Preterm Labour (PTL)

Introduction

Preterm birth is defined as birth occurring before 37 completed weeks of gestation. It remains the leading cause of perinatal morbidity and mortality in Canada and developed nations. For MCCQE1 preparation, understanding the triage, diagnosis, and management of preterm labour according to SOGC (Society of Obstetricians and Gynaecologists of Canada) guidelines is essential.

Definitions

• Preterm Labour: Regular uterine contractions accompanied by cervical change (dilation and/or effacement) occurring between 20+0 and 36+6 weeks gestation.
• Threatened Preterm Labour: Uterine contractions without documented cervical change.
• Viability: Generally considered ≥ 23-24 weeks in Canadian tertiary centres, though resuscitation decisions are complex and involve shared decision-making.

Etiology and Risk Factors

Identifying risk factors is crucial for the Health Promotion aspect of the MCCQE1.

Clinical Presentation and Diagnosis

Patients typically present with:

• Regular uterine contractions (painful or painless)
• Pelvic pressure
• Menstrual-like cramping
• Low backache
• Change in vaginal discharge (mucous plug, bloody show, watery fluid)

Diagnostic Approach

Management

The primary goals of management in the Canadian context are not necessarily to stop labour indefinitely, but to delay delivery long enough (48 hours) to administer corticosteroids and transfer the patient to a facility with appropriate neonatal intensive care (NICU).

Summary of Drugs and Dosages

Prevention of Preterm Birth

Secondary prevention is a high-yield topic for MCCQE1.

1. Progesterone Supplementation:

   • Vaginal Progesterone: Indicated for asymptomatic women with a short cervix (< 25 mm) on mid-trimester TVUS.
   • Evidence supports use to reduce PTB risk < 34 weeks.

2. Cervical Cerclage:

   • History-indicated: History of ≥1 second-trimester losses related to painless cervical dilation.
   • Ultrasound-indicated: Short cervix (< 25 mm) before 24 weeks in a woman with a prior spontaneous PTB.
   • Rescue cerclage: Dilated cervix on physical exam (before viability).

Canadian Guidelines (SOGC) Highlights

• Tocolysis: SOGC recommends Nifedipine or Indomethacin as first-line agents. Beta-mimetics (Ritodrine/Terbutaline) are generally no longer used due to maternal side effects.
• Magnesium Sulfate: Specifically recommended for neuroprotection in pregnancies < 32 weeks where birth is imminent.
• Transfer: CanMEDS Collaborator role involves recognizing when to transfer a patient to a Level III centre (Perinatal Centre) if the local hospital cannot support a preterm neonate (usually < 32-34 weeks depending on the province).

Key Points to Remember for MCCQE1

• Diagnosis: Requires contractions AND cervical change.
• Most useful test: Fetal Fibronectin (fFN) has a high negative predictive value (good for ruling OUT PTL).
• Steroids: Betamethasone is the single most effective intervention to improve neonatal outcomes.
• Indomethacin: Do not use after 32 weeks (ductus arteriosus closure).
• Magnesium Sulfate: Think “Neuroprotection” for < 32 weeks.
• Antibiotics: Only for GBS prophylaxis (unless PPROM or chorioamnionitis is present). Do not use antibiotics to prolong pregnancy in intact membranes (associated with worse outcomes).

Sample Question

# Sample Question

A 31-year-old G2P1 woman presents to a rural community hospital at 30 weeks gestation with complaints of regular abdominal tightening. Her pregnancy has been uncomplicated. On examination, her vital signs are stable. Fetal heart rate is 145 bpm with moderate variability. Sterile speculum examination reveals no pooling of fluid, but the cervix appears visually dilated. A digital exam confirms the cervix is 4 cm dilated and 80% effaced. The vertex is at -2 station. The nearest tertiary care centre with a NICU is 2 hours away by ambulance.

Which one of the following is the most appropriate immediate pharmacologic intervention to reduce the risk of cerebral palsy in the newborn?

• A. Intramuscular Betamethasone
• B. Oral Nifedipine
• C. Intravenous Magnesium Sulfate
• D. Intravenous Ampicillin
• E. Rectal Indomethacin

Explanation

The correct answer is:

• C. Intravenous Magnesium Sulfate

Detailed Explanation:

• C. Intravenous Magnesium Sulfate: The SOGC guidelines recommend the administration of Magnesium Sulfate for fetal neuroprotection in women with imminent preterm birth at < 32 weeks gestation. This intervention significantly reduces the risk of cerebral palsy (CP) and gross motor dysfunction in surviving infants. Given that this patient is 30 weeks and in active labour (4 cm dilated), delivery is likely imminent or she requires transfer; MgSO4 is indicated specifically for neuroprotection.

• A. Intramuscular Betamethasone: While absolutely indicated for fetal lung maturity (reducing RDS, IVH, NEC), the question specifically asks for the intervention to reduce the risk of cerebral palsy. Steroids primarily target lung maturation and mortality reduction, though they have some IVH benefit. MgSO4 is the specific answer for CP reduction.

• B. Oral Nifedipine: This is a tocolytic agent. While it might be used to delay delivery to allow for transfer and steroid administration, it does not directly reduce the risk of cerebral palsy.

• D. Intravenous Ampicillin: This would be indicated for GBS prophylaxis if her status is unknown, but it does not reduce the risk of cerebral palsy.

• E. Rectal Indomethacin: This is a tocolytic agent. Like Nifedipine, it is used to delay delivery but does not provide neuroprotection.

References

1. SOGC Clinical Practice Guideline No. 317: Diagnosis and Management of Spontaneous Preterm Labour.
2. SOGC Clinical Practice Guideline No. 259: Magnesium Sulphate for Fetal Neuroprotection.
3. SOGC Clinical Practice Guideline No. 223: Antenatal Corticosteroid Therapy for Fetal Maturation.
4. Medical Council of Canada. (n.d.). MCCQE Part I Clinical Decision-Making and Multiple-Choice Questions.
5. Smith, G. (2023). Obstetrics & Gynaecology for the MCCQE1. Toronto Medical Publishers.