Table 1 Clinical characteristics of the study population

Maternal age (year)

29 ± 6

30.5 ± 5.7

28.1 ± 4.9

Adolescent pregnancy ≤ 19 year

2 (4.9%)

1 (5.9%)

1 (6.7%)

Advanced maternal age ≥ 35 year

7 (17.1%)

4 (23.5%)

2 (13.3%)

Parity

Nulliparous

13 (31.7%)

8 (47.1%)

7 (46.7%)

Multiparous

28 (68.3%)

9 (52.9%)

8 (53.3%)

Miscarriage

1 (0–4)

1 (0–2)

1 (0–2)

2nd trimester pregnancy loss history

2.3 ± 0.5 ^a,c

0.6 ± 0.5 ^c

0.5 ± 0.4 ^a

Preterm birth history

2.4 ± 0.6 ^a,c

1.4 ± 0.6 ^b,c

0.6 ± 0.5 ^a,b

Smoking

9 (21.9%)

5 (29.4%)

5 (33.3%)

Gestational age at cerclage (weeks)

16.1 ± 2.7 ^a

18.1 ± 3.5 ^b

20.6 ± 1.5 ^a,b

Conization history

2 (4.9%)

2 (11.8%)

3 (20%)

Cervical length (mm)

27.3 ± 6.9 ^a,c

17.1 ± 3.4 ^c

17.2 ± 6.3 ^a