In fetuses with vertex cephalic presentation, the fetal head position identifies the relationship between fetal occiput and maternal pelvis.

 

Ultrasound determination of fetal head position

Abstract: In fetuses with vertex cephalic presentation, the fetal head position identifies the relationship between fetal occiput and maternal pelvis. Traditionally, fetal head position is determined during labor by digital examination, through the palpation of the fetal sutures and fontanels. However, many studies demonstrated that clinical diagnosis of the fetal head position is subjective, inaccurate and poorly reproducible. Intrapartum ultrasound has demonstrated higher reproducibility and accuracy in determination of fetal head position in comparison to digital examination, independently of the level of expertise of operators. The sonographic assessment of fetal head position is best performed by transabdominal ultrasound. The scan begins with the transducer placed transversely at the umbilical level on the maternal abdomen to determine the fetal spine position. The transducer is then moved downwards until it reaches the suprapubic region and visualizes the fetal head. The fetal occiput position can be determined by visualizing various landmarks: the fetal orbits, in cases of occiput posterior position, and the fetal occiput, the midline of fetal brain, and/or the cerebellum for the other positions. If the fetal head is particularly low in the birth canal, a transperineal approach may be helpful for the visualization of fetal intracranial structures and thus for determination of the fetal head position.

Keywords: Fetal head position, intrapartum ultrasound, occiput position, transabdominal ultrasound, transperineal ultrasound

Authors: Elena Brunelli1,2, MD, Aly Youssef1,2, MD, PhD

1- Obstetric Unit, IRCCS Azienda Ospedaliero-Universitaria di Bologna
2 - Alma Mater Studiorum - Università di Bologna

Reviewers: Karen Fung-Kee-Fung

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Introduction

Cephalic presentation is present when the fetal head is the lowermost fetal structure in the maternal pelvis. In most cases of cephalic presentations, the fetal neck is fully flexed resulting in a vertex presentation. In vertex presentation, the denominator (i.e. the part of the fetus used to define the position inside the maternal pelvis) is the occiput. The fetal head position in vertex presentation is defined by the relationship between fetal occiput and maternal pelvis. The fetal occiput may be oriented towards the anterior, posterior, or lateral side of the maternal pelvis. Thus, the variations in fetal cephalic presentation can be described comparing the position of the occiput to a clockface: 


-    Occiput positions > 09.30 h and < 02.30 h identify occiput anterior (OA) positions,
-    Occiput positions > 03.30 h and < 08.30 h identify occiput posterior (OP) positions,
-    Occiput positions ≥ 02.30 h and ≤ 03.30 h identify left occiput transverse (OT) positions,
-    Occiput positions ≥ 08.30 h and ≤ 09.30 h right OT positions(1-3).


In labor, any fetal position different from OA is considered a malposition (4), although in most cases, especially early in labor, it is a transient finding. 


Traditionally, fetal head position is determined during labor, after cervical dilatation, by digital examination, through the palpation of the fetal sutures and fontanels. However, current evidence demonstrated that clinical diagnosis of fetal head position is subjective and inaccurate.(5) Intrapartum ultrasound is highly reproducible and accurate in the determination of fetal head position regardless of the level of expertise of operators (6). In such context, some international guidelines released specific recommendations on the use of ultrasound in labor ward. In particular, the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) recommends the use of sonography as an adjunctive method and not as a substitute of digital vaginal examination, in particularly in difficult situations such as delayed or arrested labor and before performing an operative vaginal delivery (2). The Royal College of Obstetricians and Gynaecologists (RCOG) recommends the use of ultrasound to assess the fetal head position prior to assisted vaginal birth where uncertainty exists following clinical examination. (7)

Technique

The sonographic assessment of fetal head position is ideally performed by transabdominal ultrasound. A systematic approach allows a standardized examination. This should include the following steps: (8) (https://www.youtube.com/watch?v=nMLvEcHE17g
1.    Placing the ultrasound transducer transversely on the maternal abdomen at level of the maternal umbilicus, visualizing the fetal chest or abdomen and thus allowing the determination the fetal spine position in the axial plane.
2.    The transducer is then moved downwards on the maternal abdomen to the suprapubic region, visualizing the fetal head. In case of fetal occiput posterior position, the diagnosis is already easy on the transverse plane as the operator will see the fetal orbits anteriorly facing the transducer. For other positions, visualizing the fetal intracranial structures help to accurately define the occiput position according to the above mentioned clockface classification. These structures include the midline cerebral echo, fetal thalami and cerebellum.
3.    Lastly, a clockwise rotation of the transducer on the suprapubic region, thus obtaining the sagittal view can help in visualizing the transition from the cervical spine to the fetal occiput in cases of occiput anterior position. In addition, the sagittal view is useful in the assessment of fetal head flexion both in occiput anterior and posterior positions, thus helping in the diagnosis of fetal cephalic malpresentations.(9)
If the fetal head is particularly low in the birth canal, a transperineal approach can be useful after the transabdominal scan for the visualization of fetal head structures and thus for determination of fetal head position. For this purpose, the visualization of the choroid plexi, which diverge towards the occiput, can be of help.(10) 

References

1.    Akmal S, Tsoi E, Nicolaides KH. Intrapartum sonography to determine fetal occipital position: interobserver agreement. Ultrasound Obstet Gynecol. 2004;24(4):421-4.
2.    Ghi T, Eggebo T, Lees C, Kalache K, Rozenberg P, Youssef A, et al. ISUOG Practice Guidelines: intrapartum ultrasound. Ultrasound Obstet Gynecol. 2018;52(1):128-39.
3.    Ghi T, Dall'Asta A. Sonographic evaluation of the fetal head position and attitude during labor. Am J Obstet Gynecol. 2022.
4.    Cunningham FG, Leveno KJ, Bloom SL, et al. Labor and delivery. In: Cunningham FG, Leveno KJ, Bloom SL, et al, eds. Williams obstetrics, 24th ed. New York, NY: McGraw-Hill;2014;p. 433–586.
5.    Akmal S, Kametas N, Tsoi E, Hargreaves C, Nicolaides KH. Comparison of transvaginal digital examination with intrapartum sonography to determine fetal head position before instrumental delivery. Ultrasound Obstet Gynecol. 2003;21(5):437-40.
6.    Malvasi A, Tinelli A, Barbera A, Eggebo TM, Mynbaev OA, Bochicchio M, et al. Occiput posterior position diagnosis: vaginal examination or intrapartum sonography? A clinical review. J Matern Fetal Neonatal Med. 2014;27(5):520-6.
7.    Murphy DJ, Strachan BK, Bahl R, RCOG. Assisted Vaginal Birth: Green-top Guideline No. 26. BJOG. 2020;127(9):e70-e112.
8.    Youssef A, Ghi T, Pilu G. How to perform ultrasound in labor: assessment of fetal occiput position. Ultrasound Obstet Gynecol. 2013;41(4):476-8.
9.    Bellussi F, Ghi T, Youssef A, Salsi G, Giorgetta F, Parma D, et al. The use of intrapartum ultrasound to diagnose malpositions and cephalic malpresentations. Am J Obstet Gynecol. 2017;217(6):633-41.
10.    Ghi T, Bellussi F, Eggebo T, Tondi F, Pacella G, Salsi G, et al. Sonographic assessment of fetal occiput position during the second stage of labor: how reliable is the transperineal approach? J Matern Fetal Neonatal Med. 2015;28(16):1985-8.

The article should be cited as: Brunelli E, Youssef A: Ultrasound determination of fetal head position in labor, Visual Encyclopedia of Ultrasound in Obstetric and Gynecology, www.isuog.org, April 2024. 


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