Study population
The detailed inclusion and exclusion criteria are shown in Fig. 1. A total of 20,345 FET cycles were performed from January 2015 to December 2019. Cycles with cleavage ET (n = 9512), more than one blastocyst transfer (n = 365), oocyte donation (n = 139), PGT (n = 224) and incomplete important data (n = 79) were excluded. We excluded natural cycles (n = 3265), GnRH-agonist + artificial cycles (n = 762) and ovarian stimulation cycles (n = 1980). Cycles with EMT ≤ 7.5 mm were excluded (n = 162). A total of 1672 cycles with singleton live births from single blastocyst transfer were included for analysis.
Cycles were allocated to four groups according to the duration of estrogen treatment before single frozen blastocyst transfer ①≤12 days (n = 306), ②13–15 days (n = 620), ③16–18 days (n = 471), ④≥19 days (n = 275).
Baseline characteristics
The details of the baseline and cycle characteristics among the groups are described in Table 1. The endometrial thickness was different among groups on the first day of progesterone administration (≤ 12 days, 9.5 (8.7, 10.5); 13–15 days, 9.0 (8.3, 10.0); 16–18 days, 8.5 (8.0, 9.3); ≥19 days, 8.1 (7.5, 9.0), P < 0.001). Apart from this, there were no significant differences in other basic characteristics, including maternal age at oocyte retrieval, paternal age, body mass index (BMI), duration of infertility, type of infertility, gravidity, parity, causes of infertility, basal serum FSH level, AMH, basal antral follicle count, serum estradiol on the trigger day of the controlled ovarian hyperstimulation (COH) cycles, the rate of IVF and the rate of good-quality blastocyst transfer (a good-quality blastocyst is defined as a B3-B4 or B5 embryo ≥ BB(AA, AB, BA, BB)) according to the grading scale proposed by Gardner[24]).
Table 1
Baseline and cycle characteristics among groups.
Parameters | Duration of estrogen treatment before embryo transfer (days) | P-value |
≤ 12 (n = 306) | 13–15 (n = 620) | 16–18 (n = 471) | ≥ 19 (n = 275) |
Maternal age at oocytes retrieved(year) | 30.0(28.0,33.0) | 30.0(27.0,33.0) | 30.0(28.0,32.0) | 29.0(27.0,32.0) | 0.06 |
Paternal age(year) | 31.0(28.0,34.0) | 30.0(28.0,34.0) | 30.0(28.0,33.0) | 30.0(27.0,34.0) | 0.17 |
BMI(kg/m2) | 24.1(21.7,26.2) | 23.6(21.5,25.8) | 24.1(22.0,26.1) | 23.4(21.4,25.4) | 0.07 |
Duration of infertility(year) | 3.0(1.0,5.0) | 3.0(1.0,5.0) | 3.0(1.0,4.0) | 3.0(1.0,4.0) | 0.07 |
Type of infertility (%) | | | | | 0.13 |
Primary infertility | 52.0(159/306) | 45.6(283/620) | 46.5(219/471) | 42.5(117/275) | |
Secondary infertility | 48.0(147/306) | 54.4(337/620) | 53.5(252/471) | 57.5(158/275) | |
Gravidity | 0(0,1) | 1(0,2) | 1(0,2) | 1(0,2) | 0.19 |
Parity | 0(0,1) | 0(0,1) | 0(0,1)0 | 0(0,1) | 0.61 |
Causes of infertility (%) | | | | | 0.06 |
Tubal factor | 28.4(87/306) | 28.5(177/620) | 31.0(146/471) | 37.5(103/275) | |
Male factor | 20.3(62/306) | 26.0(161/620) | 25.3(119/471) | 20.4(56/275) | |
Male + female factors | 21.9(67/306) | 16.9(105/620) | 16.8(79/471) | 18.9(52/275) | |
Others | 29.4(90/306) | 28.5(177/620) | 27.0(127/471) | 23.3(64/275) | |
Basal serum FSH level(IU/L) | 6.2(4.9,7.4) | 6.1(4.7,7.2) | 6.0(4.7,7.4) | 6.4(5.0,7.3) | 0.31 |
AMH(ng/ml) | 4.6(2.1,6.6) | 4.8(2.9,6.2) | 4.9(2.1,6.8) | 5.0(2.4,6.3) | 0.11 |
Basal antral follicle count | 20.0(13.0,24.0) | 20.0(15.0,24.0) | 22.0(15.0,24.0) | 22.0(17.0,24.0) | 0.15 |
Serum estradiol on the trigger day of the COH cycles (pg/ml) | 3033.0(1866.5,4532.5) | 3077.9(2197.5,4607.5) | 3191.5(2123.5,4637.3) | 3090.2(2091.5,4167.5) | 0.39 |
