Residency · Residency · Obstetrics Gynecology
Preterm Labor: Diagnosis, Tocolysis, and Neuroprotection
Definitions and Epidemiology
Preterm birth is defined as delivery before 37 weeks and 0 days of gestation and is subcategorized by severity: extremely preterm (before 28 weeks), very preterm (28 to 31 weeks 6 days), moderate preterm (32 to 33 weeks 6 days), and late preterm (34 to 36 weeks 6 days). The preterm birth rate in the United States is approximately 10 to 12%, and preterm birth remains the leading cause of neonatal morbidity and mortality worldwide. Approximately 70% of preterm births are spontaneous, resulting from preterm labor or preterm premature rupture of membranes (PPROM), while the remaining 30% are medically indicated for conditions such as preeclampsia, intrauterine growth restriction, or placental abnormalities.
Risk Factors
The strongest predictor of spontaneous preterm birth is a history of prior spontaneous preterm delivery, which confers a recurrence risk of 15 to 30%. Other significant risk factors include a short cervical length (below 25 mm at 16 to 24 weeks), multiple gestation, uterine anomalies (septate or bicornuate uterus), prior cervical surgery (LEEP or cone biopsy), periodontal disease, infections (bacterial vaginosis, urinary tract infection, chorioamnionitis), low BMI, smoking, substance use, and a short interpregnancy interval of less than 18 months. Black women face approximately twice the risk of preterm birth compared to white women, a disparity likely driven by systemic racism, chronic stress, and social determinants of health rather than inherent biological differences.
Diagnosis of Preterm Labor
Clinical Criteria
Preterm labor is diagnosed when regular uterine contractions (four or more in 20 minutes, or eight or more in 60 minutes) occur between 20 and 36 weeks 6 days of gestation, accompanied by documented cervical change or an initial cervical dilation of 1 cm or greater and/or effacement of 80% or more.
Adjunctive Diagnostic Tools
Fetal fibronectin (fFN) is a glycoprotein normally found at the choriodecidual interface that becomes detectable in cervicovaginal secretions when this interface is disrupted. It is collected via swab before digital cervical examination, between 22 and 34 weeks. Its greatest clinical utility lies in its very high negative predictive value: a negative fFN result excludes delivery within 7 to 14 days with more than 99% certainty, which can prevent unnecessary hospitalizations and interventions. A positive result is less definitive, with only approximately 20% of women with a positive fFN delivering within 14 days. The test is invalidated by recent intercourse, vaginal bleeding, digital examination, or lubricant use. Transvaginal cervical length (TVCL) measurement is the other key adjunctive tool. A cervical length below 20 mm at the time of presentation significantly increases the likelihood of preterm delivery, while a length of 30 mm or greater virtually excludes imminent delivery. The combination of a short cervix and a positive fFN provides the highest positive predictive value.
<image>Transvaginal ultrasound images showing cervical length measurement comparing a normal cervix (>30mm, closed internal os) with a short cervix (<20mm with funneling at the internal os) in a patient presenting with preterm contractions</image>
Tocolysis
Goals
Tocolysis does not prevent preterm birth. Its primary goal is to buy a 48-hour window to allow administration of antenatal corticosteroids for fetal lung maturity and magnesium sulfate for neuroprotection. A secondary goal is to facilitate maternal transfer to a facility with an appropriate-level NICU.
First-Line Agents
Nifedipine (Calcium Channel Blocker)
Nifedipine is the most commonly used first-line tocolytic. The loading dose is 20 to 30 mg orally, followed by 10 to 20 mg every 4 to 6 hours for maintenance. Side effects include hypotension, headache, flushing, and dizziness. It should be avoided in patients with significant maternal hypotension and must never be administered sublingually due to the risk of precipitous hypotension.
Indomethacin (NSAID/Prostaglandin Synthetase Inhibitor)
Indomethacin is loaded at 50 to 100 mg orally or rectally, followed by 25 to 50 mg every 6 hours. Its use must be limited to gestational ages below 32 weeks because of the risk of premature closure of the fetal ductus arteriosus, and the duration of treatment should not exceed 48 to 72 hours. Amniotic fluid volume should be monitored because of the risk of oligohydramnios. Indomethacin is particularly effective as a first-line agent at early gestational ages.
Second-Line / Alternative Agents
Magnesium Sulfate (as tocolytic)
Magnesium sulfate has been widely used as a tocolytic, with a loading dose of 4 to 6 g IV followed by maintenance at 2 to 3 g per hour. However, the evidence supporting its tocolytic efficacy is weak, and it carries significant risks including respiratory depression and cardiac arrest at high levels. Its primary role in current practice is fetal neuroprotection rather than tocolysis.
