24 hour rhythm to the onset of preterm labour


Correspondence: Dr S. W. Lindow, Hull Maternity Hospital, Hedon Road, Hull HU9 5LX, UK.


Objective To describe the time of onset of contractions which result in preterm delivery to investigate if there is a diurnal influence.

Design The admission register for the neonatal unit was used to identify admissions for prematurity over a three-year period. Obstetric case records were then reviewed to obtain the recorded time of onset of contractions when delivery was preceded by spontaneous labour.

Setting A maternity hospital in the United Kingdom.

Main outcome measures The time of onset of spontaneous contractions which result in preterm labour and delivery.

Results Four hundred and twenty-five women in preterm labour were studied. A significant diurnal rhythm in the timing of onset of contractions was noted with 42% of deliveries occurring in labour which commenced between midnight and 0600 hours. Subgroup analysis indicated that there was a significant rhythm in second trimester preterm labours, male and female babies and that this rhythm was present during both the winter and summer months.

Conclusion The periodicity of preterm labour onset demonstrates a rhythm which is similar to the rhythm in the onset of labour at term. Preterm labour most commonly begins between midnight and 0600 hours.


Preterm labour, leading to preterm delivery, represents a leading cause of perinatal mortality and morbidity in the developed world. The mechanism of the onset of preterm labour, despite its importance, remains unknown and is a subject of intense research activity. Similarly, while the mechanism of the onset of labour at term is also poorly understood, it may provide clues to the origin of preterm labour. The onset of term labour is known to demonstrate a 24-hour rhythm with the peak of onset of contractions occurring at 01.001.

Whether preterm labour represents an early maturation of a physiological process or, alternatively, an entirely pathological process, is uncertain. It has also been suggested that late onset preterm labour may represent a physiological mechanism whilst early preterm labour is a pathological mechanism. We considered that it may be possible to clarify the physiological or pathological nature of preterm labour by studying the 24-hour rhythm to the onset of preterm labour to determine if the periodicity observed in term labour onset is similar.

This study was undertaken to document the timing of onset of contractions when preterm labour resulted in delivery, and to compare the periodicity of early preterm labour (24–32 weeks of gestation) and late preterm labour (33–37 weeks of gestation).


The neonatal unit admission register at Hull Maternity Hospital was studied to identify women who had babies admitted to the neonatal unit at a gestational age of < 37 weeks over a three-year period (1992–1994). A total of 11,796 deliveries were recorded during the three years, and 624 of these (5.3%) were neonatal admissions. The recorded time of onset of contractions was obtained. Only precise times recorded by the medical or midwifery staff were included in this analysis. The first contraction time was approximated to the nearest hour.

Three hundred and sixty-three sets of notes (58%) contained an appropriate record of labour pains. In a number of cases (n= 62) details were also taken from additional pregnancies associated with preterm labour which were in the maternity records. This produced a total of 425 cases for analysis.

Statistical analysis was undertaken using a χ2 analysis with the observed number of deliveries in each three-hour period compared with a uniform number of deliveries in each three-hour period.


The total number (n= 425) of cases was divided into two groups: 23–32 weeks of gestation (n= 169) and 33 weeks to 37 weeks of gestation (n= 256). According to the time of onset of contractions, the cases were regrouped into eight, three-hourly periods (Fig. 1). Comparing three-hourly totals there is a significant difference between the number of women who went into labour during the night periods compared with those during day time (χ2= 71.28, P≤ 0.00001).

Figure 1.

The timing of the onset of preterm labour [——total (n= 425); ––– 33–37 weeks of gestation (n= 256); …. 24–32 weeks of gestation (n= 169)]; inline image= timing of delivery (n= 425).

When the early preterm deliveries of ≤ 32 weeks and the late preterm deliveries (≥ 33 weeks) were analysed separately, there was still a significant rhythm evident (χ2=51.73, P≤ 0.00001 and 27.8, P≤ 0.001, respectively). There was no significant difference between the rhythm of the two gestational ages. Similarly there was no significant difference between rhythm for singleton or multiple births, (χ2= 9.8, NS) or the sex of the child (χ2= 14.1, NS) (Table 1).

Table 1.  Time of first contraction in labours which result in preterm birth. Values are given as n, unless otherwise indicated.
 Time of day (hours)  
Characteristics of pregnancy0–23–56–89–1112–1415–1718–2021–23TOTALSignificance
  1. All analysis by χ2 test.

