7849 fresh transfer cycles from 6942 patients were included in the present study. The mean age of the patients was 31.44±4.38 years. The transfer depth ranged from 4-25 mm. The 10%, 25%, 50%, 75% and 90% percentile of the distance was 7, 9, 11, 14, 17 mm respectively. Using quartiles as cut-off values, the cycles were divided into four groups (quartile1-4). In 1735 cycles (22.1%), embryos were transferred at or within 9 mm of the fundus (quartile 1). In 2557 cycles (32.6%), embryos were transferred at a distance between 9.1 and 11 mm from the fundal endometrial surface (quartile 2). In 1933 cycles (24.6%), embryos were transferred at a distance >11 mm but ≤14 mm from the fundal endometrial surface (quartile 3). And finally, in 1624 cycles (20.7%), embryos were transferred at a distance>14 mm from the fundus (quartile 4).
The baseline characteristics of patients receiving transfer were summarized in Table 1. The overall baseline characteristics were similar between groups. However, the patients in quartile 2 were of longer duration of infertility, whereas the patients in quartile 4 had fewer previous attempts of transfer, lower proportion of PCOS, lower basal LH and more AFC than other three groups, as demonstrated by the post hoc test. In addition, significant heterogeneity in basal FSH levels was noted among groups, but the absolute differences were rather small.
Table 1
Baseline characteristics and ovarian stimulation parameters.
| Transfer depth quartiles | |
Variables | Quartile 1 (≤9 mm, n=1735) | Quartile 2 (9.1-11 mm, n=2557) | Quartile 3 (11.1-13.9 mm, n=1933) | Quartile 4 (≥14 mm, n=1624) | P |
Female`s age, year | 31.29±4.51 | 31.45±4.35 | 31.51±4.41 | 31.50±4.23 | 0.400 |
Male`s age, year | 33.28±5.11 | 33.45±5.04 | 33.60±5.09 | 33.40±4.85 | 0.279 |
Duration of infertility, year | 4.612±3.33ab | 4.743±3.41b | 4.440±3.22a | 4.645±3.11ab | 0.024 |
Primary/Secondary (%) | 804/931(46.3/53.7) | 1236/1321(48.3/51.7) | 917/1016(47.4/52.6) | 813/811(50.1/49.9) | 0.169 |
Previous attempt of ET | 0.390±0.86a | 0.370±0.85a | 0.375±0.89a | 0.303±0.80b | 0.016 |
BMI, kg/cm2 | 21.08±2.78 | 21.17±2.66 | 21.08±2.92 | 21.19±2.70 | 0.461 |
Additional etiologies | | | | | |
PCOS (%) | 115/1735(6.6)a | 159/2557(6.2)ab | 101/1933(5.2)ab | 75/1624(4.6)b | 0.039 |
Endometriosis (%) | 203/1735(11.7) | 285/2557(11.1) | 237/1933(12.3) | 216/1624(13.3) | 0.200 |
Hydrosalpinix (%) | 75/1735(4.3) | 84/2557(3.3) | 80/1933(4.1) | 46/1624(2.8) | 0.054 |
Male (%) | 222/1735(12.8) | 379/2557(14.8) | 279/1933(14.4) | 253/1624(15.6) | 0.121 |
Basal FSH, IU/l | 7.512±2.42ab | 7.656±2.60b | 7.633±2.45b | 7.420±2.88a | 0.017 |
Basal LH IU/l | 4.92±3.06a | 4.72±2.85ab | 4.73±2.74ab | 4.59±2.57b | 0.009 |
Basal PRL, ng/ml | 15.50±9.79 | 15.01±9.47 | 15.52±12.5 | 15.47±9.52 | 0.279 |
Basal E2, pg/ml | 45.16±34.7 | 43.45±25.5 | 43.74±24.4 | 43.41±23.8 | 0.173 |
Basal T, ng/ml | 0.44±1.20 | 0.53±2.41 | 0.54±2.48 | 0.54±2.99 | 0.556 |
Basal P, ng/ml | 0.85±1.73 | 0.77±1.07 | 0.79±1.51 | 0.83±1.68 | 0.211 |
AFC | 7.91±4.38a | 7.83±4.21a | 7.80±4.19a | 8.19±4.13b | 0.025 |
a,b,c Groups do not share common letters differ significantly, P<0.0125 for chi square multiple comparison, P< 0.05 for other analyses |
BMI, body mass index; PCOS, polycystic ovarian syndrome; AFC, antral follicle count |
Numbers are means ± SD |
Table 2 presented the ovarian stimulation characteristics and IVF outcomes in the cycles studied. Besides the catheter tip-fundus distance, significant differences were also noted in GnRH analogues, E2 level on the day of hCG and endometrial thickness and endometrial type on the day of hCG among groups. But the starting and total dose, the oocytes yielded, and the number and quality of embryos transferred were comparable among groups. The clinical pregnancy rates and ectopic pregnancy rates were also similar among groups as evaluated with bivariate analyses.
