Obstetrics has developed from the digital examination of the birth canal and fetal presentation [1]. However, digital vaginal examination is neither a highly reproducible nor reliable technique to determine the fetal head station or position [2, 3]. In addition, digital vaginal examinations are poorly tolerated [4, 5].

Given the poor reliability and pain of digital vaginal examination, the alternative of using intrapartum ultrasound (ITU) for measuring fetal head station, fetal head position, caput, and molding in a sonopartogram has been suggested [6]. The most used ultrasound measurements for the fetal head station are the angle of progression (AOP) and the head-perineum distance (HPD). The AOP is the angle between the long axis of the pubic bone and a line from the anterior edge of the pubis drawn tangential to the deepest bony part of the fetal skull in the birth canal [7]. The HPD is the shortest distance from the outer bony limit of the fetal skull to the perineum [8]. Both measurements are performed transperineally [9].

The position of the fetal head can be determined by both transperineal and transabdominal ultrasound. The fetal head position is classified based on the position of the fetal occiput. In transperineal ultrasound, the observation of the choroid plexuses reveals where the occiput is located. In transabdominal ultrasound, the position of the fetal head will be evaluated based on the observation of the orbits, the cerebellum, or the interhemispheric line [9].

So far, ITU techniques have been described only in semi-recumbent position [9]. However, in women with epidurals, flexible sacrum birthing positions, compared to semi-recumbent position, have resulted in a shorter second stage of labor and may be more comfortable for many mothers [10, 11]. Additionally, lateral birthing positions have been associated with a higher rate of intact postpartum perineum and a lower rate of episiotomies [12].

No study so far has evaluated a maternal position other than semi-recumbent when performing ITU. The aim of the present study was to test the hypothesis that ITU measurements can be obtained in maternal lateral positions during the second stage of labor.

A total of 34 women participated in the study. None refused to participate in the study. ITU was performed, and position of the fetal head, AOP, and HPD in maternal lateral position with and without contraction were obtained in all women (100%) (Table 1).

Maternal age was 34.00 (31.75 to 36) years, and BMI was 23.95 ($\pm$3.58) kg/m${}^2$. There were 23 (67.6%) nulliparous and 11 (32.4%) parous women. The ITU to birth time was 90.91 ($\pm$43.47) minutes. No baby was admitted to the NICU (Table 2).

There were 9 instrumental births and 1 cesarean birth. The cesarean and 3 instrumental births were indicated due to non-reassuring CTG. The other 6 instrumental births were due to prolonged second stage of labor.

To analyze the time from ITU to birth in correlation with ultrasound measurements, only births with reassuring CTG were used. ITU to birth time was shorter in parous women (61.56 $\pm$ 46.30 versus 104.43 $\pm$ 39.38 minutes; p = 0.030). There was a moderate correlation between HPD2 and ITU to birth time (r = 0.466; p = 0.009) (Fig. 3). This correlation remained significant after adjusting for confounders using an analysis of covariance.

Fig. 3.

HPD2 and ITU to birth time. Correlation between HPD2 and ITU to birth time (r = 0.466; p = 0.009).

There was also a moderate correlation between dHPD and ITU to birth time in nulliparous women (r = –0.473; p = 0.035) (Fig. 4). The rest of correlations between sonographic measurements and ITU to birth time were not statistically significant.

Fig. 4.

dHPD and ITU to birth time in nulliparous women. Correlation between dHPD and ITU to birth time in nulliparous women (r = –0.473; p = 0.035).

There was a very high correlation between AOP and HPD with and without contraction (r = –0.816; p 0.001) (Fig. 5).

Fig. 5.

AOP and HPD in maternal lateral position. Correlation between AOP and HPD (r = –0.816; p 0.001).

Our results show that performing ITU in maternal side-lying positions is possible in women during the second stage of labor. This practice can lead to greater comfort for women by avoiding unnecessary changes in their position to perform labor progress examinations during the second stage of labor.

