Maternal Body Mass Index and Risk of Obstetric Anal Sphincter Injury

Table 1

Risk factors for any anal sphincter injury among primiparous women with singleton vaginal cephalic births at term. OR for each variable studied in relation to anal sphincter injury was adjusted for all other variables and for year of delivery.

	Reference population (𝑛)	Anal Sphincter injury (𝑛)	%	Adjusted OR	95% CI
Maternal body mass index
Unknown	50521	3351	6.6	0.96	0.93–1.00
Less than 18.5	12154	784	6.5	1.31	1.23–1.41
18.5–24.9	260037	17251	6.6	1.00	Reference
25–29.9	82638	5592	6.8	0.92	0.89–0.95
30–34.9	22733	1443	6.3	0.87	0.82–0.92
35–39.9	6362	371	5.8	0.77	0.69–0.85
40 or more	2037	123	6.0	0.75	0.65–0.90
Maternal age, y
Younger than 20	18327	395	2.1	0.35	0.32–0.39
20–24	98749	4276	4.2	0.65	0.63–0.68
25–29	162898	11483	6.6	1.00	Reference
30–34	118037	9716	7.6	1.13	1.10–1.16
35–39	33528	2702	7.5	1.07	1.03–1.12
40–44	4803	337	6.6	0.91	0.81–1.02
45 or older	140	6	4.1	0.52	0.22–12.3
Fetal head position
Occipitoanterior position	420006	27004	6.4	1.00	Reference
Occipitoposterior position	13955	1596	11.4	1.36	1.29–1.44
Other	2521	315	12.5	1.54	1.34–1.74
Instrumental delivery
No	368519	18170	4.9	1.00	Reference
Yes	67963	10745	15.8	2.74	2.67–2.81
Infant birth weight
Less than 3000	55550	1424	2.5	0.80	0.75–0.85
3000–3499	167059	7415	4.2	1.00	Reference
3500–3999	155742	11893	7.1	1.37	1.33–1.41
4000–4499	50373	6556	11.5	2.67	2.58–2.77
4500–4999	7117	1445	16.9	6.33	5.97–6.92
5000 or more	641	182	22.1	11.56	9.95–13.44