Induction of Labour. Guideline of the DGGG, OEGGG and SGGG (S2k, AWMF Registry No. 015-088, December 2020)

Abstract

Aim The aim of this official guideline published and coordinated by the German Society of Gynaecology and Obstetrics (DGGG) in cooperation with the Austrian Society of Gynaecology and Obstetrics (OEGGG) and the Swiss Society of Gynaecology and Obstetrics (SGGG) is to provide a consensus-based overview of the indications, methods and general management of induction of labour by evaluating the relevant literature.

Methods This S2k guideline was developed using a structured consensus process which included representative members from various professions; the guideline was commissioned by the guidelines commission of the DGGG, OEGGG and SGGG.

Recommendations The guideline provides recommendations on the indications, management, methods, monitoring and special situations occurring in the context of inducing labour.

Key words

guideline - induction - indication - methods - prostaglandin

I  Guideline Information

Guidelines programme of the DGGG, OEGGG and SGGG

For information on the guidelines programme, please refer to the end of this guideline.

Citation format

Induction of Labour. Guideline of the DGGG, OEGGG and SGGG (S2k, AWMF Registry No. 015-088, December 2020). Geburtsh Frauenheilk 2021; 81: 870–895

Guideline documents

The complete long version in German and a slide version of these guidelines together with a list of the conflicts of interest of all of the authors are available on the homepage of the AWMF: http://www.awmf.org/leitlinien/detail/ll/015-088.html

Guideline authors

See [Tables 1] and [2].

Table 1 Lead author and/or coordinating guideline authors.

Author	AWMF professional society
Prof. Dr. med. Sven Kehl	German Society of Gynaecology and Obstetrics [Deutsche Gesellschaft für Gynäkologie und Geburtshilfe e. V.] (DGGG)
Prof. Dr. med. Michael Abou-Dakn	German Society of Gynaecology and Obstetrics [Deutsche Gesellschaft für Gynäkologie und Geburtshilfe e. V.] (DGGG)

Table 2 Contributing guideline authors (listed alphabetically).

Author Mandate holder	DGGG working group/AWMF/non-AWMF professional society/organisation/association
Prof. Dr. med. Michael Abou-Dakn	German Society of Gynaecology and Obstetrics [Deutsche Gesellschaft für Gynäkologie und Geburtshilfe e. V.] (DGGG) Obstetrics and Prenatal Medicine Working Group in the DGGG [Arbeitsgemeinschaft für Geburtshilfe und Pränatalmedizin in der DGGG e. V.] (AGG)
Prof. Dr. med. Irene Hösli	Academy for Fetomaternal Medicine [Akademie für fetomaternale Medizin] (AFMM)
Prof. Dr. med. Sven Kehl	German Society of Gynaecology and Obstetrics [Deutsche Gesellschaft für Gynäkologie und Geburtshilfe e. V.] (DGGG) Obstetrics and Prenatal Medicine Working Group in the DGGG [Arbeitsgemeinschaft für Geburtshilfe und Pränatalmedizin in der DGGG e. V.] (AGG)
PD Dr. med. Ulrich Pecks	German Society for Perinatal Medicine [Deutsche Gesellschaft für Perinatale Medizin e. V.] (DGPM)
PD Dr. med. Philipp Reif	Austrian Society of Gynaecology and Obstetrics [Österreichische Gesellschaft für Gynäkologie und Geburtshilfe] (ÖGGGG)
Prof. Dr. med. Ralf L. Schild	German Society of Ultrasound in Medicine [Deutsche Gesellschaft für Ultraschall in der Medizin e. V.] (DEGUM)
Prof. Dr. med. Markus Schmidt	German Society for Prenatal and Obstetric Medicine [Deutsche Gesellschaft für Pränatal- und Geburtsmedizin] (DGPGM)
PD Dr. med. Dagmar Schmitz	Academy for Ethics in Medicine [Akademie für Ethik in der Medizin e. V.] (AEM)
Prof. Dr. Christiane Schwarz	German Midwifery Society [Deutsche Gesellschaft für Hebammenwissenschaft] (DGHWi)
Prof. Dr. med. Daniel Surbek	Swiss Society of Gynaecology and Obstetrics [Schweizer Gesellschaft für Gynäkologie und Geburtshilfe] (SGGG)