Terbutaline (Beta-2 Agonist)
Terbutaline is administered at 0.25 mg subcutaneously every 20 to 30 minutes for a maximum of 3 doses. The FDA has issued a black box warning against prolonged use beyond 48 to 72 hours due to the risk of serious maternal cardiac events. It remains useful for acute tocolysis, such as achieving uterine relaxation in the operating room. Side effects include tachycardia, tremor, hypokalemia, and pulmonary edema.
| Tocolytic Agent | Class | Dosing | Key Limitations |
|---|---|---|---|
| Nifedipine | Calcium channel blocker | 20-30 mg PO load, then 10-20 mg q4-6h | Avoid with hypotension; do not give sublingually |
| Indomethacin | NSAID | 50-100 mg load, then 25-50 mg q6h | Use only <32 weeks; limit to 48-72 hours (ductal closure risk) |
| Magnesium sulfate | — | 4-6 g IV load, then 2-3 g/hr | Weak tocolytic efficacy; primary role is neuroprotection |
| Terbutaline | Beta-2 agonist | 0.25 mg SQ q20-30min (max 3 doses) | FDA black box: no use >48-72 hours; cardiac risk |
Contraindications to Tocolysis
Tocolysis is contraindicated when there is intrauterine fetal demise, a lethal fetal anomaly, severe preeclampsia or eclampsia, chorioamnionitis, significant maternal hemorrhage or hemodynamic instability, advanced cervical dilation (greater than 4 to 6 cm, where tocolysis is unlikely to be effective), or fetal compromise requiring delivery.
Antenatal Corticosteroids
Indications and Timing
Betamethasone (12 mg IM for two doses, 24 hours apart) is the preferred corticosteroid regimen, with dexamethasone (6 mg IM for four doses, 12 hours apart) as an alternative. Corticosteroids are indicated at 24 weeks 0 days through 33 weeks 6 days when delivery is anticipated within 7 days. Late preterm steroids (34 weeks 0 days through 36 weeks 6 days) may be given as a single course of betamethasone if delivery is anticipated within 7 days and the patient has not received a prior course. The ALPS trial demonstrated reduced neonatal respiratory morbidity with late preterm steroids, though the trade-off includes increased neonatal hypoglycemia and potential concerns about childhood metabolic risk.
Effects
Antenatal corticosteroids reduce respiratory distress syndrome by approximately 50%, intraventricular hemorrhage by approximately 50%, and decrease rates of necrotizing enterocolitis and neonatal death. The optimal benefit window is 24 hours to 7 days after course completion, though even an incomplete course provides some benefit.
Repeat Courses
A single rescue course may be considered if 14 or more days have elapsed since the prior course, the gestational age remains below 34 weeks, and delivery again appears imminent. ACOG does not recommend serial or multiple repeat courses because of the association with reduced fetal growth.
<image>Timeline diagram showing the optimal window for antenatal corticosteroid administration, with betamethasone dosing schedule (two IM doses 24 hours apart), peak benefit window (24 hours to 7 days), and rescue course criteria</image>
Magnesium Sulfate for Fetal Neuroprotection
Magnesium sulfate is indicated for fetal neuroprotection when delivery is anticipated within 24 hours at a gestational age below 32 weeks. The loading dose is 4 to 6 g IV over 20 to 30 minutes, followed by a maintenance infusion of 1 to 2 g per hour. The infusion is continued until delivery or for up to 12 to 24 hours, and is discontinued if delivery is no longer considered imminent. This recommendation is based on several randomized trials (ACTOMgSO4, PREMAG, and the BEAM trial) that collectively demonstrated an approximately 30% reduction in cerebral palsy. The number needed to treat is approximately 46 to prevent one case of cerebral palsy. Maternal monitoring for magnesium toxicity includes checking deep tendon reflexes, respiratory rate, and urine output.
Preterm Premature Rupture of Membranes (PPROM)
Diagnosis
The diagnosis of PPROM is based on a history of fluid gush or persistent leaking, confirmed by sterile speculum examination showing pooling of amniotic fluid in the posterior vaginal fornix. Confirmatory tests include the nitrazine test (amniotic fluid turns nitrazine paper blue because its pH exceeds 6.5, though false positives can occur with blood, semen, or bacterial vaginosis), ferning (the arborization pattern seen when amniotic fluid is dried on a glass slide), and commercial tests such as AmniSure (which detects PAMG-1) and ROM Plus (which detects IGFBP-1 and AFP). Digital cervical examination should be avoided unless delivery appears imminent, as it increases infection risk and shortens the latency period.
