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Postpartum haemorrhage (PPH) is the one of the leading causes of maternal morbidity and mortality worldwide, with the World Health Organization (WHO) estimating 140 000 maternal deaths every year due to PPH . Between 1990 and 2015 there has been a decrease in the number of women dying in the perinatal period (all-cause mortality) with 385 death per 100 000 live births in 1990 decreasing to 216 per 100 000 live births; however, a significant gap in the mortality in developed and developing countries persists . In 2015, the maternal mortality rate in low-income countries was 479 per 100 000 live births versus 13 per 100 000 births in high-income countries . In the UK, PPH is the third most common direct cause of maternal death and was attributable for 0.78 deaths per 100 000 maternities between 2014 and 2016 . Maternal death, however, can only be seen as the tip of the iceberg, with 492 cases of morbidities reported in Scotland during 2012 and many other unreported cases of morbidities such as post-traumatic stress syndrome .
Placental adhesive spectrum disorders (PASD) are on the increase. Histologically, the placenta may be adherent to the myometrium without intervening decidua (acreta), invade the myometrium (increta) and/or extend beyond the myometrium and seen via the serosa of the uterus or invade into adjacent tissues like the bladder or parametrium (percreta). Since there are difficulties in defining each entity by ultrasound or by histology and also due to the possibility of histology showing different degrees of invasion in the same case, PASD is the term now commonly used and the previous terminology of morbidly adherent placenta is no longer used. The main contributor towards PSAD is previous caesarean section (CS). With the global increase in CS, the incidence of PASD and related morbidity and mortality is on the increase.
Operative vaginal birth rate has been stable in the United Kingdom at about 10%–13% [3, 4]. The caesarean section at full dilation as an alternative approach to instrument delivery has a high maternal and neonatal morbidity, but failed instrument vaginal deliveries (FID), which lead to caesarean sections, are associated with potentially serious maternal and fetal complications such as angular tears, postpartum haemorrhage, difficulty in delivery of the fetal head, fetal ischaemic-hypoxic injuries, birth trauma and perinatal deaths. Therefore, it is important to identify factors which can help to predict successful operative vaginal delivery.
There are various established risk factors which increase the chances of instrument delivery, including advanced maternal age, high body mass index (BMI; >30), high birth weight (>4.0 kg) and epidural analgesia.