The following professional societies/working groups/organisations/associations were interested in contributing to the guideline text and participating in the consensus conference and nominated representatives to contribute and to attend the conference:

Abbreviations

CI: confidence interval

CTG: cardiotocography

g: grams

GDM: gestational diabetes mellitus

GW: week of gestation

ICSI: intracytoplasmic sperm injection

IUFD: intrauterine foetal death

IUGR: intrauterine growth restriction

LGA: large for gestational age

µg: microgrammes

mg: milligrammes

PROM: premature rupture of membranes

PPROM: preterm premature rupture of membranes

RCT: randomised controlled trial

RR: risk ratio

SDP: single deepest pocket

SGA: small for gestational age

s/p: status post

WHO: World Health Organisation

II  Guideline Application

Purpose and objectives

The induction of labour is one of the most commonly performed procedures in routine obstetric care. Despite sufficient available evidence, there is considerable heterogeneity in the approaches used, which is why the indications to induce labour and the different methods used together with their advantages, disadvantages and risks are presented here.

Targeted areas of patient care

• Inpatient care

• Day-patient care

• Outpatient care

Targeted patient group

This guideline is aimed at pregnant women.

Target user groups/target audience

This guideline is aimed at the following groups of users:

• gynaecologists and obstetricians

• midwives

Adoption and period of validity

The validity of this guideline was confirmed by the executive boards/heads of the participating medical professional societies, working groups, organisations and associations as well as by the boards of the DGGG, the SGGG and the OEGGG as well as the board of the DGGG/SGGG and OEGGG Guidelines Commission in July 2020 and was thus approved in its entirety. This guideline is valid from 1st December 2020 through to 1st December 2025. Because of the contents of this guideline, this period of validity is only an estimate. If changes are urgently required, the guideline may be updated earlier. Similarly, if the guideline continues to reflect the current state of knowledge, its period of validity may be extended.

III  Methodology

Basic principles

The method used to prepare this guideline was determined by the class to which this guideline was assigned. The AWMF Guidance Manual (version 1.0) has set out the respective rules and requirements for different classes of guidelines. Guidelines are differentiated into lowest (S1), intermediate (S2), and highest (S3) class. The lowest class is defined as consisting of a set of recommendations for action compiled by a non-representative group of experts. In 2004, the S2 class was divided into two subclasses: a systematic evidence-based subclass (S2e) and a structural consensus-based subclass (S2k). The highest S3 class combines both approaches.

This guideline has been classified as: S2k

Grading of recommendations

The grading of evidence based on the systematic search, selection, evaluation and synthesis of an evidence base which is then used to grade recommendations is not envisaged for S2k guidelines. The different individual statements and recommendations are only differentiated linguistically, not by the use of symbols ([Table 3]):

Table 3 Grading of recommendations (based on Lomotan et al., Qual Saf Health Care 2010).

Description of binding character	Expression
Strong recommendation with highly binding character	must/must not
Regular recommendation with moderately binding character	should/should not
Open recommendation with limited binding character	may/may not

Statements

Expositions or explanations of specific facts, circumstances or problems without any direct recommendations for action included in this guideline are referred to as “statements”.

Achieving consensus and strength of consensus

At structured NIH-type consensus-based conferences (S2k/S3 level), authorised participants attending the session vote on draft statements and recommendations. This can lead to significant changes in the wording, etc. Finally, the extent of consensus is determined based on the number of participants ([Table 4]).

Table 4 Level of consensus based on extent of agreement.

Symbol	Level of consensus	Extent of agreement in percent
+++	Strong consensus	> 95% of participants agree
++	Consensus	> 75 – 95% of participants agree
+	Majority agreement	> 50 – 75% of participants agree
–	No consensus	< 51% of participants agree

Expert consensus

As the term already indicates, this refers to consensus decisions taken specifically with regard to recommendations/statements issued without a prior systematic search of the literature (S2k) or where evidence is lacking (S2e/S3). The term “expert consensus” (EC) used here is synonymous with terms used in other guidelines such as “good clinical practice” (GCP) or “clinical consensus point” (CCP). The strength of the recommendation is graded as previously described in the chapter on the grading of recommendations; it is only expressed semantically (“must”/“must not” or “should”/“should not” or “may”/“may not”) without the use of symbols.

