Labour and Childbirth: Why the Uterus Goes into Labour

Labour remains challenging moment for the skilled birth attendant as he or she would be striving to ensure safe mother and healthy baby during delivery. Most pregnant women appear to be happier as they approach the last day of their third trimester due to the joy of having their baby in their arms. While some are embattled psychologically with fear of pain.  In this article, we shall consider why the uterus goes into labour and what labour is all abour.

What is labour?

Labour is the physiological process involving regular and rhythmic uterine contractions and retractions with progressive cervical dilation, in which the foetus, placenta and membranes are expelled from the uterus through the birth canal after 24th weeks of gestation.

It can also be defined as the process by which a woman gives birth.  The whole process is achieved by the rhythmic contraction and retraction of the uterine fibres and progressive dilatation of the cervical Os. 

The foetus is considered to be viable — capable of surviving outside the uterus after 24th weeks of gestation. Thus, evacuation of the product of conception by the contraction and retraction of the uterus before the 24th week of gestation is not labour but abortion . The uterine contractions can occur naturally and spontaneously or through artificial use of medicines.

Labour is considered to be normal when it occurs spontaneously at term (37-40 weeks of gestation) with the foetus presenting by vertex, the whole process completed by unaided naturally maternal efforts, does not exceed 24 hours without any complications to both mother and baby. 

It is also said to be established when there are  regular painful rhythmic uterine contraction and cervical dilatation of about 4 cm in primigravida and 3cm in multigravida. 

Causes of onset of  labour

The uterus has the intrinsic power to expel its contents before term as cases of miscarriage, preterm labour or induction before term. You might wonder why throughout pregnancy, the uterus remains sedated. 

The actual cause of onset of  labour is not properly known. However, reasons why the uterus remains quiescent during pregnancy and triggers off contraction at term could be associated with the following:

  • Uterine muscle is inhibited or completely blocked from stimulation by high levels of progestrone during pregnancy. Experts have noted that large doses of progestrone postpone labour in rabbit.
  • Towards term the level of progestrone falls, oestrogen rises and opposes the progesterone block of uterine activity. Oestrogen which triggers hyperplasia and hypertrophy of uterus during pregnancy is also believed to initiate the synthesis of prostaglandins by the decidua cells. The prostaglandins act locally on myometrial fibres and are stimulated to contract.
  • The maternal decidua, myometrium and fetal membranes are associated with the synthesis of prostaglandins that trigger contractions. 
  • Receptors for oxytocin found in myometrium, decidua, placenta and foetal membranes. The oxytocin receptors in the myometrium directly stimulate muscle contractions, resulting in childbirth.
  • Increased contractility due to over-stretching and over-distenstion which occur at term and in the obstetric conditions such as multiple pregnancy, and hydramnios had been implicated.
  • Pressure of the presenting part on the cervical nerves could cause contractions at term.
  • Braxton Hicks contractions which increase in amplitude towards term may cause the uterine contractions.
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Probable/warning/premonitory signs of labour

These are signs that show that labour is likely to occur soonest and may include the following:

Lightening

This occurs towards the last week of the third trimester of pregnancy when women experience a sign of relief as the gravid uterus no longer crowds the lungs because of the sinking down of the lower uterine segment into the pelvis, causing the presenting part to descend. 

This is usually referred to as “Give of the pelvis“. Lightening occurs because of the widening of the joints and ligaments of pelvic joints, symphysis pubis and the softening, relaxation and sagging of the pelvic floor muscles in combination with the  formation of the lower uterine pole. This physiologic effect is caused by pregnant hormones particularly the relaxin  synthesized by the placenta. 

Frequency of micturition: 

This occurs during the first trimester due to pressure of weighing the pregnant  uterus on the urinary bladder resulting in frequent urge to micturate but it gets relieved during the second trimester.  

During the third trimester, there is Increased irresistible  urge to void because the foetal head has descended,  causing greater pressure on the bladder (lightening). 

Braxton Hicks’ contractions

These ‘practice’ contractions begin from the 16th week of pregnancy and improve the uterine blood flow to the placenta. They also help in the formation of the lower uterine pole towards the last trimester of pregnancy. 

This is Nature’s way  of training and sensitising the uterus for the anticipatory future functions (delivery). 

Reduction in the Amniotic fluid:  Amniocentesis helps to detect reduction in liquor, which may account for the drop in the maternal weight towards term or that the uterus is approaching labour.

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Effacement of the cervix:  This refers to when the cervix is “ripped” and ready to go into delivery. It implies that the cervix can cooperatively dilate with commensurate uterine contractions.   

