creation date: 2026-01-13 10:04
tags: Pathologies
Preterm Labour
Background
Definitions
Preterm labour refers to labour which will result in preterm birth.
Preterm birth is defined as a birth that occurs before 37 completed weeks (<259 days) of gestation.
Etiology and Pathogenesis
The four primary processes are:
Stress-induced premature activation of maternal or fetal hypothalamic-pituitary-adrenal axis
- Activation of GPA axis results in cascading hormonal changes
- Stress-related CRH mimics physiological accumulation of CRH which initiates increased levels of glucocorticoids and prostaglandins (earlier than normal) eventually culminating in premature birth
Exaggerated inflammatory response/infection and/or altered genital tract microbiome
- Infection or cellular stress results in intra-amniotic inflammation
- Some bacteria produce phospholipase A2 (prostaglandin synthesis) and endotoxin which stimulate uterine contractions
- Some bacteria produce enzymes that degrade fetal membranes leading to preterm prelabour rupture of membrane (PPROM)
Decidual hemorrhage
- Damaged decidual blood vessels present as vaginal bleeding or retroplacental hematoma
- Following placental abruption, decidual tissue factor activates coagulation cascade
- Thrombin binds to protease-activated receptors 1 and 3 which results in uterine contractions
Pathological uterine distension
- Multiple gestations, polyhydramnios, etc. stretches the myometrium which results in upregulation of several factors that causes uterine contractions and cervical dilation
Clinical Presentation
Signs & Symptoms
Clinical findings of true labour are the same as term labour:
- Regular, usually painful contractions resulting in cervical change
Prodromal features include:
- Menstrual-like cramping
- Mild, irregular contractions (Braxton Hicks contractions)
- Low back acne
- Pressure sensation in the vagina or pelvis
- Vaginal discharge of mucus (may be clear, pink, or slightly bloody; mucus plug)
- Spotting, light bleeding
Rupture of the fetal membrane before the onset of uterine contractions may occur (preterm prelabour rupture of membranes; PPROM).
History & Physical Exam
Review of past and present obstetrics and medical history, including risk factors for preterm birth should be made. Estimation of gestational age should be made.
Attempt to elicit a cause of preterm contractions as identification of an underlying complication (eg. abruption) may require specific intervention.
The following assessments should be performed:
- Contraction frequency, duration, and intensity
- Examination of uterus for firmness, tenderness, fetal size, and fetal position
- Fetal heart rate monitoring
- Speculum/digital cervical exam for cervical dilation, presence and amount of uterine bleeding, fetal membrane status, swabbing for fetal fibronectin should it be indicated
Risk factors
Risk factors for preterm birth include:
- History of prior preterm birth
- Preterm prelabor rupture of membranes
- Shortened cervical length
- Intrauterine or vaginal infection
- Placental abruption
- Placenta previa
- Multifetal gestation
- Abnormally high or low amniotic fluid volume
- Conditions associated with uteroplacental insufficiency (eg, hypertensive disorders of pregnancy, diabetes mellitus, autoimmune disease)
- Evidence of uteroplacental insufficiency (eg, fetal growth restriction, oligohydramnios, abnormal umbilical artery Doppler velocimetry)
- Extremes of maternal age (40 years or older)
- Poor nutrition
- Low maternal body mass index
- Inadequate prenatal care
- Substance use
Diagnosis
Criteria
The following criteria is used to avoid overdiagnosis before labour is established:
Uterine contractions (≥6 in 60 minutes) plus one of the following:
- Cervical dilation ≥3 cm
- Cervical length <20 mm on TVUS
- Cervical length 20-29 mm on TVUS and positive fFN
Work-up
Ultrasound
Transvaginal ultrasound may be indicated if diagnosis is unclear. Obstetrics ultrasound may be performed for additional information which may guide treatment.
Laboratory studies
- Retrovaginal group B streptococcal culture (if not completed within 5 weeks prior)
- Urine culture for asymptomatic bacteriuria and pyelonephritis
- Fetal fibronectin (fFN) if preganacy <34 weeks with intact membranes, cervical dilation <3 cm, and cervical length 20-29 mm on TVUS (distinguishes true preterm labour from false labour)
- Testing for STI depending on risk factors
Prelabour rupture of membranes
In cases where a rupture of membranes occurred prior to preterm labour, confirmation of PPROM can be made using:
- Nitrazine paper (pH) - amniotic fluid has pH of 7.0-7.3 (normal vaginal pH 3.8-4.2)
- Fern test - ferning pattern seen microscopically in fluid form the posterior vaginal fornix
Differential
Diagnoses include:
- Braxton-Hicks contractions (ie. false labour)
- Placental abruption
- Uterine rupture
Red Flags / Complications
Neonatal complications associated with prematurity. Further complications may be present depending on underlying etiology of preterm labour.
Management
Prevention
Several prevention strategies can be employed for those at high risk of preterm birth or if there are signs of impending preterm birth:
- Progesterone 200 mcg VAGINALLY daily (if asymptomatic with cervical length ≤2.5 mm at week 18-24; begin at 16-20 if prior preterm birth)
- Cervical length screening
- Rescue cerclage for cervical insufficiency with dilatation <4 cm
Triage
Patients found to not be in labour without complications after 4-6 hours are discharged and counselled to follow up in 1-2 weeks or if signs/symptoms of labour or complications arise.
Patients found to be in labour and ≥34 weeks of gestation, labour and delivery are managed as normal as the benefits of inhibition of labour does not outweigh the complications/cost.
Patients <34 weeks of gestation are admitted for further observation and treatment.
Treatment of preterm labour <34 weeks
Antenatal corticosteroids
A course of betametasone or dexamethasone is administered to mitigate risk of incomplete lung development.
Tocolytics
Inhibition of acute labour in order for corticosteroids to achieve its maximum effect.
GBS prophylaxis
Antibiotics are used against early-onset neonatal group B streptococcal infection. This is indicated unless the patient had a recent negative GBS culture (5 weeks).
Neuroprotection
Magnesium sulfate is administered if gestational age is <32 weeks.