IV  Guideline

1  Purpose of inducing labour

The decision to induce labour must be considered very carefully, as inducing labour affects the natural course of the pregnancy. The assumed benefits must be weighed against the potential disadvantages. In general, the guiding principle must be that inducing labour will achieve a better perinatal outcome for mother and child than expectant management of the pregnancy.

2  Indications for inducing labour

2.1  Exceeding the due date, post-term/prolonged pregnancy

As the gestational age increases, the risk of mortality and morbidity, which is very low in absolute numbers, steadily increases [1], [2], [3], and randomised studies on inducing labour carried out in recent years show that inducing labour from week 39 + 0 of gestation is associated with a lower rate of caesarean sections [4], [5], [6], [7], [8].

As part of antenatal care, information must be provided about individual risk situations (e.g., maternal age, nicotine abuse, parity, Bishop score, obesity, estimated foetal weight) and the respective individual advantages and disadvantages of terminating the pregnancy must be described and weighed up in consultation with the pregnant woman (cf. 3.1.). The low absolute risk of intrauterine foetal death may be higher in certain situations (e.g., a maternal age of more than 40 years) [9]. If the pregnancy is continued, the pregnancyʼs progress should be monitored more closely from week 40 + 0 (e.g., every 3 – 5 days). In German-speaking countries, monitoring to detect potential early-stage risks is usually done with CTG and sonography. However, there are no prospective data on monitoring pregnancies who have just passed their due date, which is why some countries do not monitor pregnant women between week 40 + 0 and week 41 + 0 of gestation [10].

2.2  (Preterm) premature rupture of membranes

2.2.1  Preterm premature rupture of membranes (PPROM)

2.2.2  Premature rupture of membranes (PROM)

2.3  Gestational diabetes

Detailed recommendations are given in the German-language AWMF S3 guideline no. 057-008 “Gestational Diabetes Mellitus (GDM): Diagnosis, Therapy and Follow-up Care” [11].

2.4  Abnormal amniotic fluid volumes

2.4.1  Oligohydramnios

However, from week 39 + 0 of gestation, the leeway to decide whether terminating the pregnancy is indicated in cases with isolated oligohydramnios and additional risk factors is greater – even though there is no clear evidence on this matter.

2.4.2  Polyhydramnios

2.5  SGA foetus or intrauterine growth restriction

Up to 70% of SGA (small for gestational age) foetuses are constitutionally small and have a normal perinatal outcome [12], [13]. In contrast, intrauterine growth restriction (IUGR) is associated with higher perinatal morbidity and mortality rates and may be identified when additional anomalies (pathological Doppler sonography, oligohydramnios, no interval growth, estimated weight < 3rd percentile) are present. The risk of intrauterine foetal death increases with increasing gestational age, which is why preterm termination of the pregnancy should be discussed with the patient.

As regards the rationale behind the recommendations on inducing labour, we refer interested parties to the S2k guideline no. 015-080 “Intrauterine Growth Restriction” [14].

2.6  Intrahepatic cholestasis of pregnancy

The level of bile acid concentrations at an undefined time point in pregnancy is a predictive marker for stillbirth and neonatal complications, and the cut-off value in different studies varies between > 40 µmol/L and > 100 µmol/L [15], [16], [17], [18], [19]. Based on the best available evidence, induction of labour may be recommended between week 34 + 0 and week 36 + 6 of gestation if bile acid concentrations are > 100 µmol/L [20].

2.7  Hypertensive disorders of pregnancy

The definitions of hypertensive disorders of pregnancy are presented in detail in the S2k guideline no. 015/018 “Hypertensive Disorders of Pregnancy: Diagnosis and Therapy” [21]. The pregnancy may be terminated by inducing labour for hypertensive disorder of pregnancy after due consideration of additional foetal and obstetric factors.

In cases with HELLP syndrome, the foetus should be delivered from week 34 + 0 of gestation (at the latest); inducing labour may be an option once the overall situation has been carefully considered.

2.8  Suspicion of (non-diabetogenic) macrosomia

Foetal macrosomia is a risk factor for birth complications. Inducing labour must therefore reduce the risk of shoulder dystocia with the potential for a subsequent lesion of the brachial plexus and the risk of surgical intervention.

