ORGANUM
Towards the end of pregnancy, as the time of labour approaches, the cervix must undergo cervical effacement (thinning) and dilation (widening) to facilitate and accommodate the delivery of the fetus. Complete dilation of the cervix is achieved when the cervix dilates to 10 cm. How rapidly a female will progress to full dilation is dependent on several factors, including her parity, medical history, pelvic anatomy, the size of the fetus and the position of the fetus at the time of labor.
Multigravida women progress to full dilation more quickly than primigravida. If the pelvic anatomy of the mother is too narrow or has any other anatomic anomaly, cephalopelvic disproportion between the fetus and the mother may lead to prolonged or arrest of labor. If the fetus is in the breech position, this may also lead to labor difficulties and hinder the ability of the cervix to dilate fully.
Effacement and dilation of the cervix from 0 to 10cm
Latent phase : 0-6 cm
Active phase : 6-10 cm
Delivery of the fetus
Expulsion of the placenta
The latent phase should not > 20 hours in a primigravida female or 14 hours in a multigravida female. If the mother does not fully dilate by these average time estimates, she is said to be having an arrest of the active phase of Stage 1 labor.
A positive feedback loop requiring oxytocin plays a pivotal role in the progression of the first stage of labour. Oxytocin is made in the hypothalamus and released into the bloodstream by the posterior pituitary gland, where it is transported to the uterus and stimulates uterine contractions. These contractions result in the descent of the fetus, with the descending fetal head activating stretch receptors on the cervix; these send signals to the hypothalamus via afferent nerves, stimulating the additional release of oxytocin. This loop results in progressively intensifying contractions and dilation over time.
This exam is known as a digital cervical exam. The obstetrician places two fingers inside the vagina and measures the width of the opening of the external os of the cervix using fingers to approximate the distance:
1cm : One fingers width
3cm : Width of two fingers
4cm : Two fingers are approx. 1 cm apart
7cm : Widest stretch of the two fingers
10cm (full): The cervix should not be present in front of the presenting fetal part (typically the head).
Ultrasound may play a new role in measuring cervical dilation; still, at the moment, the digital cervical exam remains the gold standard.
There is a risk of vaginal bleeding, discomfort and iatrogenic amniotic membrane rupture.
The bishop scoring system is known as the bishop score; it is an important criterion utilised to determine the probability of successful labour induction. It is a clinical tool used to assess the cervix in pregnant women that can be obtained at the time of induction by a digital cervical exam. There is a maximum score of 13, with a higher score (defined as 6+) indicating a more favorable cervix for vaginal delivery and a higher chance of the women undergoing cesarean delivery. Many clinicians use a simplified scoring system that includes only cervical dilation, effacement, and station, with each factor scoring 0 to 3 points. In this shortened modification, a score of >5 is considered favourable
Score | Dilation (cm) | Position of cervix | Effacement (%) | Station -3 to +3 | Cervical consistency |
|---|---|---|---|---|---|
0 | Closed | Posterior | 0-30 | -3 | Firm |
1 | 1-2 | Midposition | 40-50 | -2 | Medium |
2 | 3-4 | Anterior | 60-70 | -1 to 0 | Soft |
3 | 5-6 | - | 80 | +1, +2 | - |
The measure of how dilated the cervix is in centimeters; estimating the average diameter of the cervical os via digital cervical examination.
The estimation of the thinning or shortening of the cervix expressed as a percentage of a non-laboring average cervix. Zero percent effacement means the cervix has not thinned at all and is of average length, 50% effacement represents a cervix at half of the expected length, and 100% effacement represents subcentimeter cervical thinning.
The position of the fetal presenting relative to the ischial spines of the ischial spines of the maternal pelvis, which are halfway between the pelvic inlet and the outlet. At 0 station, the fetal head is at the level of the ischial spines. The bishop score divided the distance above and below this level into thirds, by which the position of the presenting fetal part is assigned negative numbers above and positive numbers below the 0 station: -3 to 3. The American College of Obs and Gyne (ACOG) redefined the division of fetal station from -5 to +5, using cm instead of dividing the distance from the ischial spines into thirds.
Refers to the position of the cervix
This represents the feel of the cervix on examination. A firm cervix is typically characterised as having a consistency similar to the tip of nose, whereas a soft cervix has a consistency similar to the labium of the oral cavity.
Failure of the cervix to dilate to an appropriate amount in a certain amount of time can lead to protracted or arrested labor:
Protracted latent phase (dilation <6cm)
> 20 hours (primigravida)
> 14 hours (multigravida)
Protracted active phase (dilation 6+ cm)
Dilation progresses less than 1cm/hour
Arrested active phase
No cervical change for 4+ hours with adequate contractions
No cervical change for 6+ hours with inadequate contractions
The absence of cervical change for greater than 4 hours in the presence of adequate contractions or six hours with inadequate contractions is considered an arrest of labor and may warrant clinical intervention.
In those with a bishop of less than 6 or arrest of labor, we can use pharmacological, mechanical or surgical methods.
Pharmacologic methods
Misoprostol : Synthetic prostaglandin E1 analog administered orally or vaginally
Dinoprostone: Prostaglandin E2 agent administered as a vaginal insert or cervical gel
Mifepristone: Progesterone antagonist administered orally
Oxytocin: Does not directly induce cervical dilation nor ripening per sae, but causes the uterus to contract more fully. This pushes the fetal head against the cervix, stimulating the positive feedback loop.
Mechanical methods
Hygroscopic dilations: these absorb endocervical fluids, allowing the device to expand within the endocervix, leading to dilation
Ballon catheter: A specialised balloon catheter or foley catheter may be used. The catheter is introduced to the endocervix and inflated with saline. The balloon rests on the internal OS, by being retracted, and applies direct pressure to the cervix, mimicking fetal head pressure against the cervix and thus oxytocin and prostaglandin may be released due to the cervical tissue response
Surgical methods
Membrane stripping: inserting a finger through the internal cervical OS and moving it in a circle, detaching the inferior pole of the membranes from the lower uterine segment
Amniotomy: Artificial rupture of the membranes
There are some non-pharmacological methods, such as herbal (primrose oil and red raspberry leaves), breast stimulation (oxytocin release), sexual intercourse (promotes release of oxytocin, stimulation of lower uterine via penetration may result in the local release of prostaglandins) and acupuncture (via oxytocin and prostaglandins). The clinician should verbalise to the patient that these non-pharmacological methods continue to maintain an uncertain efficacy role in labour induction and cervical ripening.
It is well known that the cervical length is a strong indicator of whether or not a birth will be preterm. The shorter the cervical length, the higher the risk of spontaneous preterm birth. For clinical ease, the cutoff above which a cervix can be considered normal is 25mm (2.5 cm). A meta-analysis of 158,000 women who underwent a transvaginal ultrasound between 16 and 24 weeks showed a mean cervical length of 3.8 cm.