Fertilization method (%) | | | | | 0.50 |
IVF | 68.6(210/306) | 65.0(403/620) | 68.4(322/471) | 69.1(190/275) | |
ICSI | 31.4(96/306) | 35.0(217/620) | 31.6(149/471) | 30.9(85/275) | |
Endometrial thickness on the first day of progesterone administration (mm) | 9.5(8.7,10.5) | 9.0(8.3,10.0) | 8.5(8.0,9.3) | 8.1(7.5,9.0) | < 0.001 |
The rate of good-quality blastocyst transfer* | 63.1(193/306) | 63.9(396/620) | 67.9(320/471) | 61.5(169/275) | 0.27 |
Note: Data are presented as median (P25,P75) for continuous variable and %(n/N) for categorical variable. *A good-quality blastocyst defined as B3-B4 or B5 embryo ≥ BB(AA, AB,BA,BB) according to the grading scale proposed by Gardner. |
Table 2
Comparison of neonatal and perinatal outcomes among groups.
| Duration of estrogen treatment before embryo transfer (days) | P-value |
≤ 12 (n = 306) | 13–15 (n = 620) | 16–18 (n = 471) | ≥ 19 (n = 275) |
Neonatal outcomes | | | | | |
Neonatal birth weight (g) | 3500(3200,3850) | 3500(3200,3895) | 3400(3100,3800) | 3500(3100,3800) | 0.38 |
Neonatal sex (%) | | | | | 0.45 |
Male | 57.5(176/306) | 55.6(345/620) | 57.1(269/471) | 51.6(142/275) | |
Female | 42.5(130/306) | 44.4(275/620) | 42.9(202/471) | 48.4(133/275) | |
Gestational weeks at delivery (week) | 39(38,40) | 39(38,40) | 39(38,40) | 39(38,40) | 0.16 |
Preterm birth (%) | 6.2(19/306) | 6.3(39/620) | 8.3(39/471) | 4.4(12/275) | 0.20 |
Low birth weight (%) | 5.6(17/306) | 4.8(30/620) | 6.8(32/471) | 5.5(15/275) | 0.58 |
Small-for-gestational age (%) | 7.8(24/306) | 4.8(30/620) | 5.7(27/471) | 7.6(21/275) | 0.20 |
Macrosomia (%) | 14.4(44/306) | 18.7(116/620) | 16.6(78/471) | 13.8(38/275) | 0.20 |
Large-for-gestational age (%) | 22.9(70/306) | 26.3(163/620) | 22.7(107/471) | 21.5(59/275) | 0.34 |
Neonatal malformation (%) | 0.7(2/306) | 0.5(3/620) | 0.4(2/471) | 0.4(1/275) | 0.96 |
Pregnancy-related complications | | | | | |
Hypertensive states of pregnancy (%) | 5.6(17/306) | 6.1(38/620) | 8.7(41/471) | 8.0(22/275) | 0.24 |
Gestational diabetes mellitus (%) | 3.9(12/306) | 8.1(50/620) | 7.0(33/471) | 5.5(15/275) | 0.09 |
Placenta previa (%) | 1.3(4/306) | 1.0(6/620) | 0.4(2/471) | 1.1(3/275) | 0.54 |
Placental abruption (%) | 0.7(2/306) | 1.0(6/620) | 0.2(1/471) | 0.7(1/275) | 0.49 |
Premature rupture (%) | 2.9(9/306) | 3.1(19/620) | 2.5(12/471) | 1.1(3/275) | 0.37 |
Note: Data are presented as median (P25,P75) for continuous variable and %(n/N) for categorical variable . |
Neonatal And Perinatal Outcomes
The main outcome measure was the rate of SGA among the four groups, which was 7.8% (24/306), 4.8% (30/620), 5.7% (27/471), and 7.6% (21/275), with no statistical significance (P = 0.20). The mean neonatal birth weight was comparable among groups (≤ 12 days, 3500 (3200, 3850); 13–15 days, 3500 (3200, 3895); 16–18 days, 3400 (3100, 3800); ≥19 days, 3500 (3100, 3800), P = 0.38). The neonatal sex ratio (P = 0.45) and gestational weeks at delivery were similar between the groups (P = 0.16). The preterm birth rates among the four groups were 6.2% (19/306), 6.3% (39/620), 8.3% (39/471), and 4.4% (12/275), with no statistical significance (P = 0.20). The rates of LBW, macrosomia, LGA and neonatal malformation were comparable among the groups (P = 0.58, P = 0.20, P = 0.34, P = 0.96). Pregnancy-related complications, including hypertensive states of pregnancy, gestational diabetes mellitus, placenta previa, placental abruption and premature rupture, were similar among the groups (P = 0.24, P = 0.09, P = 0.54, P = 0.49, P = 0.37).
