Is IVF/ICSI an Independent Risk Factor for Spontaneous Preterm Birth in Singletons? A Population-Based Cohort Study

Table 1

Characteristics of women included in the analysis, Slovenia, 2002–2015.

Characteristic	Spontaneous conception		IVF/ICSI		p value	All births
	n = 261 881		n = 5 837			n = 267 718
Maternal age (years)	n		N		<0.001	n
< 25	40150	15.3	79	1.4		40 229	15.0
25 – 29	97348	37.2	1006	17.2		98 354	36.7
30 – 34	87586	33.4	2496	42.8		90 082	33.6
35 – 39	31625	12.1	1782	30.5		33 407	12.5
40 ≥	5172	2.0	474	8.1		5 646	2.1
Maternal education					<0.001
Primary or less	11 174	4,3	197	3.4		11 371	4.2
Vocational	35 525	13,6	779	13.3		36 304	13.6
Secondary or professional	89 836	34,3	1 849	31.7		91 685	34.2
Tertiary	92 322	35,3	2 599	44.5		94 921	35.5
Not stated	33 024	12.6	413	7.1		33 437	12.5
Maternal BMI					<0.001
< 18.5 kg/m^2	13513	5.2	233	4.0		13 746	5.1
18.5–24.9 kg/m^2	180295	68.8	3891	66.7		184 186	68.8
25–29.9 kg/m^2	46616	17.8	1137	19.5		47 753	17.8
30 kg/m^2≥	21362	8.2	574	9.8		21 936	8.2
Missing data	95	0.0	2	0.0		97	0,0
Parity					<0.001
0	127684	48.8	4345	74.4		132 029	49.3
1	97868	37.4	1353	23.2		99 221	37.1
2≥	36329	13.9	139	2.4		36 468	13.6
Smoking during pregnancy					<0.001
No	233049	89.0	5368	92.0		238 417	89.1
Yes	28832	11.0	469	8.0		29 301	10.9
Cervical excision procedure					<0.001
No	255164	97.4	5566	95.4		260 730	97.4
Cold-knife	2570	1.0	119	2.0		2 689	1.0
Other – LLETZ	4147	1.6	152	2.6		4 299	1.6
Resection of uterine septum					<0.001
No	252770	96.5	4753	81.4		257 523	96.2
Yes	9111	3.5	1084	18.6		10 195	3.8
Other uterine malformation					<0.001
No	260118	99.3	5744	98.4		265 862	99.3
Yes	1763	0.7	93	1.6		1 856	0.7
Bleeding in pregnancy					<0.001
No	243991	93.2	5035	86.3		249 026	93.0
Yes	17890	6.8	802	13.7		18 692	7.0
Preeclampsia / HELLP					<0.001
No	257528	98.3	5685	97.4		263 213	98.3
Yes	4353	1.7	152	2.6		4 505	1.7
Maternal heart, renal or pulmonary illness					0,181
No	257136	98.2	5745	98.4		262 881	98.2
Yes	4745	1.8	92	1.6		4 837	1.8
Gestational age at birth (weeks)					<0.001
< 28	761	0.3	56	1.0		817	0.3
28 to 31 and 6/7	950	0.4	47	0.8		997	0.4
32 to 33 and 6/7	1229	0.5	48	0.8		1 277	0.5
34 to 36 and 6/7	8216	3.1	298	5.1		8 514	3.2
37 ≥	250725	95.7	5388	92.3		256 113	95.7

BMI: body mass index.
LLETZ: large loop excision of transformation zone.
HELLP syndrome: syndrome with hemolysis, elevated liver enzymes, and low platelet count.