Management by Gestational Age
Before 23 weeks, management involves counseling about prognosis and a shared decision between expectant management and delivery based on patient values. Between 23 and 33 weeks 6 days, expectant management is recommended with latency antibiotics, corticosteroids, and magnesium sulfate if below 32 weeks, with delivery indicated for infection, abruption, or non-reassuring fetal status. At 34 weeks 0 days through 36 weeks 6 days, delivery is recommended per ACOG guidelines, with consideration of late preterm steroids before delivery. At 37 weeks or beyond, delivery by induction should proceed.
| Gestational Age | PPROM Management | Key Interventions |
|---|---|---|
| <23 weeks | Counseling; shared decision-making | Expectant management vs. delivery |
| 23+0 to 33+6 weeks | Expectant management | Latency antibiotics, corticosteroids, MgSO4 if <32 wks |
| 34+0 to 36+6 weeks | Delivery recommended | Consider late preterm steroids |
| ≥37 weeks | Delivery by induction | — |
Latency Antibiotics
Latency antibiotics prolong the interval from rupture to delivery by approximately one week and reduce the incidence of chorioamnionitis. The standard regimen is ampicillin 2 g IV every 6 hours plus erythromycin 250 mg IV every 6 hours for 48 hours, followed by amoxicillin 250 mg orally every 8 hours plus erythromycin 250 mg orally every 6 hours for 5 additional days (7 days total). An increasingly used alternative substitutes single-dose azithromycin for erythromycin. Amoxicillin-clavulanate must be avoided because of its association with increased necrotizing enterocolitis.
Surveillance During Expectant Management
Monitoring includes temperature and vital signs every 4 to 8 hours, daily fetal heart rate monitoring with nonstress testing, serial white blood cell counts and clinical assessment for signs of chorioamnionitis, and monitoring for cord prolapse, abruption, and labor.
Progesterone Supplementation for Prevention
Vaginal Progesterone
Vaginal progesterone is indicated for singleton gestations with a short cervix (below 25 mm) discovered on transvaginal cervical length measurement at 16 to 24 weeks. The dose is 200 mg vaginal suppository or 90 mg vaginal gel nightly until 36 weeks. Vaginal progesterone has not been shown to be effective in twin gestations with short cervix.
17-Alpha Hydroxyprogesterone Caproate (17-OHPC)
17-OHPC was previously recommended for women with a history of prior spontaneous preterm birth, based on the Meis trial. However, the larger PROLONG trial (2019) failed to replicate that benefit, and the FDA withdrew approval in 2023. ACOG currently does not recommend routine 17-OHPC use. Vaginal progesterone may be considered for women with a history of prior preterm birth who also have a concurrent short cervix.
Clinical Pearls
A negative fetal fibronectin is one of the most clinically useful test results in obstetrics. It reliably excludes imminent delivery and can prevent unnecessary admissions, tocolysis, and transfers.
Tocolysis buys time -- typically 48 hours -- for corticosteroid administration and maternal transfer. It does not prevent preterm birth, and this limitation should be clearly understood.
Nifedipine and magnesium sulfate should not be given simultaneously due to the risk of neuromuscular blockade and severe hypotension.
Late preterm steroids reduce respiratory distress syndrome but increase neonatal hypoglycemia. The risks and benefits should be carefully weighed in each clinical scenario.
After the PROLONG trial and subsequent FDA action, 17-OHPC is no longer a recommended standard for preterm birth prevention. The field has shifted toward vaginal progesterone and cerclage for at-risk patients.
Chorioamnionitis is the most important contraindication to continued expectant management of PPROM. Clinical signs including maternal fever, uterine tenderness, fetal tachycardia, and purulent discharge should prompt delivery.
Digital cervical examination in PPROM shortens the latency period and increases infection risk. It should be avoided unless delivery is planned.
References
- ACOG Practice Bulletin No. 234: Prediction and Prevention of Spontaneous Preterm Birth (2021)
- ACOG Practice Bulletin No. 217: Prelabor Rupture of Membranes (2020)
- ACOG Committee Opinion No. 713: Antenatal Corticosteroid Therapy for Fetal Maturation (2017, reaffirmed 2023)
- Rouse DJ et al. A randomized, controlled trial of magnesium sulfate for the prevention of cerebral palsy (BEAM Trial). N Engl J Med. 2008;359:895-905
- Gyamfi-Bannerman C et al. Antenatal betamethasone for women at risk for late preterm delivery (ALPS). N Engl J Med. 2016;374:1311-1320
- Blackwell SC et al. 17-OHPC to prevent recurrent preterm birth (PROLONG). JAMA. 2020;323:2060-2069
