Table 1 Baseline characteristics of all FET cycles

Age of embryo transfer (years),mean (SD),

33.63±3.51

33.84±3.79

33.69±3.65

0.065

0.205

0.674

Age of oocyte retrieval (years),mean (SD),

33.12±3.43

33.29±3.85

33.17±3.57

0.138

0.316

0.720

Duration of infertility (years),mean (SD),

3.57±1.28

3.62±1.35

3.69±1.62

0.472

0.352

0.219

BMI of women,mean (SD),

17.53±3.07

21.61±1.73

26.37±1.88

< 0.001

< 0.001

< 0.001

Antral follicle count (n),mean (SD),

12.41±5.64

12.72±5.68

12.67±6.07

0.072

0.782

0.265

Infertility causes, n (%)

0.291

0.988

0.494

 Tubal infertility

790 (60.09%)

3917 (62.87%)

762 (62.98%)

 Male factor infertility

83 (6.32%)

373 (5.98%)

75 (6.18%)

 Unexplained infertiltiy

70 (5.31%)

318 (5.11%)

60 (4.98%)

 Combined*

372 (28.28%)

1622 (26.04%)

313 (25.86%)

Basal FSH (mIU/mL),median (min-max)

5.77 (0.18–9.99)

5.55 (0.63–9.97)

5.43 (0.43–9.99)

< 0.001

0.008

< 0.001

Basal LH (mIU/mL),median (min-max)

5.02 (0.13–17.96)

4.63 (0.15–18.32)

3.74 (0.42–16.22)

< 0.001

< 0.001

< 0.001

Basal E2 (pg/mL),median (min-max)

35.00 (10.00–86.00)

33.00 (10.00–88.00)

32.00 (10.00–92.00)

0.017

0.154

0.009

Basal P (ng/mL),median (min-max)

0.30 (0.10–0.90)

0.30 (0.10–0.80)

0.30 (0.10–0.90)

0.991

0.126

0.326

Type of infertility, n (%)

0.083

0.295

0.069

 Primary

840 (63.89%)

4107 (65.93%)

808 (66.79%)

 Secondary

475 (36.11%)

2123 (34.07%)

402 (33.21%)

Type of FET cycle, n (%)

0.031

< 0.001

< 0.001

 HRT

325 (24.71%)

1373 (22.04%)

313 (25.87%)

 Natural cycle

290 (22.05%)

1303 (20.91%)

169 (13.97%)

 Ovarian stimulation

700 (53.24%)

3554 (57.05%)

728 (60.16%)

Endometrial thickness on transfer day,mean (SD)

10.31±3.32

10.47±2.92

10.44±2.24

0.078

0.686

0.253

Fertilization method, n (%)

0.088

0.245

0.112

 IVF

838 (63.73%)

3860 (61.96%)

743 (61.41%)

 ICSI

326 (24.79%)

1720 (27.61%)

356 (29.42%)

 Half IVF+half ICSI

151 (11.48%)

650 (10.43%)

111 (9.17%)