Preterm labor is defined as the presence of uterine contractions with sufficient frequency and intensity to cause progressive cervical effacement and dilation, occurring between 20 weeks' and 37 weeks' gestation (before the 37th week). This condition can happen with appropriately grown babies, some small for gestational age (SGA), and a few large for gestational age (LGA).
In a typical pregnancy, labor begins around 40 weeks' gestation, requiring more than 4 contractions per hour for cervical dilation to occur. In preterm labor, there is appreciable cervical effacement or dilation (at least 2 cm) without previous examinations.
Uterine contractions in preterm labor might not always be painful and can present as abdominal tightening, lower back pain, or pelvic pressure.
Preterm labor is a leading cause of neonatal morbidity and mortality, contributing to 25% of neonatal deaths unrelated to congenital anomalies, occurring in 10-15% of all pregnancies.
Causes of Preterm Labor
While the exact cause remains unclear, preterm labor is believed to be multifactorial, with spontaneous labor and elective causes both playing a role.
Spontaneous Causes:
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Unknown causes (up to 40% of cases)
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Multiple gestation (increased risk with higher multiples)
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Maternal infection (e.g., pyelonephritis, chorioamnionitis, urinary tract infections)
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Premature rupture of membranes
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Polyhydramnios
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Maternal factors (e.g., short stature, age < 18 or > 35, low socioeconomic status, previous preterm labor)
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Uterine abnormalities (e.g., bicornuate uterus, fibroids)
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Cervical insufficiency (especially after cone biopsy)
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Substance abuse (e.g., alcohol, cigarette smoking)
Elective Causes:
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Hypertensive disorders (e.g., pre-eclampsia, chronic hypertension)
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Maternal diseases (e.g., renal or heart disease)
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Placental abnormalities (e.g., placenta previa, abruptio placentae)
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Rhesus incompatibility
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Fetal disorders (e.g., congenital abnormalities, intrauterine growth restriction (IUGR))
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Thyrotoxicosis in the fetus
Risk Factors Associated with Preterm Labor
Obstetric Complications:
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Previous preterm or low birth weight infant
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Short pregnancy interval (< 3 months)
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Inadequate or excessive weight gain
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Previous cervical lacerations or uterine surgeries (e.g., C-section)
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Multiple pregnancy
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Non-Caucasian race
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Increased maternal age (<18 or >40)
Medical Complications:
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Pulmonary or systemic hypertension
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Renal or heart disease
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Infections (e.g., urinary tract infection, genital tract infections)
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Cigarette smoking and alcoholism
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Severe anemia
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Malnutrition or obesity
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Trauma or burns
Surgical Complications:
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Previous abdominal surgery
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Cervical conization or uterine surgery
Signs and Symptoms of Preterm Labor
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Regular uterine contractions: More than two contractions per half-hour
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Cervical changes: Dilatation or effacement of at least 1 cm, or dilation ≥ 2 cm on admission
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Vaginal bleeding: Often presenting as a bloody show; significant bleeding requires assessment for placenta previa or abruptio placentae
Other indicators of preterm labor include:
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Blood-stained show
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Palpable uterine activity
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Engagement of presenting part
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Bulging membranes
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Rupture of membranes
Diagnostic Investigations
Laboratory tests to consider in suspected preterm labor:
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Blood slide: For malaria parasites, if the mother is from a malarial endemic region
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Urine sample: For microscopy, culture, and sensitivity (including testing for urinary tract infections)
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Swabs: For gonorrhoea, Chlamydia, and gram stain (from high vaginal and endocervical swabs)
Fetal fibronectin can be tested before cervical examination if preterm labor is suspected. Additional investigations may include:
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Ultrasound: For fetal assessment, including biophysical profiling, amniotic fluid volume, fetal weight estimation, and presentation
Treatment of Preterm Labor
Management depends on gestational age and the clinical presentation. The two main categories are expectant management and active management.
Goals of Management:
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Early identification of risk factors
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Timely diagnosis of preterm labor
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Evaluation of fetal well-being
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Initiation of tocolysis and corticosteroids
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Continuous monitoring of maternal and fetal status
Conservative Management:
For cervix less than 2 cm dilated, management includes:
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Bed rest, especially for mothers at risk of preterm labor
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Use of sedatives to ensure complete bed rest
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Administration of tocolytics (to relax the uterus)
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Corticosteroids (e.g., dexamethasone or betamethasone) to enhance fetal lung maturity
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Treat underlying causes (e.g., infections, dehydration)
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Avoidance of strenuous activity and stress
Active Management:
If cervical dilation exceeds 2 cm, or if fetal distress is present, the management involves:
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Corticosteroids to improve fetal lung development
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Antibiotics to prevent infection
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Rupture of membranes (except in intrauterine fetal demise (IUFD))
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Labor monitoring, with caesarean section if necessary
Corticosteroid Administration
Corticosteroids are essential for fetal lung maturity and improving neonatal outcomes. The two most commonly used corticosteroids are:
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Betamethasone: 12 mg IM every 24 hours for two doses
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Dexamethasone: 6 mg IM every 12 hours for four doses
These corticosteroids should be used before 34 weeks' gestation, especially for fetuses with a weight between 600 g and 2500 g. Their use is not recommended if there is an existing infection.
Use of Tocolytics
Tocolytics are medications that aim to delay labor by relaxing the uterus, providing time for fetal lung maturity and further interventions. Tocolytics are typically used to prolong labor by 48 hours.