Table 2
Outcome of ovarian stimulation, fertilization and embryo transfer.
| Transfer depth quartiles | |
Variables | Quartile 1 (≤9 mm,n=1735) | Quartile 2 (9.1-11 mm, n=2557) | Quartile 3 (11.1-13.9 mm, n=1933) | Quartile 4 (≥14 mm, n=1624) | P |
Antagonist/agonist (%) | 349/1386(20.1/79.9)a | 518/2039(20.3/79.7)a | 350/1583(18.1/81.9)ab | 232/1392(14.3/85.7)b | <0.001 |
Starting dose of stimulation, IU | 208.93±33.60 | 208.70±34.33 | 208.89±33.63 | 208.79±32.32 | 0.996 |
Total dose of gonadotropin, IU | 2324.32±624.65 | 2320.07±621.09 | 2301.97±609.31 | 2351.75±604.94 | 0.119 |
E2 level on the day of hCG, pg/ml | 2832.97±1568.52ab | 2719.56±1576.04a | 2756.70±1578.52ab | 2864.02±1609.90b | 0.014 |
Progesterone elevation, ng/ml | 311/1735(17.9) | 432/2557(16.9) | 306/1933(15.8) | 283/1624(17.4) | 0.368 |
Endometrial thickness, mm | 10.02±3.13a | 10.49±3.18b | 10.76±2.34c | 11.63±2.69d | <0.001 |
Endometrial pattern | | | | | |
A/B/C (%) | 354/1247/134(20.4/71.9/7.1)a | 517/1787/253(20.2/69.9/9.9)ab | 365/1341/227(18.9/69.4/11.7)b | 271/1058/295(16.7/65.1/18.2)c | <0.001 |
Number of oocyte retrieved | 10.63±5.44 | 10.48±5.60 | 10.63±5.72 | 10.96±5.43 | 0.052 |
ICSI/IVF (%) | 455/1280(26.2/73.8) | 704/1853(27.5/72.5) | 536/1397(27.7/72.3) | 455/1169(28/72) | 0.65 |
Blastocyst/cleavage transferred (%) | 149/1586(8.6/91.4) | 215/2342(8.4/91.6) | 158/1775(8.2/91.8) | 149/1475(9.2/90.8) | 0.746 |
Number of embryos transferred | | | | | |
One/two/three (%) | 361/1305/69(20.8/75.2/4) | 512/1940/105(20/75.9/4.1) | 400/1452/81(20.7/75.1/4.2) | 318/1267/39(19.6/78/2.4) | 0.060 |
At least one top-quality embryo transferred (%) | 407/1735(23.5) | 612/2557(23.9) | 441/1933(22.8) | 428/1624(26.4) | 0.083 |
Transfer depth, mm | 6.754±1.27a | 10.01±0.80b | 12.83±0.79c | 17.61±2.87d | <0.001 |
Implantation rate, % | 45.35±41.79 | 43.35±41.22 | 44.20±41.83 | 42.64±41.71 | 0.248 |
Ectopic pregnancy* (%) | 10/1104(0.9) | 24/1596(1.5) | 15/1212(1.2) | 10/997(1) | 0.496 |
Clinical pregnancy (%) | 1038/1735(59.8) | 1498/2557(58.6) | 1129/1933(58.4) | 921/1624(56.7) | 0.335 |
OR for clinical pregnancy | Ref. | 0.896(0.785-1.024) | 0.856(0.742-0.987) | 0.704(0.604-0.821) | <0.001** |
a,b,c,d Groups do not share common letters differ significantly, P<0.0125 for chi square multiple comparison, P< 0.05 for other analyses |
*Ectopic pregnancy rate=ectopic pregnancies /(chemical pregnancies +clinical pregnancies +ectopic pregnancies) |
**P for trend, ORs were adjusted for female`s age, duration of infertility, hydrosalpinx, the number of oocyte retrieved, starting dose of stimulation, type of GnRH analogues, the number of embryos transferred, endometrial thickness, endometrial pattern, progesterone elevation, the development stage of transferred embryos, the presence of at least one good-quality embryo transferred and providers of embryo transfer. |
Adjusted for aforementioned confounding factors, multivariate analyses revealed a decrease in clinical pregnancy rates in quartile 3 and quartile 4 with quartile 1 as reference. The odds ratios (OR) for clinical pregnancy comparing quartile 3 and quartile 4 with quartile 1 were 0.86(95%CI: 0.74-0.99) and 0.70(95%CI: 0.60-0.82) respectively. (Table 2)