2.9  Termination on request/no medical reason

Studies carried out in recent years have shown that inducing labour in women with no risk factors/no medical indications from week 39 + 0 of gestation was not associated with a higher risk of caesarean section compared to expectant management [4], [5], [22]. These findings have led to international guidelines presenting the option of inducing labour from week 39 + 0 of gestation to primipara with no risk factors [23]. Because of the potentially higher neonatal morbidity (hyperbilirubinemia, hypoglycaemia), labour should not be induced before week 39 + 0 of gestation.

3  Measures taken before inducing labour

3.1  Information, consultation and documentation

Inducing labour is a medical intervention, which is why providing information and advice/consultation with an individual risk-benefit analysis is necessary. The aim when consulting with the patient is to arrive at a joint decision which is supported by the pregnant woman. The indications for inducing labour must be reviewed, possible contraindications must be excluded, and the method used to induce labour must be chosen after all factors have been taken into consideration. The time frame, possible progress, and outcome as well as other options (e.g., deferring the decision and waiting) should be discussed with the pregnant woman. As some methods have only been approved for certain settings, the specifically approved indications and contraindications should be known. However, a procedure may be used if there is sufficient existing evidence, even if it has not been approved. But this requires appropriate information to be given to the patient and the patient to consent. The consent should be recorded in writing.

Inducing labour in a pregnant woman who has previously had a caesarean section requires high levels of information and advice/consultation. The decision that induction of labour is indicated must be based on very strict criteria, and possible alternatives (e.g. repeat caesarean section) must be pointed out. The patient must be informed about the risks (primarily uterine rupture and the corresponding consequences for mother and child) associated with inducing labour after a previous delivery by caesarean section and must be given extensive detailed advice. Moreover, as almost all procedures for inducing labour have not been approved in this setting and are contraindicated, the patient must give her consent in writing.

3.2  Risk-benefit analysis, contraindications

In certain situations, the risks of inducing labour outweigh the intended benefit. Medication given to induce labour when the patient is already having regular contractions may lead to overstimulation and have a negative impact on the babyʼs condition. In addition, the standard contraindications for vaginal delivery (e.g., transverse presentation, total placenta praevia, presenting umbilical cord/umbilical cord prolapse, active herpes simplex, s/p uterine rupture and s/p caesarean section with a vertical uterotomy) must be taken into account.

3.3  Predicting the success of induced labour

Factors which have a positive impact on the induction of labour include a ripe cervix, a prior history of vaginal delivery, and premature rupture of membranes [24]. Cervical ripening is evaluated clinically and described using the Bishop score. The Bishop score is one of the most important predictors for successful induction of labour and should therefore be ascertained and recorded. Unfavourable factors for inducing labour include a low Bishop score, nulliparity, a high body mass index, previous caesarean section, and low gestational age.

4  Methods for inducing labour

A number of mechanical procedures and medications are available to induce labour. The choice of method to induce labour is determined, first and foremost, by the cervical findings (Bishop score, cf. 3.3). Other factors which must be taken into account when selecting the appropriate method include whether (premature) rupture of membranes has occurred and whether the mother has a previous history of caesarean section.

4.1  Mechanical induction of labour

4.1.1  Membrane sweep

A membrane sweep is a procedure during which one or two fingers are inserted into the uterine cervix to separate the lower part of the amniotic sac from the lower uterine segment using circular movements of the fingers.

4.1.2  Amniotomy

Amniotomy (also known as artificial rupture of membranes) is carried out when the cervix is ripe and the cervical os has begun to open. The aim of this intervention is to trigger the release of endogenous prostaglandins.

4.1.3  Balloon catheter

Balloon catheters are used for cervical ripening and to induce labour when the cervix is still unripe. Both single and double catheters with fill volumes ranging from 30 to 100 ml can be used. The pressure of the liquid-filled balloon(s) on the uterine cervix results in the endogenous release of prostaglandins, which may lead to cervical ripening and the onset of labour.

4.1.4  Hygroscopic dilators

Hygroscopic cervical dilators are placed in the uterine cervix and lead to dilation of the cervix through osmotic dehydration of cervical cells with endogenous release of prostaglandins. They are mostly used to induce abortion in the early weeks of pregnancy [25].

4.2  Pharmaceutical induction of labour

4.2.1  Oxytocin

Oxytocin is administered intravenously and acts by inducing contractions. The benefits of oxytocin are that it can be easily controlled as it floods the body very quickly (onset of effect occurs after around 3 – 10 minutes) and is also eliminated quickly from the bloodstream (its half-life is 3 – 6 minutes). It is approved to induce labour when the cervix is ripe.