In the second stage of labor, multiple digital vaginal examinations are often performed to assess the descent and rotation of the fetal head [13]. It is known that a high number of vaginal exams can increase adverse events such as chorioamnionitis or febrile morbidity [14, 15]. ITU makes it possible to observe labor progression in a less invasive and better-tolerated way [4]. In our study, it was possible to obtain ultrasound measurements to assess the progression and position of the fetal head in 100% of the women. None refused to participate in the study, which showed not only the high capacity to obtain information, but the high acceptability by women of intrapartum ultrasound.

Regarding maternal lateral positions in childbirth, multiple benefits have been reported, especially in terms of a lower rate of perineal tears [10, 12]. Although it is widely accepted that women have to be free to decide in which position they feel most comfortable during childbirth, obstetricians and midwives are not trained to perform vaginal examinations in positions other than semi-recumbent [11, 16, 17]. Currently, many women must modify their position to perform examinations during childbirth, which could be avoided using ITU techniques. In our study, we were able to assess AOP, HPD, and fetal head position in all women. But in the event that it is not possible to determine the AOP or HPD in a woman, given the high correlation between both measurements, obtaining one of them would be sufficient to have reproducible and reliable information.

It is known that the AOP and HPD measured in semi-recumbent position by ITU at the beginning of the second stage of labor can predict spontaneous vaginal birth, although no cut-off values have been established for these predictors [18]. Our study was not designed to show whether the values of the ITU measurements in lateral positions were a predictor of vaginal birth. Still, we found a moderate correlation between HPD2 and ITU to birth time in women with a reassuring CTG and between dHPD and ITU to birth time in nulliparous women. This correlation between ITU measurements and time to birth leads us to believe that through determinations of ITU measurements in maternal lateral positions, the progress of labor can be monitored in the same way that can be performed in semi-recumbent position [9].

With regard to maternal positions, not only during the second stage of labor but also during the first stage, women should be encouraged to adopt the positions in which they are most comfortable [19]. Position changes can favor labor progression and be helpful in non-reassuring cardiotocographic monitoring [20]. Using ITU, it might be possible to monitor labor progress regardless of the maternal position. Therefore, we believe that it would be appropriate to study intrapartum ultrasound in other maternal positions and the relationship between ultrasound measurements in various positions and at different stages of labor.

In case that ITU became the standard clinical practice, it is possible that the rate of vaginal examinations, failed instruments, and even the rate of cesarean births could be reduced by studying the progression of labor with greater accuracy. So far, studies have failed to show these benefits, but no harm has been found either [18, 21, 22, 23]. In addition, it is an examination that is well accepted, harmless, and without contraindications [9].

Our study is the first to describe the ITU technique in maternal side-lying position. The main strength of our study is that it was conducted on women during the second stage of labor and that it evaluated consecutive women regardless of height and weight. Another strength of the study is having studied the time until birth since it allows showing the relation between the ITU measurements and the birth progress.

The main weakness of our study is the lack of interobserver verification of ITU measurements. Even knowing that sonographic measures are highly reproducible and reliable [24], it would have been convenient if two independent operators had performed them. Another weakness is the small sample size; since it was calculated only to show if obtaining ITU measurements in maternal lateral position was possible, it does not allow reaching other conclusions regarding perinatal outcomes.

ITU in maternal lateral position (side-lying) is a simple technique that avoids unnecessary changing of position during the second stage of labor. Our study shows that this technique obtains results for measurements of the AOP and HPD in addition to fetal head position in more than 80% of cases. In fact, we obtained these results in all the women who participated in the study. We believe that more studies are necessary and that ITU could probably end up being a valuable tool in any maternal position.

MJC designed the research study. MJC and ER performed the research. FL and JLB provided assistance and advice on methodology. MJC and JLB analyzed the data. MJC and FL wrote the manuscript. JLB supervised all the research. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.

Ethical clearance was obtained from the institutional Ethics Committee (PI-4853). Informed consent was obtained from all participants in the study.

Thanks to the women who participated in the study.

Thanks to S Franke and M Cruset for English proofreading.

This research received no external funding.

The authors declare no conflict of interest.