Continuous cervical dilatation monitoring by ultrasonic methods during labor P. L.






With the technical assistance v.




Quebec, Canada, and Cleveland, Ohio

Continuous recording of cervical dilatation during labor has been investigated in 13 pregnancies. The rwdings were obtained with an ultrasonic c@meter that cpn!inuously monitqrs cervical dilatation from thg transit time of ultrasound signals between two piezoektric crystals attaohed on the uterine cervix. A small~spring-loaded clip allowed each crystal to ‘be fixed on the rjm pf the cervical OS. Clinical accuracy was 20.6 cm. When the Jrqsound recording of cervical ditatatfon is compared tq the intrauterih pressure curve, it is char+terized by a Ras#ne ant! wave&& curve of dilctation (DWP). ihe maximal amplitude component is called cervical maximal ~astk$ty. The onset of the DWP is re!ated to’ cervical resistivity, and the encj of PW? r&ects the relaxa$ion time of cervical dilatation. The data shqw that as diiatatiin enters the active phase of labor, the plaetkii, the r&+ivity, and the duration of relaxation of the cervix @crew. The& o@vations are discussed and related to the, structural changes of the cervix during labor. (AM. J. OFMET. GYNEC;OL.132: 16, 1976.)

CLOSE AN? CONSTANT supervision of high-risk pregnancies increasingly involves the use of new and advanced technologies. The present study loaks at the contihuous recording of cervical dilatation during labor by means of ultrasonic cervimetry. Cervimetry, or cervical dilatation measurement, is done by either mechanical or magnetic or ultrasonic means. In 1956, Friedman’ authored a report based on mechanical cervimetry, which was followed by another co-authored with Von Micsky2 in 1963. Siener3+ with Wust,” from 1956 to 1972, and Krementsov,7 in 1968, also used the same method. The magnetic cervimetry From the Perinatul Labor&or)‘, Department of Obstetrics and Gynecology, Sainte-Justine Hospital, University qf Montreal, and The I$ngineering Design Center, Department of Reproductive Biology, Case Western Reset-or University. Received,for Accepted April




was first proposed by Smith8 in 1954. However, there were many drawbacks and it was only in 1974 that Rice,$ and also Kriewall,*” tried to solve these problems. The ultrasonic cervimetry was introduced from 19’74 to 1976 by Neuman, Wolfson, and Zador”-13 and later by Kok, Wallenburg, and WIadimiroff.14 A comparison of the advantages and inconveniences of each method is illustrated in Table I. Our major goal was to evaluate ultrasqnic cervimetry and to look at the characteristics of the record&e wi& respect to conventional variables of feel monitoring. In particular we looked at the relationship between dynamic changes in cervical dilatation and intrauserine pressure. We looked at both the amplitudes of the changes and the phase rela&nships between the two signals.

19, 1978.

3, 1978.

Reprint requesti; Mr. Philippe L. Moss, Sainte-Justine Hospital, 3175 CiZe Ste-Catherine Rd., Montreal, P.Q., Canada H3T 1 C5.

The ultrasonic cervimeter used was evaluated by use on a group of 13 patients.* The nine primigravidaxxl four multiparous women had ha4 no major cow

*France-Quebec Fellow and Resident, Department of Obstetrics and Gynecology, Sainte-Justine Hospital, ‘University of Montreal.

*Informed consent was obtained from patient and z+x&+&ing physician in each case. OOO2-%78/78/01



1978 ~l'hr (:. \'. Mtaby


Volume Number


132 I

Cervical dilatation monitoring by ultrasonic methods

I. Advantages

and disadvantages

of cervimetry




cervimetty Installation Patient comfortable during installation Patient comfortable after installation Convenience Measurement by stretching Possibility of digital pelvic examination Output stability

Output linearitv Recording up to full tation *Yes,

if with










Difficult No

Difficult No

Difficult No





High Yeb Yes

Low Yes No


Moderate No Diminished


Calipers can be moved patient movements

Subiective Yes-

Yes, almost No;. calipers are limited by the vaginal walls


of the


Easy No


Diminished by

Relies on signal intensity; sensors’ motions are reco.rded Yes* Not always; earth’s magnetic field interferes


Relies on signal’s transit time transmission; good stability Yes Yes


tions of pregnancy and the fetal outcome was good in each case. The installation of the transducers consisted of fixing two piezoelectric crystals (1 by 1 by 5 mm.) to the external OS of the uterine cervix. The installation took place at 3 cm. or more of dilatation.‘The crystals were fixed in places diametrically opposed to each other and held there by spring-loaded clips. The ultrasonic cervimeter in use generated an ultrasound wave* each second, and the total time elapsed from the emission of that signal by one crystal to the reception by the other was compiled and converted into a distance. The ultrasound wave velocity was considered to be constant at 1.48 mm. per microsecond.‘3 Since time, and not intensity of the signal, was the important parameter, the crystals had to rotate more than 60 degrees from one another before an error in the measurements was introduced. Migration was not possible since the clips’ teeth, when closed; pierced the cervix through and through. The dilatation values along with the fetal heart rates, the fetal electrocardiograms, and uterine contractions were recorded on an eight-channel rec0rder.t The parameters measured are described in Fig. 1. The amplitudes A, and AZ were defined as the maximum changes in dilatation and pressure, respectively. A time delay (D3) was computed from the phase relationship between those two amplitudes. Also, time delays for the onset and end of the dilatation waveform, D, and Dz, were determined by reference to the intrauterine pressure (IUP) signal. As a mean of standardization, the





