Results
Between 2005-2016, 988 988 vaginal deliveries were recorded in the MBR. The rate of OASIS was 3.5% (n=34 583). Table 1 shows maternal characteristics and delivery mode in relation to the risk of OASIS. There was an overall seven percent decrease of OASIS during 2013-2016 as compared to 2005-2008.
The risk of OASIS increased with maternal age as seen after adjusting for maternal characteristics (Table 2), with the highest risk in the age group 34-39 years, and inversely young maternal age of <20 years was significantly protective. When also adjusting for delivery mode, the association between OASIS and increasing maternal age was less pronounced, albeit still statistically significant.
Primiparous women were approximately three times as likely to attain OASIS as compared to multiparous women, and the strong association between primiparity and OASIS remained after adjustments.
In the crude model, high BMI (>30) had a considerably protective effect (cRR 0.80), but after initial adjustment for maternal characteristics and then delivery mode this association disappeared. After also adjusting for birth weight a negative association was again seen, but less pronounced (aRR 0.90). Low BMI (<18.5) showed a risk increase of OASIS as compared to women with normal BMI (cRR 1.12). Through all three steps of adjustment the risk increase among women with BMI <18.5 was maintained, and the positive association was even more noticeable (aRR 1.30). Maternal height showed an inverted relationship to risk of OASIS which became more evident after adjusting both for maternal characteristics, delivery mode and birth weight (height <155 cm: aRR 1.72, compared to height >175 cm: aRR 0.82).
Maternal smoking appeared to be negatively associated with risk of OASIS that remained after adjusted calculations, even when adjusting for birth weight (aRR 0.74). An association between increased risk of OASIS and higher maternal educational level was found, and adjustment for maternal characteristics, delivery mode, and birth weight only marginally altered the risk estimates. According to the crude estimates, no association between maternal country of birth and OASIS was found, but when adjustments were made, decreased risk (aRR 0.92) was found for women from Europe/ USA/ Canada/ Australia/ New Zeeland as compared to women from the Nordic countries, whereas women with other ethnic origin had a significant risk increase (aRR 1.31) as compared to women from the Nordic countries.
The strongest risk factor for OASIS was instrumental delivery, where the most significant risk increase was seen with the use of forceps (cRR 6.88). The prevalence of vacuum extraction (VE) was 8.3% and the use of forceps was rare at 0.2%.
Vaginal birth after previous caesarean section (VBAC) as a risk factor of OASIS was analysed separately and the risk of OASIS among secundiparous women with a vaginal birth following one previous caesarean section (2,755/29,927) was compared to the corresponding risk among primiparous women with vaginal births (25,576/429,727). This analysis showed a profound risk increase (cRR 1.60; 95% CI 1.54–1.66) of OASIS after previous CS. After adjustment for maternal characteristics the risk of OASIS remained significantly elevated (aRR 1.41; 95% CI 1.36–1.47). This risk increase could partly be explained by an increase in instrumental delivery, but after adjustment for delivery mode and birth weight the risk remained raised (aRR 1.24; 95% CI 1.20–1.29).