Table 1.

Baseline characteristics before and after matching.

	Unmatched			Matched
	Control n = 3973	Case n = 70	P value	Control n = 195	Case n = 65	P value
Ages (years)	31.0 (29.0, 35.0)	32.0 (29.8, 36.0)	0.374	32.0 (29.0, 35.0)	31.0 (29.0, 34.0)	0.718
BMI (kg/m2)	21.9 (21.1, 22.4)	21.2 (19.4, 23.2)	0.034	21.9 (20.8, 22.4)	20.6 (19.1, 23.3)	0.064
ART times	1 (1, 1)	1 (1, 2)	0.021	1 (1, 2)	1 (1, 2)	0.477
Infertility durations (years)	3 (1, 4)	3 (1, 4)	0.955	3 (1, 4)	3 (1, 4)	0.845
Infertility types			0.844			0.255
Primary (%)	66.0 (2623)	67.1 (47)		64.6 (126)	72.3 (47)
Secondary (%)	34.0 (1350)	32.9 (23)		35.4 (69)	27.7 (18)
Infertility causes			0.340			0.731
Female factor (%)	60.1 (2388)	57.1 (40)		53.8 (105)	53.8 (35)
male factor (%)	13.1 (520)	8.6 (6)		6.2 (12)	9.2 (6)
both female and male factors (%)	21.5 (853)	30.0 (21)		36.9 (72)	32.3 (21)
unexplained (%)	5.3 (212)	4.3 (3)		3.1 (6)	4.6 (3)
COS protocols			0.637			0.447
GnRH agonist protocol (%)	37.0 (1469)	34.3 (24)		37.9 (74)	36.9 (24)
GnRH antagonist protocol (%)	52.2 (2074)	51.4 (36)		47.2 (92)	53.8 (35)
Other protocola (%)	10.8 (430)	14.3 (10)		14.9 (29)	9.2 (6)
Operation types			0.159			0.830
IVF (%)	61.7 (2450)	54.3 (38)		54.4 (106)	53.8 (35)
ICSI (%)	34.0 (1353)	44.3 (31)		42.1 (82)	44.6 (29)
R-ICSI (%)	4.3 (170)	1.4 (1)		3.6 (7)	1.5 (1)

Note: BMI, body mass index; ART, assisted reproductive technique; COS, controlled ovarian stimulation; GnRH, gonadotrophin releasing hormone; IVF, in vitro fertilization; ICSI, intracytoplasmic sperm injection; R-ICSI, rescue intracytoplasmic sperm injection;

Continuous variables were presented as the median (first quartile, third quartile).

Categorical variables were presented as% (n).

P < 0.05 was considered statistically significant.

^aOther protocol: including mild stimulation and luteal phase stimulation protocols.