National large-scale initiatives have been established to address spontaneous preterm birth. ^,,,,, Progress is challenging due to the varied and multifactorial nature of spontaneous preterm birth, and a clear understanding of the etiologies and mechanisms is needed.

Cesarean section (CS) birth in labor, particularly at full cervical dilatation, has been associated with subsequent spontaneous preterm birth. ^,,,,,,,,,,,, This is especially important considering globally rising CS rates, including at full dilatation. ^,, Furthermore, preterm birth after CS has been associated with higher recurrence rates, and the use of preventive cervical cerclage may be less successful. ^,

Birthing factors reported to be associated with subsequent spontaneous preterm birth include CS in the first or second labor stages, cervical dilatation, second-stage duration, fetal station, instrumental birth (including attempt), uterine incision type, fetal disimpaction, CS method, uterine wall thickness, method of uterine closure, and unintended uterine incision extension. ^{,,,,,,,,,,,,,,}

The association of uterine incision extension and preterm birth represents the newest evidence, supporting the early hypothesis linking structural cervical damage to harm. To date, this association is limited to one study demonstrating an impact on total preterm birth <34, but not <37, and limited data at <32 and <28 weeks. It is also unclear if uterine incision extension is independent of cervical dilatation at CS. Understanding the impact of uterine incision extension has implications for future pregnancies and management, including ultrasound cervical length surveillance and cervical injury detection and consideration of effective preventative treatment. ^,,,,

We have used a comprehensive prospectively collected obstetric dataset from a single tertiary center to assess the association between mode of birth and subsequent spontaneous preterm birth. We specifically aimed to determine how risk was affected by increasing cervical dilatation and uterine incision extension, appropriately controlling for potential confounders.

This observational cohort study conducted at one tertiary hospital included pregnancies with 2 consecutive live births, where the first (index) was a term cephalic singleton and the second the first subsequent live birth from 20 weeks, both births occurring between January 1, 2008 and December 31, 2021. Cases were ascertained and data extracted from the hospital maternity database, a high-quality resource used for research and reporting. ^,

Mode of birth was classified as spontaneous vaginal, operative vaginal using forceps or ventouse (sequential instrumental births included with the successful method), and CS. If the index birth was by CS, clinical notes were reviewed to confirm cervical dilatation and extract information on uterine incision extensions. CS and cervical dilatation were grouped for analysis as prelabor CS, in-labor CS with cervical dilatation ≤4 cm, 5 to 7 cm, 8 to 9 cm, and full (10 cm) dilatation.

Uterine incision extension was recorded as present or absent, and by site. Intentional surgical extension of the uterotomy (but not initial incision) to a “U”, “smiley”, “J”, or “T” incision to facilitate birth was included. Major extension was defined as involving the cervix, vagina, ureter, bladder, broad ligament, or uterine artery. Where the operative report did not detail an extension, it was noted as absent. The veracity of uterine incision extension data was confirmed by an audit of 100 records, 50 at full dilatation from this study and 50 from a previous cohort from the same center, with data extraction by an experienced clinician blinded to cohort, undertaken as the rate of extension was comparatively low in this study. This audit’s findings were consistent with the original data extraction.

Assistance to deliver the fetal head was defined as change of surgeon, vaginal assistance (‘push up’), breech extraction, shoulder traction, or administration of tocolytic drug.

If the subsequent birth occurred preterm (<37 weeks), this was defined as either spontaneous or provider-initiated based on available routinely collected data and clinical note review. Spontaneous preterm birth included birth after spontaneous onset of labor, including following preterm premature rupture of the membranes (even if birth followed induction of labor or emergency CS). Provider-initiated preterm birth included birth following induction of labor or emergency or elective CS where these were not a consequence of preterm premature rupture of the membranes. These definitions are consistent with the recently defined core outcome set for preterm birth in Aotearoa New Zealand.

This study is reported in accordance with STengthening the Reporting of OBservational studies in Epidemiology (STROBE) guidance.

Ethics approval was obtained from the Auckland Health Research Ethics Committee (AH25293) and included a waiver of consent for use of patient records.

Data management and analysis were undertaken using STATA v18.0. Frequency data are presented as proportions and compared using chi-squared and Fisher’s exact tests. Continuous data are presented as mean (standard deviation) and median (interquartile range) and compared using Student’s t test or analysis of variance and Kruskal-Wallis tests, as appropriate for parametric and nonparametric data.