4.2.2  Prostaglandin E2 (dinoprostone)

In contrast to oxytocin, prostaglandins take effect through cervix-ripening and contraction-inducing mechanisms, which is why they may also be used in cases with unripe cervix.

4.2.3  Prostaglandin E1 analogues (misoprostol)

Misoprostol may be administered orally or vaginally and may be administered both to women with an unripe cervix and to women with a ripe cervix. The use of misoprostol to induce labour is now well researched, and it appears to be the most effective pharmaceutical to induce labour when the cervix is unripe.

4.3  Other methods to induce labour

4.3.1  Castor oil

A “contractions cocktail” using castor oil was the first drug-based procedure used to induce labour in the first half of the last century [26], [27], [28]. Because of its side effects (especially diarrhoea), castor oil came to be used less and less over time and was finally replaced by oxytocin. The effect of castor oil is based on ricinoleic acid, the active ingredient of castor oil. Ricinoleic acid affects the prostaglandin receptors of the uterine and abdominal muscle cells, which is why it can lead to uterine contractions in addition to the typical symptoms associated with the use of laxatives. Although castor oil has been used to induce labour since almost one hundred years, evidence is insufficient.

4.3.2  Other methods

Many other methods are also being used to induce labour. They include, among others, the insertion of a clove oil tampon, sexual intercourse [29], [30], acupuncture [31], nipple stimulation, and homeopathic methods such as the administration of caulophyllum. As these alternative methods have not been sufficiently studied, they are not recommended as a means of inducing labour and should only be used in the context of scientific trials.

5  Monitoring the induction of labour

There are no evidence-based recommendations on how precise the monitoring of induction of labour must be. The general consensus is that a CTG should be carried out at least 30 minutes before starting induction of labour.

Mechanical methods (cf. 4.1.) lead to cervical ripening through endogenous release of prostaglandins, which is why side effects such as uterine overstimulation are not anticipated [32] and CTG examinations are not mandatory during the course of labour. CTG examinations are recommended in cases of vaginal bleeding or other anomalies.

6  Special situations when inducing labour

6.1  Obesity

Maternal obesity is not an indication per se for inducing labour if there are no other obstetric or medical indications [33]. However, obese pregnant women have an increased risk of needing labour to be induced because of existing co-morbidities or newly occurring complications of pregnancy. Moreover, maternal obesity is associated with a higher rate of antenatal, intrapartum and postpartum complications [34], [35].

6.2  Breech presentation

The data on inducing labour in cases with foetal breech presentation are based on a few non-randomised controlled studies. A few cases series have shown that when the patient is selected carefully, the risk for mother and infant is low [36], [37], [38], [39].

6.3  Twin pregnancy

There are no evidence-based data about the right time to deliver dichorionic twin pregnancies. The studies which have been carried out to date are limited by different factors [40], [41], [42], [43], [44]. While the prospective risk of intrauterine foetal death (IUFD) between week 37 + 0 and week 37 + 6 of gestation appears to be equivalent to the risk of neonatal mortality, the IUFD risk exceeds the risk of neonatal mortality from week 38 + 0 of gestation [45]. The risk of neonatal morbidity (e.g., respiratory distress syndromes, admissions to a neonatal intensive care unit …) is lower in later weeks of gestation. But as these data are still controversially discussed because of the lack of information on ultrasound examinations, antepartum monitoring, the mode of delivery, and the quality of neonatal care, it is difficult to give a general recommendation to induce labour from week 37 + 0 of gestation in dichorionic twin pregnancies. For this reason, various professional societies recommend inducing labour from week 37 + 0 or 38 + 0 of gestation [46], [47].

There are no RCTs which have investigated the optimum time point to begin inducing labour for monochorionic diamniotic twin pregnancies. There appears to be a trend that after the 36th week of gestation the risk of IUFD is higher than the risk of neonatal mortality, but because of the above-mentioned criticisms, the data are limited [45]. Nevertheless, the potentially higher risk of intrauterine death should be discussed [47], [48], [49], [50], [51], even though there is no unambiguous evidence for inducing labour from week 36 + 0 of gestation for monochorionic diamniotic twin pregnancies [52], [53]. Because of this, some international professional societies recommend delivering twins by week 37 + 6 of gestation [46], [51].