Regarding SGA, to adjust for the influence of confounding factors, we conducted a multiple logistic regression analysis. The included factors were maternal age (continuous variable), body mass index (continuous variable), duration of infertility (continuous variable), type of infertility (primary/secondary infertility), infertility diagnosis(tubal/male/both/others), basal antral follicle count (continuous variable), basal serum FSH level (continuous variable), gravidity (continuous variable), parity (continuous variable), fertilization method (IVF/ICSI), endometrial thickness on the first day of progesterone administration (continuous variable) and blastocyst quality (good-quality/non-good-quality).
After adjustments for confounding factors, taking ‘≤12 days group’ as the reference group, the duration of estrogen treatment did not affect the rate of singleton SGA (13–15 days, AOR = 1.37,95% CI = 0.70–2.70, P = 0.36; 16–18 days, AOR = 0.74༌95% CI = 0.40–1.36, P = 0.34; ≥19 days, AOR = 0.81, 95% CI = 0.44–1.49, P = 0.50). The results of logistic regression analysis indicated that maternal BMI was a significant predictor for SGA (AOR = 0.93, 95% CI = 0.88–0.99, P = 0.04). The specific AOR values with their 95% CIs are presented in Table 3.
Table 3
Multivariable logistic regression analysis to account for confounding variables for SGA.
| Adjusted OR (95%CI ) | P-value |
Maternal age(year) | 1.03(0.97–1.09) | 0.36 |
Body mass index(kg/m2) | 0.93(0.88–0.99) | 0.04 |
Duration of Infertility (year) | 0.92(0.83–1.01) | 0.09 |
Type of infertility(Primary/ Secondary infertility) | 0.69(0.38–1.24) | 0.21 |
Infertility diagnosis(Tubal/ Male/ both/Others) | 1.05(0.88–1.26) | 0.57 |
Basal antral follicle count | 0.99(0.97–1.03) | 0.93 |
Basal serum FSH level(IU/L) | 1.03(0.95–1.11) | 0.43 |
Gravidity | 0.82(0.62–1.10) | 0.18 |
Parity | 1.32(0.90–1.96) | 0.16 |
Fertilization method(IVF/ICSI) | 1.03(0.94–1.12) | 0.57 |
Endometrial thickness on the first day of progesterone administration (mm) | 0.85(0.72–1.01) | 0.07 |
Blastocyst quality(good-quality*/ Non-good-quality) | 1.09(0.70–1.71) | 0.70 |
Duration of estrogen treatment (days) | | 0.67 |
≤ 12 (reference ) | 1 | |
13–15 | 1.37(0.70–2.70) | 0.36 |
16–18 | 0.74(0.40–1.36) | 0.34 |
≥ 19 | 0.81(0.44–1.49) | 0.50 |
Note: Variables entered in the logistics regression model listed. CI = confidence interval. *A good-quality blastocyst defined as B3-B4 or B5 embryo ≥ BB(AA, AB,BA,BB) according to the grading scale proposed by Gardner. |