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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.
There is an overall increase in plasma, red blood cells (RBCs) and total blood volume. Plasma volume increases by 15% during the first trimester; accelerates in the second trimester; peaks at around 32 weeks, reaching up to 50% above non-pregnant levels; and stays elevated until term. It returns to non-pregnant levels by 6 days post-delivery. There is often a sharp rise of up to 1 litre in plasma volume within the maternal circulation at 24 hours after delivery.