The baseline comparison group was vaginal birth in the index pregnancy. Outcome data are presented as crude and adjusted relative risks (RRs) with 95% confidence intervals (CIs). Spontaneous preterm birth is presented at thresholds <37, <32, and <28 weeks. Prioritized maternal ethnicity is presented according to the New Zealand Ministry of Health recommendations with Māori, the indigenous people of New Zealand, as the referent group. Spontaneous preterm birth among Māori is presented by cervical dilatation compared to vaginal birth or prelabor CS.

Multivariable models were developed to determine the independent association between mode of index birth, cervical dilatation, and uterine incision extension and the primary outcome of subsequent spontaneous preterm birth. Variables were included in models according to their association with the predictor and outcome variables. Full models, where numbers of preterm births allow, or reduced models, excluding variables not significantly associated with the outcome and not confounders (using the rule that their exclusion did not change the coefficient for spontaneous preterm birth by more than 10%), are presented. Socioeconomic deprivation quintile was excluded from the final model as it was neither significant nor a confounder and was missing for 3% of births. Models exploring uterine incision extensions are limited to CS index births, with prelabor CS as the referent group.

Multivariable modeling was undertaken using generalized linear models outputting RRs and CIs. Missing data are presented in tables and not imputed. Statistical significance was determined at P <.05.

20,499 pairs of live births, where the index birth was a term cephalic singleton, were identified. Of these, 20,031 (97.7%) were consecutive births ( Table 1 ). Of the index births, 6058 (30.0%) were CS including 2139 (10.4%) prelabor, 1302 (6.4%) in-labor at 0 to 4 cm cervical dilatation, 943 (4.6%) at 5 to 7 cm, 743 (3.6%) at 8 to 9 cm, and 931 (4.5%) at full cervical dilatation ( Table 2 ).

Table 1

Demographic and clinical factors of index and subsequent birth by mode of index birth

