Table 1 Baseline characteristics according to serum progesterone levels on the day of embryo transfer
| Â | Progesterone quartiles (ng/mL) | P-value | |||
|---|---|---|---|---|---|
Q1 (< 7.9) | Q2 (7.9–9.6) | Q3 (9.7–12.0) | Q4 (≥ 12.1) | ||
No. of cycles | 905 | 894 | 938 | 922 | Â |
Maternal age (years) | 35.1 ± 5.3 | 35.1 ± 5.2 | 34.8 ± 5.3 | 35.5 ± 5.3 | 0.052 |
Maternal BMI (kg/m2) | 21.92 ± 2.73 | 22.07 ± 2.98 | 21.75 ± 2.78 | 21.70 ± 2.87 | 0.057 |
Duration of infertility (years) | 3.7 ± 2.8 | 3.9 ± 3.0 | 3.6 ± 2.9 | 3.6 ± 2.7 | 0.067 |
Primary infertility, n (%) | 469 (51.8) | 438 (49.0) | 449 (47.9) | 425 (46.1) | 0.097 |
Infertility diagnosis, n (%) | Â | Â | Â | Â | 0.192 |
 Tubal | 438 (48.4) | 418 (46.8) | 446 (47.5) | 426 (46.2) |  |
 Male | 94 (10.4) | 86 (9.6) | 85 (9.1) | 80 (8.7) |  |
 Ovulatory | 150 (16.6) | 138 (15.4) | 170 (18.1) | 159 (17.2) |  |
 Endometriosis | 68 (7.5) | 69 (7.7) | 70 (7.5) | 50 (5.4) |  |
 Unexplained | 96 (10.6) | 112 (12.5) | 108 (11.5) | 119 (12.9) |  |
 Combined | 59 (6.5) | 71 (7.9) | 59 (6.3) | 88 (9.5) |  |
Ovarian stimulation protocol, n (%) |  |  |  |  | < 0.001 |
 Mild stimulation | 187 (20.7) | 134 (15.0) | 152 (16.2) | 121 (13.1) |  |
 GnRH-agonist short | 50 (5.5) | 49 (5.5) | 30 (3.2) | 39 (4.2) |  |
 Progestin-primed ovarian stimulation | 668 (73.8) | 711 (79.5) | 756 (80.6) | 762 (82.6) |  |
Fertilization method, n (%) | Â | Â | Â | Â | 0.537 |
 IVF | 575 (63.5) | 565 (63.2) | 618 (65.9) | 594 (64.4) |  |
 ICSI | 206 (22.8) | 223 (24.9) | 199 (21.2) | 217 (23.5) |  |
 IVF + ICSI | 124 (13.7) | 106 (11.9) | 121 (12.9) | 111 (12.0) |  |
Duration of estrogen use (days) | 12.7 ± 3.3 | 12.8 ± 3.2 | 12.8 ± 3.2 | 13.3 ± 3.5 | 0.001 |
Endometrial thickness (mm) | 9.96 ± 1.97 | 9.93 ± 1.80 | 9.89 ± 1.92 | 9.66 ± 1.86 | 0.002 |
Progesterone level on embryo transfer day (ng/mL) | 6.3 ± 1.4 | 8.8 ± 0.5 | 10.8 ± 0.7 | 15.3 ± 3.8 | < 0.001 |
No. of embryos transferred, n (%) | Â | Â | Â | Â | 0.165 |
 Single | 227 (25.1) | 216 (24.2) | 197 (21.0) | 207 (22.5) |  |
 Double | 678 (74.9) | 678 (75.8) | 741 (79.0) | 715 (77.5) |  |
Embryo stage at transfer, n (%) | Â | Â | Â | Â | 0.010 |
 Cleavage | 798 (88.2) | 769 (86.0) | 842 (89.8) | 784 (85.0) |  |
 Blastocyst | 107 (11.8) | 125 (14.0) | 96 (10.2) | 138 (15.0) |  |
Transfer of ≥ 1 top-quality embryos (%) | 191 (21.1) | 157 (17.6) | 170 (18.1) | 168 (18.2) | 0.209 |
Duration of embryo cryopreservation (years) | 0.49 ± 0.50 | 0.46 ± 0.36 | 0.47 ± 0.42 | 0.52 ± 0.55 | 0.525 |