6.4  Previous caesarean section

6.4.1  Information and advice/consultation

Inducing labour after a previous caesarean section places high demands on the information and advice provided to patients (cf. 3.1.). In view of the high risks involved compared to spontaneous labour (e.g., a higher rate of caesarean sections (OR 1.52, 95% CI: 1.26 – 1.83 [54]), the decision whether induction of labour is indicated must be made very carefully. Nevertheless, the overwhelming majority of affected pregnant women meet the requirements for an attempt to delivery vaginally and must be informed about that (cf. 3.2.).

If the previous caesarean section was not a horizontal incision, particularly if it consists of a fundal uterotomy or a classic C-section or an inverted T incision, no attempt at vaginal delivery should be made [55], [56], [57].

6.4.2  Methods of inducing labour after previous caesarean section

6.4.3  Induction of labour after previous caesarean section

7  Setting for inducing labour

8  Induction of abortion/induction of labour for intrauterine foetal death

8.1  Induction of abortion in the 1st trimester

It is important to differentiate between induction of abortion for disorders of pregnancy (e.g., missed abortion, anembryonic pregnancy, gestational trophoblastic disease) and termination of a viable pregnancy in accordance with the German legal requirement of compulsory ethical counselling (in Austria: legal provisions permitting abortion within the first 3 months of pregnancy as outlined in § 97 of the Austrian Penal Code; in Switzerland: non-punishable termination of pregnancy in accordance with Article 119, Para. 2 of the Swiss Penal Code) or for criminological or medical reasons. Before terminating the pregnancy, an ultrasound examination must be carried out to determine the duration of the pregnancy.

The patient should be informed about the advantages and disadvantages of the respective methods (cf. [Table 5]) [58].

Table 5 Advantages and disadvantages of the different methods used to induce abortion in the 1st trimester of pregnancy [58].

	Medication	Surgery
Invasive procedure	usually not	yes
Anaesthesia	not required	required
Timing can be predicted	timing uncertain	can be scheduled
Success rate	approx. 95%	approx. 99%
Bleeding	moderate	slight
Follow-up care	required to exclude incomplete abortion	often not required
Approach	more than one session	single session

8.2  Induction of abortion/induction of labour for intrauterine foetal death in the second and third trimester of pregnancy

In German-speaking countries, medication is used to terminate a pregnancy from week 14 + 0 of gestation at the latest. The sequential administration of mifepristone and misoprostol has been reported to be superior to applying only misoprostol, even in the 2nd trimester. There are many variations of dosages and intervals between applications.

Below are examples of recommended regimens:

• mifepristone 200 mg administered orally, followed after 24 – 48 hours by:

  • misoprostol 400 µg, buccal administration every 3 hours (maximum of 5 doses) [59], or

  • misoprostol 400 µg, (oral, vaginal or sublingual administration) every 3 hours (maximum of 5 doses) [60], or

  • misoprostol 800 µg, vaginal administration every 3 hours (maximum of 5 doses) [59], or

  • misoprostol 800 µg, vaginal administration, followed by 400 µg misoprostol every 3 hours, administered vaginally or sublingually (maximum of 5 doses) [60].

In the 3rd trimester, it is recommended that misoprostol doses given after the administration of mifepristone are lower:

• misoprostol 200 µg, (vaginal, sublingual or buccal administration) every 4 – 6 hours [61]

• misoprostol 100 µg, (vaginal, sublingual or buccal administration) every 4 – 6 hours [61]

If the foetus has not been aborted/delivered after 5 doses of misoprostol, most international guidelines recommend pausing further drug treatment for 12 hours; the current NICE recommendations do not stipulate a pause [62].

After abortion/delivery has occurred, the placenta should be carefully inspected and an ultrasound examination of the uterine cavity should be carried out; additional curettage is not required. But curettage should be carried out if there is a suspicion of retained placental fragments or postpartum haemorrhage.

Conflict of Interest/Interessenkonflikt

The conflicts of interest of all of the authors are listed in the German-language long version of the guideline./Die Interessenkonflikte der Autoren sind in der Langfassung der Leitlinie aufgelistet.

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Publication History

Received: 25 May 2021

Accepted after revision: 27 May 2021

Article published online:
09 August 2021