The association among fetal head position, fetal head rotation and descent during the progress of labor: a clinical study of an ultrasound-based longitudinal cohort study in nulliparous women

Introduction

Fetal head position is a vital factor during labor. The continuous changing of fetal head position is an important indicator for the labor progress, and influencing decision-making for clinicians, especially for the prolonged or stagnant second stage of labor [1], 2]. Since Calkins et al. [3] reported his observation of the fetal head rotation during the labor, this phenomenon has been widely recognized in the clinical diagnosis and treatments. The determination of fetal head rotation was based on vaginal examination by obstetricians [4]. This method was simple and feasible, whereas, its accuracy had a great dependency on the experience of the performer with subtle subjective bias. The errors were inevitable, especially when fetal bone sutures overlap significantly. It has been postulated that the error rate of fetal head position via vaginal examination was as high as 68.5 % in the first stage of labor [5]. Besides, the frequent vaginal examination is a risk of amniotic cavity infection and we should avoid vaginal examinations by multiple healthcare professionals.

An increasing amount of evidence considered ultrasound might be a potent tool for determining fetal indexes during labor, not only for its objective and accuracy, but also for its learning course is much shorter than clinical examination [6], 7]. To date, the role of fetal head rotation via ultrasound on labor process and clinical decision making has not been clearly elucidated. In 2021, Hjartardóttir et al. [8] conducted a research on the fetal head rotation during the active phase of labor in primiparous women, and found that the proportion of occipital posterior position in the active phase was highest, reaching to 52.5 %. Recently, clinical practice guideline and recommendation by the World Association of Perinatal Medicine (WAPM) and the Perinatal Medicine Foundation (PMF) indicated that obstetricians could be competent in basic ultrasound techniques in order to recognize acute clinical emergencies as well as to improve the recognition of labor arrest [9].

Given the differences of pelvic types among Chinese and European women, it’s legitimate to pustulate that process of fetal head rotation during labor might be quite different in Chinese population. In this study, we aimed to characterize the process of fetal head rotation and its effects on the labor as well as pregnancy outcomes.

Subjects and methods

Ultrasound measurement

Ultrasound measurements were performed at the latency stage (1 cm ≤cervical dilation <6 cm), active stage (cervical dilation ≥6 cm), and the first 15 min at the second stage of labor. Fetal head position, AOP and HPD were measured via transvaginal or/and transabdominal ultrasound. Fetal head position was determined using multiple indexes (including fetal spine, orbit, nasal bridge, eyeball, midline echo of the brain, and midline angle of the brain) as the previously published literature indicated [9]. In brief, fetal head position was divided into four groups according to the position of fetal brain midline: occiput anterior (OA: fetal brain midline at the area from 09:30 to 02:30 clockwise), left occiput transverse (LOT: fetal brain midline at the area from 02:30 to 03:30 clockwise), occiput posterior (OP: fetal brain midline at the area from 03:30 to 08:30 clockwise), and right occiput transverse (ROT: fetal brain midline at the area from 08:30 to 09:30 clockwise). The diagrammatic sketch was described previously and displayed in Figure 1.

Figure 1:

Schematic diagram of the clock dial for fetal position.

The standard AOP measurement should display the binary structure of the pubic symphysis with the maximum diameter and the lowest point of the skull cortex in the same section (Figure 2) [10], [11], [12]. Participants were encouraged to empty their bladder before measurement, and excessive residual urine detected by ultrasound would be catheterized if necessary. Ultrasound probe was placed at the pubic symphysis and pressed tightly against the vaginal opening at a longitudinal angle to obtain a standard section. A midsagittal image was obtained at the uterine contractions. Each patient was measured three times to take the average value. All the measurements were performed by two senior physicians, and images did not meet the standard were discarded.

Figure 2:

The standard image of angle of progression.

Results

The correlation of uterine orifice dilation and fetal head position

After eliminating the unsatisfied images, 102 valid data were obtained. A total of 97 data were obtained from 41 vaginal deliveries. Thirty six were at the latency stage, 32 were at the active stage and 29 were at the second stage of labor. According to the fetal head position detected at the uterine orifice dilation, occiput transverse position occupied the most part, with 38.9 % LOT and 19.4 % ROT (Table 2). At the active stage, more patients were presented with OA position and the percentage of OT position was decreased. As the uterine orifice dilated, AOP was increasing. While the HPD displayed a decreasing incline. The correlation of fetal head position, uterine orifice dilation, and the progress of labor were displayed in Figure 3.

Table 2:

Dilation of cervix and fetal head position in 41 cases of vaginal delivery.

