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).