*This ultrasonic technique is well within currently able safe limits (CSA Standards). tBeckman RM Dynograph, Beckman Instruments Fullerton, California.



the intersection of the signal with a calculated baseline. This baseline was obtained by adding 5 per cent of the amplitudes A, or AZ to their graphic baseline.

Observations and results Of the 13 cases studied, five failed to give any results. The main reasons for such a low’success rate (61.5 per cent) were difficulties of installation on a cervix incompletely effaced and sensors falling down during parallel digital pelvic examinations. One failure was due to technical difficulties. By comparing 21 digital examinations from the nine remaining patients with the data obtained via the cervimeter at that time, we found a mean difference of 0.6 cm. with a standard error of kO.2 cm. Digital examinations had a net tendency to give a higher result. On the recorded data, the apparition of dilatation wave patterns (DWP) in concomitance with the contractions was not observed in primigravid women before the effacement of the cervix was completed. In the multiparous women, the DWP appeared before completion of the effacement. A typical continuous recording showing dilatation wave patterns is illustrated in Fig. 2. A comparison of intrauterine contractions and cervical dilatation was done on 100 contractions in a group of five primigravid women. The values’of the various parameters measured (A,, D,, Dz, and D3) were divided into two groups, depending whether they were values obtained in the period preceding or following 4 cm. of dilatation.



of 4 cm.

was related

to the


indicating it as a good point of demarcation. The maximal amplitude on the dilatation wave patterns (DWP), defined as A,, was found to follow, in time,






intensity (Ds).


of time

contractions delay


(As) was





et al.

September Am. j. Ohstet.





+ 5XAI



’ !+%A2


Fig. 1. Determination of the various the maximal amplitudes of dilatation Dz for the delay between the onset tion wave pattern (DWP) compared for the delay between A, and AZ.

parameters: A 1 and A2 for and contraction ; D, and and the end of the dilatato the contraction; and D3

Table II. Duration of the different dilatation wave pattern*

Before4cm. After 4 cm.

4.2 t 0.8 7.2 + 0.8



phases of the

0.0 f 1.5 4.6 2 1.1 13.7 f 2.0

4.2 k 0.7

7.2 r 0.5 15.8 r 1.3

*All results are in means ? standard errors of 100 contractions. ?A11 the data after 4 cm. are statistically different from those before 4 cm., except for D,; significance at the 1 per cent level. pendent of labor progression (Table II). The DWP amplitude A, could go up to 40 mm. in the active phase of labor and it showed a tendency to augment with progressing labor, being up to two times higher after a dilatation of 4 cm. than before that dilatation. The delay between the onset of the contraction and DWP, D1, varied from 0 to 22 seconds and generally was longer during the active phase of labor than it was before that phase. The time delay D2 between the end of the contraction and DWP varied from - 12 to 55 seconds and was longer in late labor.

Comment The mean difference of 0.6 cm. between the digital pelvic examinations and the cervimeter values is an expected result and correlates with the observations of Zador, Neuman, and Wolfson.13 The discrepancy originates from the fact that a digital examination stretches the cervix. This permits an evaluation of cervical ripeness and dilatation. The cervimeter measures only the degree of opening of an unstretched cervix. This is well illustrated in situations where a speculum or an amnioscope is inserted into the vagina. A 3 cm. cervical dilatation can easily be found to be less than 2

1, 197X Gynetol.