Demographic and clinical factors	Total		Vaginal birth						Cesarean birth										P value
	n	20,499	Unassisted vaginal birth		Ventouse		Forceps		Prelabor CS		In labor 0–4 cm		In labor 5–7 cm		In labor 8–9 cm		In labor 10 cm
			n	10,309	n	2708	n	1424	n	2139	n	1302	n	943	n	743	n	931
	n/% Median/IQR mean/SD		n	%	n	%	n	%	n	%	n	%	n	%	n	%	n	%
Ethnicity
Māori	1255	6.1	830	8.1	107	4.0	45	3.2	87	4.1	60	4.6	57	6.0	29	3.9	40	4.3
Pacific people	2587	12.6	1926	18.7	142	5.2	71	5.0	132	6.2	119	9.1	80	8.5	59	7.9	58	6.2
Indian	1409	6.9	598	5.8	226	8.3	110	7.7	126	5.9	131	10.1	74	7.8	66	8.9	78	8.4
Other Asian	4497	21.9	2402	23.3	627	23.2	285	20.0	327	15.3	297	22.8	207	22.0	176	23.7	176	18.9
NZ European	7720	37.7	3208	31.1	1164	43.0	692	48.6	1085	50.7	507	38.9	360	38.2	417	56.1	417	44.8
Other European	2203	10.7	928	9.0	339	12.5	174	12.2	297	13.9	130	10.0	115	12.2	123	16.6	123	13.2
MELAA/other/not stated	828	4.0	417	4.0	103	3.8	47	3.3	85	4.0	58	4.5	50	5.3	39	5.2	39	4.2	<.001
Birthweight index baby (g), mean(SD)	3437	474	3399	461	3397	435	3457	445	3442	525	3449	521	3573	487	3611	480	3628	452	.0001
Gestation at index birth (wks)
37	1432	7.0	804	7.8	138	5.1	68	4.8	217	10.1	103	7.9	28	3.0	26	3.5	48	5.2
38	3908	19.1	1971	19.1	466	17.2	212	14.9	695	32.5	225	17.3	121	12.8	88	11.8	130	14.0
39	5708	27.8	2954	28.7	727	26.8	381	26.8	743	34.7	286	22.0	224	23.8	176	23.7	217	23.3
40	5779	28.2	2976	28.9	828	30.6	482	33.8	278	13.0	362	27.8	281	29.8	249	33.5	323	34.7
41	3286	16.0	1431	13.9	511	18.9	249	17.5	186	8.7	275	21.1	267	28.3	183	24.6	184	19.8
42	380	1.9	170	1.6	37	1.4	32	2.2	20	0.9	49	3.8	22	2.3	21	2.8	29	3.1
43	6	0.0	3	0.0	1	0.0	0	0.0	0	0.0	2	0.2	0	0.0	0	0.0	0	0.0	<.001
Parity at index birth
Nulliparous	16,579	80.9	7474	72.5	2559	94.5	1345	94.5	1571	73.4	1200	92.2	858	91.0	695	93.5	877	94.2
Multiparous	3920	19.1	2835	27.5	149	5.5	79	5.5	568	26.6	102	7.8	85	9.0	48	6.5	54	5.8	<.001
Change in parity from index to subsequent pregnancy
1	20,031	97.7	9979	96.8	2663	98.3	1397	98.1	2122	99.2	1285	98.7	928	98.4	738	99.3	919	98.7
2	395	1.9	269	2.6	41	1.5	24	1.7	17	0.8	15	1.2	14	1.5	4	0.5	11	1.2
>2	73	0.4	61	0.6	4	0.1	3	0.2	0	0.0	2	0.2	1	0.1	1	0.1	1	0.1	<.001
Age at subsequent birth (y)
<20	182	0.9	148	1.4	13	0.5	7	0.5	2	0.1	2	0.2	5	0.5	2	0.3	3	0.3
20–34	13,224	64.5	7305	70.9	1715	63.3	818	57.4	999	46.7	794	61.0	580	61.5	453	61.0	560	60.2
≥35	7093	34.6	2856	27.7	980	36.2	599	42.1	1138	53.2	506	38.9	358	38.0	288	38.8	368	39.5	<.001
Socioeconomic quintile subsequent birth
1 (least deprived)	3968	19.4	1654	16.0	568	21.0	330	23.2	581	27.2	292	22.4	199	21.1	157	21.1	187	20.1
2	4124	20.1	1828	17.7	618	22.8	312	21.9	517	24.2	297	22.8	196	20.8	153	20.6	203	21.8
3	3939	19.2	1935	18.8	562	20.8	277	19.5	395	18.5	246	18.9	191	20.3	143	19.2	190	20.4
4	4070	19.9	2194	21.3	501	18.5	263	18.5	337	15.8	232	17.8	189	20.0	159	21.4	195	20.9
5 (most deprived)	3717	18.1	2304	22.3	389	14.4	195	13.7	238	11.1	205	15.7	142	15.1	118	15.9	126	13.5	<.001
missing data	681	3.3	394	3.8	69	2.5	48	3.4	71	3.3	30	2.3	26	2.8	13	1.7	30	3.2
Interpregnancy interval in days (median [IQR])	891	678, 1228	887	661, 1255	895	693, 1173	877	683, 1202	888	690, 1209	903	700, 1182	898	690, 1254	905	705, 1207	916	710, 1238	.22
Gestation at subsequent birth (wks)
<25	32	0.2	13	0.1	4	0.1	0	0.0	1	0.0	1	0.1	3	0.3	3	0.4	7	0.8	<.001
<28	58	0.3	20	0.2	8	0.3	1	0.1	5	0.2	3	0.2	5	0.5	4	0.5	12	1.3	<.001
<32	137	0.7	58	0.6	15	0.6	2	0.1	14	0.7	10	0.8	8	0.8	10	1.3	20	2.1	<.001
<37	1022	5.0	492	4.8	103	3.8	61	4.3	138	6.5	66	5.1	52	5.5	49	6.6	61	6.6	<.001
≥37	19,477	95.0	9817	95.2	2605	96.2	1363	95.7	2001	93.5	1236	94.9	891	94.5	694	93.4	870	93.4	<.001
BMI at subsequent birth (n=20,337) (median [IQR])	24	21, 28	24	21, 29	23	21, 26	23	21, 27	24	21, 28	24	22, 29	24	22, 28	24	22, 28	24	22, 28	.0001
Smoking at booking subsequent birth (n=20,489)
	963	4.7	738	7.2	62	2.3	24	1.7	56	2.6	30	2.3	24	2.5	12	1.6	17	1.8	<.001

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