Total no. of patients8989544236412945425< 0.00001
≥ 32 weeks384320158141417169< 0.00001
33–37 weeks5146342728271528256< 0.001
≥ 28 weeks131163168755< 0.05
Summer (May-July)342216101081014124< 0.0001
Winter (Nov-Jan)333518141416811149< 0.00001
Male infants4254242723271431242< 0.00001
Female infants4735301513141514183< 0.00001
Singleton pregnancy8178453231362838369< 0.00001
Multiple pregnancy811910551756NS
Previous preterm birth23201610891410110< 0.05
First preterm birth6669383228321535315< 0.00001
Idiopathic preterm labour3735221513181317170< 0.0001
Predisposing causes5254312726231638254< 0.00001
Primiparous4143232118171225200< 0.00001
Multiparous4846312118241720225< 0.00001
Time of delivery5272555462355639425NS

No difference between the periodicity of labour onset was noted when comparing women who were delivered in the summer months (May-August) with those who were delivered in the winter months (November-February). Both groups demonstrated a tendency for nocturnal onset of labour.

Similarly there was no difference in the rhythm when there was a history of preterm deliveries in the past when compared with a first episode, or if there were complicating events resulting in the onset of labour compared with idiopathic preterm labour (Table 1). There was no significant rhythm to the timing of delivery in the same group of patients (Table 1, Fig. 1).


The analysis of preterm births in Hull Maternity Hospital has indicated that there is a diurnal rhythm to the onset of preterm labour. This rhythm is present in both early and late preterm labour, male and female fetuses, single births and winter and summer deliveries. This rhythm is also present in women in their first preterm delivery compared with those with a recurrent problem, and it is also evident in women with known predisposing causes to preterm labour, compared with those in idiopathic preterm labour.

The magnitude of this difference is such that 42% of the births occurred in women who went into labour in the six-hour period between midnight and 0600 hours. This rhythm to the onset of labour has been described with a similar increase in the frequency in the onset of labour in women at term who go into labour during the early hours of the morning1.

It could be hypothesised that preterm labour represents a premature maturation of a physiological mechanism or, alternatively, that it could be due to an entirely pathological mechanism or mechanisms. It has also been suggested that late preterm labour may be physiological and early preterm labour, particularly that occurring in the second trimester, is more likely to be pathological2.

By examining the subtle rhythms which occur in the onset of preterm labour, it would appear from our data that the mechanism which causes the onset of contractions, which lead to both preterm and term labour, is influenced by diurnal factors. Furthermore, our results indicate that the onset of labour between 23 and 37 weeks has the same diurnal influences as those, which occur in term labour. The rhythm of time on delivery did not demonstrate a significant diurnal pattern, which may be expected as operative intervention, tocolysis and variation in labour duration are all factors which can alter the periodicity of the onset of labour.

Uterine contractions occurring before labour have been studied using a mobile tocodynamometer and data storing unit which documented the hourly number of contractions which occurred prior to labour from 24 weeks onwards3. It was found that there is a strong diurnal variation to non-labour uterine contractions which increases as gestation increases. Contraction frequency reached a maximum in the early hours of the morning. The authors also noted that rest decreased uterine activity, while sexual intercourse increased it. A similar nocturnal rise in non-labour uterine activity was noted by Main et al.4. The aetiology of the nocturnal rise in uterine activity before labour or the nocturnal predominance of contractions which represents the onset of labour is unknown.

Oxytocin is known to have a diurnal rhythm which demonstrates a peak of activity at midnight. This is evident in the mid trimester and third trimester5. Further evidence of the involvement of oxytocin comes from studies in pregnant rhesus monkeys. Honnibier et al.6 found that the administration of oxytocin antagonist inhibited spontaneous nocturnal myometrial contractions. Further work in pregnant rhesus macaques indicates that despite increases in maternal plasma and amniotic fluid prostaglandin levels as delivery approached, there were no demonstrable day-night differences. Pre-labour uterine activity rhythms were not related to prostaglandin levels7. In addition to oxytocin, oestrogen-progesterone ratios8 and beta-endorphin5 have been shown to demonstrate a diurnal rhythm.

Studies of the 24-hour rhythm of corticotrophin releasing factor and gonadotrophin releasing hormone in the plasma of pregnant women could not demonstrate a rhythm9. Salivary steroid hormone profiles in late pregnancy demonstrated evidence of a rhythm in cortisol and, to a lesser degree, progesterone10. Both hormones demonstrate a reduced level at 2200 hours compared with 0800 hours, with a more pronounced rhythm in the cortisol levels.

This study does not shed any light on the actual mechanisms which may be involved in preterm labour. However, the subtle rhythms which have been described in association with term labour are similar to the rhythm in both early and late preterm labour.

It could be hypothesised that the mechanism which results in the initiation of labour contractions is similar in all women who give birth after 24 weeks. This study indicates that the initiation of idiopathic preterm labour is a premature maturation of a physiological mechanism. However, the aetiological factors which are involved in the preterm labour process are not necessarily the same in each gestational age group.

In summary, there is a clear difference in the frequency between the onset of preterm labour during night time and day time hours. This periodicity is present in early preterm, late preterm, single gestations, winter or summer months and male or female fetuses. Comparison with the published data by Smolensky et al.1 on the periodicity of the onset of labour at term indicate a similarity between the two groups of patients.