To test the effect of extreme values of the transfer depth on clinical pregnancy and illustrate the trend of the change of pregnancy rates across the range of distance, we introduced 10% and 90% percentile of the transfer depth into analyses. In the six-group comparison using multivariate analysis, the ORs for clinical pregnancy of different distances (7.1-9 mm, 9.1-11 mm, 11.1-14 mm, 14.1-1.7 mm and >1.7mm) in comparison with the distance of ≤7mm was 0.91 (95%CI: 0.76-1.08), 0.89 (95%CI: 0.75-1.05), 0.84(95%CI: 0.72-0.99), 0.73 (95%CI: 0.60-0.88) and 0.64(95%CI: 0.51-0.80) respectively. A trend of decrease in clinical pregnancy with the increase of transfer depth was illustrated (Figure 1.) and the P value for trend was less than 0.001.
When the transfer depth was treated as continuous value, the OR for clinical pregnancy per millimeter increased was 0.97 (95%: 0.96-0.99) in multivariate analyses (supplemental Table 1). The ORs for clinical pregnancy of other covariates, which were included in multivariate analyses were also presented in supplementary Table 1.
To explore whether the association between embryo transfer depth and pregnancy differs across stratum of potential effect modifiers, the interaction terms of endometrial thickness×embryo transfer depth, blastocyst transfer ×embryo transfer depth, progesterone elevation ×embryo transfer and transfer provider×embryo transfer depth was introduced into the model. When endometrial thickness on the day of hCG was categorized into thin (<8 mm, n=913), normal (8-11 mm, n=3760) and thick (>11mm, n=3176) categories, the ORs for pregnancy comparing quartile 2, quartile 3 and quartile 4 of tip-to-fundus distance with quartile 1 were 1.07(95%: 0.79-1.45) fold, 1.09(95%: 0.80-1.50) fold and 1.03(95%: 0.74-1.43) fold in thick group than in normal thickness group, respectively, suggesting insignificant change of the effect with increased endometrial thickness. To the contrast, the ORs increased 1.96(95%: 1.33-2.90) fold, 1.20(95%: 0.78-1.87) fold and 1.98(95%: 1.20-3.26) fold respectively in thin group comparing with normal group, suggesting an effect modification of thin endometrium. In cycles with thin endometrium, the adjusted ORs for pregnancy comparing quartile 3 and quartile 4 with quartile 1 was 1.72 (95%: 1.21-2.47), 1.03(95%: 0.68-1.55) and 1.44 (95%: 0.90-2.30), respectively.
On the other hand, both progesterone elevation and blastocyst transfer decreased ORs. The ORs comparing quartile 2 through 4 with quartile 1 decreased 0.52 (95%: 0.32-0.85), 0.60 (95%: 0.36-1.00) and 0.55 (95%: 0.33-0.93) fold comparing blastocyst transfer with cleavage stage transfer and decreased 0.86 (95%: 0.68-1.07), 0.79 (95%: 0.60-1.03) and 0.77 (95%: 0.58-1.01) fold comparing cycles with progesterone elevation with those without. Interaction (P for interaction term was 0.25) was not detected between embryo transfer providers and catheter tip-fundus distance (supplemental Table 2).