	Dilation of cervix, cm	n	OA	LOT	OP	ROT	AOP°	HPD, cm
Latency stage	1	1	–	–	–	1	108.7	4.5
	2	18	4	7	3	4	108.9 (93.8–121.9)	4.4 (3.2–5.6)
	3	10	3	2	2	4	108.9 (93.4–129.7)	4.4 (3.6–5.4)
	4	4	1	1	1	1	121.7 (106.3–137.7)	3.3 (3.2–3.4)
	5	3	1	2	–	–	115.8 (104.7–124.5)	4.0 (3.75–4.4)
	Total	36	9 (25)	14 (38.9)	6 (16.7)	7 (19.4)
Active stage	6	13	4	8	1	–	122.2 (100.3–148.8)	3.64 (2.13–4.68)
	7	9	3	2	2	2	122.7 (98.8–141.6)	3.36 (2.75–4.06)
	8	9	4	1	2	2	128.8 (112.8–143.7)	3.0 (1.51–3.85)
	9	1	–	–	1	–	152.1	2.55
	Total	32	11 (34.3)	11 (34.3)	6 (18.6)	4 (12.5)
Full-cervical-dilatation	10	29	22 (75.9)	2 (6.9)	2 (6.9)	3 (10.3)	140.2 (113.7–169.8)	2.6 (1.14–4.92)

1. Data are expressed as quantity (percentage) and mean (minimum-maximum).

Figure 3:

The relationship between fetal position, delivery time, and dilation of the cervix. The x-axis represents the delivery time, which is 0 and calculated in reverse from the delivery time. The y-axis represents dilation of the cervix (cm).

Discussion

This study aims to conduct an observational study characterizing the role of fetal head rotation of term postpartum primiparous women via ultrasound. In this study, we found that occipital transverse is the most common fetal head position in the early stage of labor and fetal head rotation occurs mostly in the active phase and the second stage of labor by ultrasound examination during the progress of labor. The most common situation is occipito-transverse to occipito-anterior. We should give enough time to pregnancy women who tried vaginal delivery without contraindications during labor management.

In a study conducted by the School of Medicine of the National University of Iceland in 2021, it was reported that 52.5 % of active postpartum women were in the occipital posterior position [8]. In this study, based on the observed fetal rotation during different stages of labor, the proportion of postpartum women in the occipital transverse position during the latency stage was the highest, of 62.9 %. At the active stage, occipital anterior position became the most common position, while the proportion of postpartum women in the active stage was only 16.2 %. The occipital transverse position is more commonly seen in the flat pelvis, while the occipital posterior position is more commonly seen in the male type pelvis. However, proportion of pelvis reported varies in different reports. Racial and the anatomical variation may responsible for the discrepancies in fetal orientation during the active stage.

From the perspective of fetal head rotation, more than half of the OT rotated to OA in the first stage of labor, and all OPs did not rotate. After entering the second stage of labor, five cases underwent vaginal delivery after rotating form OP to OA, of which three cases underwent manual rotation of the fetal head; More than half of OTs without rotation in the first stage of labor rotate into OA in the second stage. During the active stage, two postpartum women who underwent cesarean section due to labor process factors were OT, and one postpartum woman who underwent cesarean section in the second stage was OP. The majority non-OA pregnant women may undergo vaginal delivery after rotating to OA during the active phase and the second stage of labor. Therefore, the OT and OP positions in the first stage of labor should not be considered as positive signs themselves. Only sustained non-OA position may lead to delayed delivery and difficult labor. This is also consistent with previous clinical experience.

This study categorizes the fetal orientation of pregnant women into OA, LOT, OP, and ROT according to the four regions pattern of the clock chart (Figure 1). The proportion of LOT in women with occipital transverse position is higher than that of ROT, and the anatomical and physiological mechanisms of this phenomenon are not yet clear. However, from the perspective of delivery outcomes, the majority women with occipital transverse position can rotate to OA position during labor. Thus, further research is needed to determine whether L/ROT has an impact on delivery outcomes.

In addition to rotation, the descent of the fetal head is also an important factor determining the birth process, and AOP is a good ultrasound indicator that reflects the descent of the fetal head currently [10], [13], [14], [15]. Therefore, when measuring the fetal orientation, AOP values were obtained at the same time point and we attempted to describe the relationship between the fetal orientation and AOP. When the AOP value was 95–109°, only 26.1 % women were in the occipital anterior position. When the AOP value was ≥125°, 66.7 % women were in the occipital anterior position. These results fully demonstrated that the progression of the labor process was a continuous process with synchronous descent and rotation of the fetal head.

The advantage of this study lies in the exploratory use of ultrasound technology to accurately describe the process of fetal orientation rotation during labor, linking the timing of fetal head rotation with delivery outcome and AOP angle. This provides experience for the multidimensional exploration and application of ultrasound technology in labor, laying the foundation for future large-scale ultrasound research in labor, and has positive significance for ensuring delivery safety. In addition, although some literature reported that the progress of induced labor and natural labor were different [16], there was no difference in the management of the two groups after entering the labor process. Therefore, this study included both groups of women in the observation, and we believed that the observational data obtained from two gorups may have more reference significance for clinical practice.

This study also has certain limitations. Firstly, we did not evaluate the occiput spine angle (OSA) of fetuses in non-OP position and the chin to chest angle (CCA) of fetuses in OP position, which are important ultrasound indicators used to evaluate the flexion of the fetal head in the birth canal. The two indicators were associated with cesarean section causing by labor dystocia in women with prolonged active phase of labor [17]. Secondly, due to factors such as clinical work and maternal willingness, the selection of postpartum women is not continuous. Thirdly, the monitoring of the entire labor process is time-consuming, and continuous tracking requires a lot of manpower and time. This study attempts to conduct exploratory observational measurements without interfering with normal labor process management. The sample size is limited, and no statistical differences have been found between the baseline data of each group, making it impossible to conduct accurate statistical analysis. Further confirmation is needed from large-scale ultrasound related studies during labor.