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Analysis of contractile activity

Uterine contraction is the driving force that, during birth, enables dilatation of the cervix and progression of the fetal presenting part through the pelvi-genital canal. Without going into detail, contractile uterine activity increases progressively during pregnancy due to the multiplication of muscle cells and intercellular "gap junctions", the increase in intracellular free calcium concentration Ca⁺⁺, prostaglandins and oestrogens.

L’augmentation de l’activité contractile au cours de la grossesse d’après Amorim-Costa 2016 — Figure 1.9 – The increase in contractile activity during pregnancy, according to Amorim-Costa 2016

During labour, the characteristics of uterine contractions are well known. They are:

involuntary, due to smooth muscle; the patient undergoes the contraction;
intermittent and rhythmic (4 to 5 per 10 min); the duration of contractions varies from 30 to 60 seconds during the 1^st stage of labour, then increases to reach 90 seconds during the 2^nd stage of expulsion. The resting period between two contractions ranges from 2 to 3 min.
total, involving the whole uterus, i.e. synchronous;
and painful: the pain increases as labour progresses and occurs beyond a certain intensity threshold (Polaillon threshold). This perception of pain varies between women in labour. Antenatal preparation methods may help raise this threshold…

Uterine activity is recorded together with the fetal heart rate. It is analysed in terms of the frequency, intensity and duration of contractions, and the relaxation time between them (Fig. 1.10, 1.11 and 1.12). Uterine activity is also quantified by the number of Montevideo units (MU); this corresponds to the sum of the maximum intensity of the contractions, measured in mmHg, over a ten-minute period.

Normal and abnormal values of uterine activity are given in Table 1.3, and the increase in uterine activity during labour is given in Tables 1.4 and 1.5, with mean values and their standard deviation. According to Johnson 1994^[6], Peebles 1994^[8] and McNamara 1995^[7], the nadir of the fall in SaO₂ corresponds to the end of the contraction, and the return to normal takes 2 minutes. If the recovery interval is less than 2 minutes, SaO₂ decreases progressively and the fetus becomes hypoxaemic.

Note: according to Ayres de Campos 2015a^[2], "some data suggest that in the case of spontaneous labour, it takes up to 90 seconds after a contraction for fetal oxygenation to be restored (McNamara 1995^[7]), whereas when oxytocin is used, this recovery period lasts on average 138 seconds (Peebles 1994^[8])" . These 2 figures, cited by Berveiller 2020^[4], are however not specified in the 2 original publications by McNamara and Peebles…

It is essential to obtain an accurate recording of uterine contractions throughout birth in order to correctly interpret FHR decelerations and to prevent oxytocin-induced tachysystole. The use of an internal pressure catheter in cases of major obesity or dynamic dystocia allows precise assessment of uterine activity (Boog 2011^[5]).

L’activité contractile (1) — Figure 1.10 – Contractile activity (1)

L’activité contractile (2) — Figure 1.11 – Contractile activity (2)

Des contractions bigéminées — Figure 1.12 – Bigeminal contractions

Table 1.3 – Contractile activity

Parameter	Normal	Abnormal	Term
Frequency	3 to 5 UC / 10 min	< 3 UC / 10 min	frequency hypokinesia
		> 5 UC / 10 min	tachysystole
Intensity	< 80 mmHg	≥ 80 mmHg	intensity hyperkinesia
Basal tone	< 20 mmHg	≥ 20 mmHg	uterine hypertonus
Duration	< 2 min	≥ 2 min

Table 1.4 – Contractile dynamics during dilatation

Parameter	3-4 cm	4-5 cm	6-8 cm	9-10 cm
Basal tone (mmHg)	5 ± 3	6 ± 4	8 ± 5	8 ± 5
Intensity (mmHg)	35 ± 12	42 ± 14	47 ± 16	48 ± 16
Frequency (/ 10 min)	3.8 ± 1.7	3.8 ± 1.5	4 ± 1.6	4.1 ± 1.4
Duration (s)	82 ± 31	86 ± 22	86 ± 19	83 ± 19
Uterine activity (uM)	104 ± 42	131 ± 60	143 ± 60	157 ± 69

Note: after Schaal and Martin 2004^[9].

Table 1.5 – Characteristics of contractile activity

Parameter	Dilatation	Expulsion
Duration (s)	87 ± 10	84 ± 12
Interval between UCs (s)	63 ± 35	47 ± 34
Number / 10 min	4.8 ± 0.7	5.2 ± 0.9
Uterine activity (uM)	236 ± 97	402 ± 129

Note: after Bakker 2007^[3].

References

Amorim-Costa C., Costa-Santos C., Ayres-de Campos D., et Bernardes J. Longitudinal evaluation of computerized cardiotocographic parameters throughout pregnancy in normal fetuses: A prospective cohort study. Acta Obstet Gynecol Scand, 95 (10): 1143–1152, Oct. 2016. ISSN 0001-6349, 1600-0412. doi: 10.1111/aogs.12932.
Ayres-de-Campos D., Arulkumaran S., et FIGO Intrapartum Fetal Monitoring Expert Consensus Panel. FIGO consensus guidelines on intrapartum fetal monitoring: Physiology of fetal oxygenation and the main goals of intrapartum fetal monitoring. Int J Gynaecol Obstet, 131 (1): 5–8, Oct. 2015a. ISSN 1879-3479. doi: 10.1016/j.ijgo.2015.06.018. ↩
Bakker PCAM., Kurver P. H. J., Kuik D. J., et Van Geijn H. P. Elevated uterine activity increases the risk of fetal acidosis at birth. American journal of obstetrics and gynecology, 196 (4): 313–e1, 2007. ↩
Berveiller P., Garabedian C., et Maisonneuve E. Analyse pratique du RCF: méthodologie et cas cliniques commentés. Pratique en gynécologie-obstétrique. Elsevier Masson, Issy-les-Moulineaux, 2020. ISBN 978-2-294-76845-3. ↩
Boog G. Cerebral palsy and perinatal asphyxia (II–Medicolegal implications and prevention). Gynecologie, obstetrique & fertilite, 39 (3): 146–173, 2011. ISSN 1297-9589. ↩
Johnson N., Van Oudgaarden E., Montague I., et McNamara H. The effect of oxytocin-induced hyperstimulation on fetal oxygen. BJOG, 101 (9): 805–807, Sept. 1994. ISSN 1470-0328, 1471-0528. doi: 10.1111/j.1471-0528.1994.tb11951.x. ↩
McNamara H. et Johnson N. The effect of uterine contractions on fetal oxygen saturation. BJOG: An International Journal of Obstetrics & Gynaecology, 102 (8): 644–647, 1995. ISSN 1471-0528. ↩
Peebles D. M., Spencer J. A. D., Edwards A. D., Wyatt J. S., Reynolds E. O. R., Cope M., et Delpy D. T. Relation between frequency of uterine contractions and human fetal cerebral oxygen saturation studied during labour by near infrared spectroscopy. BJOG, 101 (1): 44–48, Jan. 1994. ISSN 1470-0328, 1471-0528. doi: 10.1111/j.1471-0528.1994.tb13008.x. ↩
Schaal J.-P., Martin A., et Riethmuller D. Surveillance foetale: guide de l'enregistrement cardiotocographique et des autres moyens de surveillance du foetus. Sauramps médical, Montpellier, 2ème edition, 2004. ISBN 978-2-84023-249-0. ↩