basic oestradiol (pg/ml; median (IQR))

27.5 (20.8, 38.0)

26.9 (20.8, 37.9)

0.642

Ovarian stimulation protocol

0.019

  GnRH-a long protocol

765 (81.2)

1429 (84.8)

  GnRH-ant protocol

177 (18.8)

257 (15.2)

No of days of COS (mean (SD))

11.6 (2.7)

11.6 (2.7)

0.781

Total Gn dose administered (IU; mean (SD))

2530.5 (983)

2509.2 (975.9)

0.647

Serum LH on trigger day (mIU/ml; median (IQR))

1.3 (0.8, 2.2)

1.3 (0.8, 2.3)

0.439

Serum estradiol on trigger day (pg/ml; median (IQR))

4800 (2956, 4901)

4800 (2858, 4936)

0.055

Serum progesterone on trigger day (ng/ml; median (IQR))

1.2 (0.9, 1.8)

1.3 (0.9, 1.8)

0.819

Ovulation trigger protocol

0.056

  hCG

875 (92.9)

1597 (94.7)

  GnRH-a and hCG (dual trigger)

67 (7.1)

89 (5.3)

Method of fertilization

0.693

  IVF

685 (72.7)

1238 (73.4)

  ICSI

257 (27.3)

448 (26.6)

No of oocytes retrieved (mean (SD))

16.5 (8.0)

16.9 (8.5)

0.413

2PN fertilization (median (IQR))

9 (6, 13)

10 (6, 14)

0.127

No of embryos available for transfer (mean (IQR))

5.1 (2, 7)

5.5 (3, 8)

0.026

Blastocysts available for transfer

101 (10.7)

213 (12.6)

0.147

No of high-quality embryos (mean (IQR))

1.8 (0, 3)

2.1 (0, 3)

0.013

Ovarian stimulation cycle outcomes

0.356

  Cycle with fresh embryo transfer

116 (12.3)

229 (13.6)

  Cycle with freeze-all strategy

826 (87.7)

1457 (86.4)

Day of FET

 < 0.001

  Day 3

1305 (92.8)

2287 (84.2)

  Day 5

101 (7.2)

429 (15.8)

No of frozen thawed embryos transferred

 < 0.001

  SET

288 (20.5)

807 (29.7)

  DET

1118 (79.5)

1909 (70.3)

Assisted hatching

0.277

  Yes

191 (13.6)

403 (14.8)

  No

1215 (86.4)

2313 (85.2)

Endometrial thickness prior to FET (mm; mean (SD))

10.4 (1.9)

10.3 (1.9)

0.281

Physicians of embryo transfer

0.002

  Physician A

657 (46.7)

1111 (40.9)

  Physician B

265 (18.9)

520 (19.1)

  Physician C

276 (19.6)

605 (22.3)

  Physician D

208 (14.8)

480 (17.7)