The Effect of Additional Embryo Transfer on the Pregnancy Rate in Young Women Receiving in vitro Fertilization: A Natural Experiment Study

Figures & data

Figure 1 Eligibility for additional embryo transfer in women aged 34 and 35 according to the policy of the Ministry of Health and Welfare of Korea.

Abbreviation: ET, embryo transfer.

Table 1 Characteristics of 1909 Fresh and Frozen Embryo Transfer Cycles of 34‒35-Year-Old Women

Variables	34 Years (932 Cycles)	35 Years (977 Cycles)	P for Difference
Endometrial thickness on hCG day (mm)	9.56 ± 2.1	9.63 ± 2.18	0.839
P4 on hCG day (ng/mL)	0.83 ± 0.57	0.86 ±1.00	0.587
Number of oocytes	12.79 ± 9.04	13.3 ± 10.16	1.000
Number of fertilized eggs	8.24 ± 6	8.75 ± 6.89	0.799
Number of transferred embryos, day 2–4	1.85 ± 0.43	2.47 ± 0.69	<0.001
Number of transferred embryos, day 5–6	1.04 ± 0.19	1.75 ± 0.44	<0.001
Proportion of fresh embryo transfer	60.1% (818/1360)	63.2% (900/1425)	1.000
Proportion of blastocyst transfer	42.9% (400/932)	46.9% (458/977)	1.000
Thin endometrium (<7mm) on hCG day	5.8% (52/895)	6.0% (56/940)	0.972
Serum progesterone elevation on hCG day	13.5% (109/807)	13.1% (117/893)	0.320
Decreased ovarian reserve (retrieved oocytes <5)	21.4% (173/809)	17.1% (153/896)	0.064
Intrauterine pregnancy	45.2% (421/932)	51.3% (501/977)	0.009
Biochemical or ectopic pregnancy	4.5% (20/441)	4.5% (20/445)	0.198
Multiple pregnancy of all intrauterine pregnancy	6.9% (29/421)	30.5% (153/501)	<0.001

Notes: Continuous variables are presented as means and standard errors and binary variables are as proportions (and frequencies in parentheses). Difference in continuous variables was tested using Student t-tests. Pearson’s Chi-squared test with Yates’ continuity correction was used for categorized variables.

Abbreviations: hCG, human chorionic gonadotropin; P4, progesterone.

Table 2 Frequencies of Intrauterine Pregnancy and Multiple Pregnancy Among Intrauterine Pregnancy Cases, 1909 Cycles of 34–35-Year-Old Women

	ET Day	No Additional ET	Additional ET	P for Difference
Intrauterine pregnancy
Fresh cycles	Day 2–4	32.6% (128/393)	36.0% (62/172)	0.479
	Day 5–6	51.5% (67/130)	46.0% (75/163)	0.411
Frozen cycles	Day 2–4	28.2% (20/71)	17.2% (5/29)	0.373
	Day 5–6	52.9% (324/613)	71.7% (241/336)	<0.001
Multiple pregnancy
Fresh cycles	Day 2–4	18.8% (24/128)	27.4% (17/62)	0.191
	Day 5–6	0.0% (0/67)	36.0% (27/75)	<0.001
Frozen cycles	Day 2–4	30.0% (6/20)	0.0% (0/5)	0.289
	Day 5–6	0.0% (0/324)	44.0% (106/241)	<0.001

Notes: Additional ET cycle is defined as transfer of three day 2–4 embryos or two day 5–6 embryos which is allowed after 35th birthday. Proportions were compared using Fisher’s exact test or chi-square test. Multiple pregnancy rate was calculated only in intrauterine pregnancy cases. Two ET cycles were missing for ET day. Sum of all numerators and denominators may not equal to the total number of ET cycles due to cases with missing information.

Abbreviation: ET, embryo transfer.

Figure 2 Unadjusted and adjusted relative risk of intrauterine pregnancy for additional embryo transfer, 1909 cycles of 34–35-year-old women.

Notes: In the models for fresh cycles, thin endometrium, P4 elevation, decreased ovarian reserve, season of transfer and within-individual correlation were included as covariates. In models for frozen cycles, thin endometrium, season of transfer and within-individual correlation were included. Hollow markers represent unadjusted and solid markers represent adjusted estimates. Asterisks (*) indicate p < 0.05.

Abbreviation: ET, embryo transfer.