Effacement can also refer to the “taking up” of the cervix, which means the cervical ring has been drawn up to form part of the lower uterine segment. When the cervix shortens, the cervical Os can admit a tip of one or two fingers.  Shortening of the cervix is usually looked for in the interest of mother or baby, or when labour has to induced.  

Labour

True labour Vs False Labour

True labour is also called true pain and refers to a physiological  process characterized by regular uterine contractions, dilatation of the cervix and expulsion of fetus, placenta and membranes. 

Recognition of true labour

Regular uterine contractions: When the uterine contraction and retraction occurs in a regular interval — maintaining a consistent rhythm, the uterus is said to be in labour.

Dilatation of the cervix:  This refers to the opening of the neck of the womb. Cervical dilatation occurs in agreement with the contraction and shortening of the longitudinal and oblique muscle fibres of the upper segment.  When the cervix dilates, it becomes obvious that the uterus is definitely trying to empty its contents.

Show: “Show” refers to blood-stained mucus which is released or discharged from the cervix as it dilates. The “show” is a tenacious mucus that plugs the cervix during pregnancy. It mixes with blood from severed blood vessels around the cervix as it dilates.

However, false labour otherwise called false pain or spurious labour and refers to irregular and erratic uterine contractions lasting 2 to 3 minutes and pain a pregnant woman experienced with no accompanying changes in cervix(e.g. no “Show” or cervical dilatation). 

Difference between true labour and false labour

True labourFalse labour
Regular rhythmic uterine contractionsUterine contractions are irregular and erratic.
Duration of contraction rarely exceeds 60 seconds.Contractions may last  2 to 3 minutes
Back ache may be presentBack ache absent
Cervix is effacedCervix is not effaced
Progressive cervical dilatationCervix may not dilate
“Show” is presentNo “show”
Membranes may or may not be intact.Membranes intact 

Factors influencing labour 

There are five factors that affect the process of childbirth, and are usually remembered by this acronym: 5P’s.

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P: Passenger —This refers to: 

  1. Foetus: The way the foetus negotiates or moves through the birth canal is determined by some factors such as size of the foetal head, presentation, lie, attitude and foetal position.
  2. Membranes: Intact pulsating membranes is a good dilator of the cervix.
  3. Placenta: It rarely impairs the process of labour in normal vaginal birth. 

P: Passageway — This points to: 

  1. Bony pelvic canal: adequacy of the pelvis for parturition
  2. Cervix: a soft and thin cervix dilates faster.
  3. Pelvic floor: Yielding pelvic floor muscles aids descent and childbirth.
  4. Vagina: Distensible vaginal wall helps perineal phase of delivery.
  5. Vaginal introitus: External opening to the vagina has effects on childbirth.   

P: Powers —These refer specifically to uterine contractions, which can originate from both primary and secondary powers.

  1. Involuntary or primary powers include:
  • Braxton Hicks contractions: Painless uterine contractions before childbirth that helps in cervical effacement.
  • Painful contraction at the onset of parturition that assists in cervical dilatation.
  1. Secondary powers — These comprise of:
  • Expulsive uterine contraction of the second stage.
  • Bearing-down efforts of the woman
  • Contractions of the abdominal muscles
  • Contractions of the muscles of the diaphragm
  • Increased intra-abdominal pressure that compresses the uterus and adds to the expulsive forces. Secondary powers have no effects on cervical dilatation but are very helpful in the expulsion of the foetus.

P: positions of the labouring woman

During childbirth, women should be encouraged to adopt any position most comfortable to her. Position affects the woman’s anatomical and physiological adaptation during parturition. 

For instance, an upright position such as walking, sitting, kneeling or squatting provides several benefits. Upright position aids gravity for descent and makes uterine contractions generally stronger and more efficient in effacement and dilatation of the cervix.  Frequent changes in position during delivery relieves fatigues, increases comfort and improves circulation. 

P:Psychological response of the woman

This refers to the woman’s state of mind and perception about parturition generally. How she copes in labour depends on her personality make-up, previous experiences and/or tales about labour pain as well as her concept generally about childbirth. 

Her belief that labour pain is unbearable would definitely affect her pain threshold.

The bottom line

Labour is a process by which a woman gives birth and is characterised by regular and rhythmic uterine contractions and retractions with progressive cervical dilation as well as expulsion of baby, placenta and membranes. 

Its onset is caused by hormones (release of oxytocin and altered oestrogen progesterone ratio), biochemical(prostaglandins) and mechanical( pressure from the presenting part and over-stretched uterus) which occur at term.

To have a successful delivery, the factors influencing labour should be considered.  Every skilled birth attendant should endeavour to provide women with respectful, evidence-based maternity care irrespective of race, culture and religion.  Thanks for sharing.