Table 1.

Outcomes and comparison of potential confounders in the fresh ET and FET groups

	Fresh cycles (n = 7014)	Frozen-thaw cycles (n = 3730)	P-value
Age (yr)		35.51	35.14	$<$ 0.0001
Body mass index (kg/m ${}^2$)		25.10	24.98	0.3474
Cumulative ET (n = mean)		1.30	2.68	$<$ 0.0001
Fertilization methods (%)
	IVF	30.33	31.05	0.44
	ICSI	69.67	68.95	0.44
Syngamy check (%)
	EC	31.11	19.65	$<$ 0.0001
	NEBD	39.67	33.46	$<$ 0.0001
	2PN	29.22	46.89	$<$ 0.0001
Day 2 assessment
	Number of cell on day 2 (mean)	3.84	3.63	$<$ 0.0001
	Grade of embryos (mean)	1.87	2.00	$<$ 0.0001
Grade of embryos (%)
	Grade 1	35.56	27.35	$<$ 0.0001
	Grade 2	42.81	46.04	$<$ 0.0001
	Grade 3	20.37	26.11	$<$ 0.0001
	Grade 4	1.26	0.51	$<$ 0.0001
Life birth (%)		19.13	14.13	$<$ 0.0001
Clinical pregnancy (%)		22.48	16.25	$<$ 0.0001
Multiple pregnancy (%)		1.12	2.09	0.107
Continuous variables were examined in relation to relevant outcomes using the students t test, while binary variables were initially examined using the Chi-square test. Cumulative ET (embryo transfer) refers to previous embryo transfer cycles included in the analysis. Syngamy check was the first evaluation performed 23-24 hr after insemination/ICSI when the embryos were assessed for the presence and number of cells (EC = early cleavage, NEBD = nuclear envelope breakdown, or 2PN = 2 pro-nuclei). Embryos were evaluated again on day 2 post insemination/ICSI. Number of cells, degree of fragmentation, and multinucleation (grading of embryos) were assessed at the day 2 check