Preterm birth

Question 1

Definition of preterm birth

Answer 1

Preterm birth is defined as delivery between 20-36+6 weeks gestation

Categories:
Late preterm - 34-36+6
Moderate preterm - 32-33+6
Very preterm (<1%) - 28-31+6 weeks
Extremely preterm - <28 weeks

Question 2

Epidemiology of preterm birth

Answer 2

Leading cause of neonatal morbidity and mortality and also longer term consequences later in life (worldwide)

Accounts for ~70% of perinatal mortality in Australia

Approximately 2/3 of all preterm births occur spontaneously

Incidence in Australia = 8.1% in general population, 14.2% in First Nations population

Question 3

Risk factors for preterm birth

Answer 3

1. Previous spontaneous preterm birth
   - 1 PTB increased risk 15%
   - 2 PTBs increased risk 30%
   - >3 PTBs increased risk >50%
2. Infection
   Intra-uterine
   - Ascending from genital tract e.g. BV, Ureaplasma, gonorrhoea, chlamydia, Trichomonas vaginalis - most common
   - Transplacental from maternal blood
   - Trans-Fallopian from abdominal cavity
   - Post-procedural e.g. amniocentesis
   Extra-uterine - asymptomatic bacteriuria, pyelonephritis, periodontal disease, malaria, typhoid
3. Cervical
   - Previous TOPs, particularly if recurrent, second trimester or cervical dilatation >10mm
   - Cervical surgery - knife cone or LLETZ for cervical intra-epithelial neoplasia (volume of cervical tissue removed important)
   - Cervical trauma in previous deliveries
   - Of those women with previous excisions, a midtrimester cervical length <25 or 30mm confers a greater risk of preterm birth (positive predictive value 30-50%) compared to a longer cervix (negative predictive value 94-95%)
4. Fetal
   - Congenital or chromosomal abnormality
5. Uterine
   - Uterine abnormalities
   - Overdistension e.g. multiple pregnancy, polyhydramnios
   - Trauma (accidents, falls)
6. Demographic
   - Age <18 or >35
   - Low or high BMI
   - Ethnicity - African, SE Asian, First Nations Australian
   - Low SE –> associated factors include smoking, low pre-pregnancy weight, low pregnancy weight gain
   - Rural/remote communities
   - Low pre-pregnancy weight
   - Short pregnancy interval
   - Poverty
7. Social - smoking, EtOH, drugs, domestic violence, psychological stress
8. Other
   - Antepartum haemorrhage (found in 20% preterm births)
   - Pre-eclampsia
   - Uteroplacental insufficiency
9. Use of ART

** 2/3 of women who experience preterm birth have no identifiable risk factors **

Question 4

Mechanisms that lead to preterm birth

Answer 4

1. Hormonal changes
   - Premature activation of the fetal hypothalamic pituitary adrenal axis in response to maternal or fetal stresses
2. Infection or inflammation
   - Ascending, transplacental or trans-Fallopian tubes
   - Elicits an inflammatory response
3. Decidual haemorrhage
   - Due to abruption or smaller bleeds -> leads to generation of thrombin
4. Pathological uterine distension -> stretching of the myometrium
   - E.g. multiple pregnancy, polyhydramnios, uterine abnormalities

All four pathways leads to a common pathway of activation of inflammatory mechanisms -> release of cytokines/prostaglandins -> triggers onset of labour

Other pathogenic pathways:
- Cervical insufficiency - the pathologic dilatation or effacement of cervix in absence of uterine contractions that leads to pre-viable pregnancy loss and spontaneous preterm birth
- Structural cervical weakness:
Due to previous cervical/uterine surgery or a congenital anomaly e.g. agenesis of cervix
May be related to the above four pathways - which can initiate the biochemical changes in the cervix that leads to premature cervical shortening
- Genetic basis - if born premature or sister had a preterm birth -> higher risk preterm birth

Question 5

PPROM

Question 6

Steroids

Question 7

Cervical insufficiency