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Intrapartum assessment of fetal occiput position is of major relevance in the management of the second stage of labor. Indeed, fetal head malposition is associated with higher risk of operative delivery and maternal and perinatal morbidity. Although sonographic assessment of fetal occiput position alone may not predict the mode of delivery accurately, it may be of great value in certain clinical situations, such as before instrumental delivery. Incorrect determination of fetal head position may lead to inappropriate vacuum or forceps blade placement, increasing the risk of both fetal injury and failure of the procedure. Failed instrumental delivery followed by Cesarean section is associated with an increased decision-to-delivery interval and with a greatly increased risk of fetal trauma. In addition, some preventative measures can be undertaken in cases of posterior occiput, such as mediolateral episiotomy, which has been found to reduce significantly the risk of anal sphincter damage in these cases.
Traditionally, clinicians determine fetal head position by palpating the sutures and fontanels. However, clinical examination is highly subjective and inaccurate, with a rate of error ranging from 30% to 70%. Accuracy is particularly affected in cases of malposition, yet this is when intervention is more likely to be required. Sometimes, clinical evaluation is completely unable to locate fetal head position.
Various studies have demonstrated the higher accuracy and reproducibility of ultrasonographic determination of fetal occiput position as compared with traditional digital examination. There are good reasons to support the use of ultrasound as the gold standard in the assessment of fetal position, particularly when the result of the clinical examination is uncertain, and possibly prior to any instrumental delivery. Many approaches have been suggested to determine fetal occiput position with ultrasound: transperineal, transvaginal and transabdominal ones. Although the best approach is still unknown, the transabdominal one is the most widely used and practical approach. The aim of this article and of the accompanying figures, slides and videoclips is to provide a simple and practical guide to determining accurately and rapidly the fetal occiput position when this information is judged to be clinically relevant.
Placing the ultrasound probe horizontally on the maternal abdomen, a transverse view of the fetal trunk is obtained at the level of the fetal upper abdomen or the four-chamber view of the heart. The position of the fetal spine is then determined. The ultrasound transducer is then moved downwards until the maternal suprapubic region is reached, visualizing the fetal head. The landmarks depicting fetal occiput position are the midline cerebral echo (Figure 1), fetal thalami (Figure 1a) and cerebellum (Figure 1b) for occiput transverse and anterior positions, and the fetal orbits for occiput posterior position (Figure 2). In cases of occiput anterior positions, the fetal spine can be alternatively or additionally demonstrated in the sagittal plane and traced from the fetal thorax to the occiput (Figure 3). The sagittal plane can be useful, especially in fetuses with a deeply engaged head, in which identification of the intracranial landmarks can be challenging, and can occasionally help in the diagnosis of some fetal head malpositions.
Using these landmarks, the fetal occiput position is traditionally classified as one of the following eight categories: occiput anterior (OA), left occiput anterior (LOA), left occiput transverse (LOT), left occiput posterior (LOP), occiput posterior (OP), right occiput posterior (ROP), right occiput transverse (ROT), right occiput anterior (ROA). Alternatively, some authors have suggested classification of fetal head position by angles, categorizing it into 24 sections each of 15°, with fetal head position defined in accordance with the section occupied by the occiput.