cm. under direct view, especially if the cervix is not fully effaced. From the point of view of labor evaluation, the digital examination cannot be matched b) the cervimeter’s measurements. However, in the study of‘ labor physiology, the cervimeter is more accurate, and the continuous recording of the instantaneous dilatation as a function of time and uterine pressure is invaluable. The ultrasound recording of cervical dilatation is characterized by a baseline and a wave-shape curve of dilatation or DWP. This DWP has three time components (D1, D,, and D3) when compared to the curve of intrauterine pressure (Fig. 1). A, is the amplitude component and will be called the cervical maximal plasticity. It represents the maxima1 opening of the cervix in relation to the force of the contractions, the size of the pelvis, the degree of engagement of the head, ancl the softness of the cervix. Hence, in the active phase of dilatation, each uterine contraction tends to produce a higher A, and the cervical plasticity increases. This result is based on data from primigravid women only, due to technical difficulties and to the fact that, by the time the cervimeter and all the related monitoring instruments were installed, our multiparous subjects had already passed the 4 cm. demarcation. It is interesting to note that the peak values of uterine contraction and cervical dilatation are out of phase (Da). This is contrary to the observations of simultaneousness reported by Kok, Wallenburg, and W1adimiroff.14 The onset of an active DWP (D1) is delayed when it is compared to the onset of uterine contractions. This represents the cervical resistivity to active dilatation; and it is in agreement with the finding by Kriewall. lo In the active phase, the resistivity increases (Table II). This finding probably results from the resistance to stretching of cervical fibrous tissues. Indeed, as the cervix becomes increasingly dilated, the cervical connective tissue reaches a maximum stretching capacity and it takes more force or hydrostatic uterine pressure to reach peak dilatation. This observation differs from that of Siener.” By use of mechanical cervimetry he found a diminishing cervical resistivity. This difference may be due to the methodology. A mechanical cervimeter is much less physiologic than ultrasound cervimetry, which is practically weightless. The increase in cervical resistivity is reflected by a rise in the relaxation time of DWP (Dz) during the active phase. Since the cervix is a nonmuscular organ, the maximally stretched fibrous tissue is slow to come back to its original precontraction stretching state; and it is prevented from doing so by the descent of the fetal presenting part. The description of different phases of cervical di-

Volume Number

132 I

Cervical dilatation monitoring by ultrasonic methods


1 mink

Fig. 2. Simultaneous recording during the first stage of labor.

of cervical dilatation,

latation is essential to understand the pathologic conditions of cervical dilatation and fetal descent so often categorized as cephalopelvic disproportion. It is evident that a defect in the cervical fibrous tissue will alter the DWP just as much as will a presenting part that does not descend because of a narrow pelvis. A study of drug effect on DWP should be rewarding. It has to be noted that cervical dilatation and fetal descent can be monitored simultaneously by ultrasound.rJ


pressure, and fetal heart rate

Ultrasonic cervimetry is a new tool that may help in our understanding of labor physiology. It may be an instrument of the future for patients in whom digital examinations are inadvisable. The installation is still a handicap and progress in this direction must be made before any clinical application can be exploited.


Friedman, E. A.: Cervimetry; an objective method for study of cervical dilatation in labor, AM. J. OBSTET.






5. 6.

7. 8.




E. A., and Von Micsky,

L. 1.: Electronic cervimeters: A research instrument for the study of cervical dilatation in labor, AM. J. OBSTET. GYNECOL. 87: 789, 1963. Siener, H.: An apparatus for recording the opening of the cervix during labor, Zentralbl. Gynaekol. 78: 2069, 1956. Siener, H.: A new electromechanical apparatus for measuring labor activity by the execution of combination measurements, Arch. Gynaekol. 196: 365, 1961. Siener, H.: First stage of labor recorded by cervical tocometry, AM. J. OBSTET. GYNECOL. 86: 303, 1963. Siener, H., and Wust L.: Internal tocometry and graphic registration of cervix dilatation as a basis for calculation of labor effectiveness and soft tissue resistance, Geburtshilfe Frauenheilkd. 32: 123, 1972. Krementsov, U.: Improved technique for measurement of cervical dilatation, Biomed. Eng. (N. Y.) 2: 350, 1968. Smith, C. N.: Measurement of the forces and strains of labour and the action of certain oxytocic drugs, Intemational Congress of Obstetrics and Gynecology, Geneva, 1954, S. A. George, p. 1030.


Rice, D. A.: Mechanism and measurement of cervical dilatation, Doctoral thesis, Purdue University, Lafayette, Indiana, 1974. 10. Kriewall, T. J.: Measurement and analysis of cervical di-

latation in human parturition, 11.





Doctoral thesis, University

of Michigan, Ann Arbor, Michigan, 1974. Neuman, M. R., Wolfson, R. N., and Zador, I.: Ultrasonic transit time methods for monitoring the progress of obstetrical labor, Transactions of Professional Group in Ultrasonics-IEEE, Vol. 33, 1975. Zador, I.: Ultrasonic determination of cervical dilatation during labor. Master’s thesis, Case Western Reserve University, Cleveland, Ohio, 1974. Zador, I., Neuman, M. R., and Wolfson, R. N.: Continuous monitoring of cervical dilatation during labour by ultrasonic transit-time measurement, Med. Biol. Eng. 14: 229, 1976. Kok, F. T., Wallenburg, H. C. S., and Wladimiroff, J. W.: Ultrasonic measurement of cervical dilatation during labor, AM. J. OBSTET. GYNECOL. 126: 288, 1976. Wolfson, R. N.: An instrument for the continuous and quantitative determination of fetal descent by measurement of ultrasonic transit time, Doctoral thesis, Case Western Reserve University, Cleveland, Ohio, 1974.

Continuous cervical dilatation monitoring by ultrasonic methods during labor.

Continuous cervical dilatation monitoring by ultrasonic methods during labor P. L. MOSS, B.M.E. P. LAURON* J. F. M. R. PqUX NEUMAN With the tech...
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