Preterm Labor

BASICS

Preterm birth is defined as birth between 20 0/7 weeks to 36 6/7 weeks’ gestation

DESCRIPTION

Regular contractions occurring between 20 0/7 and 36 6/7 weeks’ gestation with either a change in effacement and cervical dilation or cervical dilation of ≥2 cm on presentation.

EPIDEMIOLOGY

Preterm birth is the leading cause of perinatal morbidity and mortality in the United States.

Incidence

10–15% of pregnancies experienced at least one episode of preterm labor.
Causes of preterm births in the US: spontaneous preterm labor makes up 40–45% of preterm births, preterm premature rupture of membranes makes up 30–35% of preterm births, medically indicated delivery due to maternal or neonatal etiology associated with 30–35% of preterm births, and multiple gestation

Prevalence

Non-Hispanic black patients are 50% more likely to have a preterm birth than Hispanic or white patients.
10% of all births in the United States in 2019 were preterm: 50% of preterm deliveries are due to spontaneous preterm labor, 25% following preterm prelabor rupture of membranes, and 25% due to maternal or fetal medical indications due to complications (1).

ETIOLOGY AND PATHOPHYSIOLOGY

Premature formation and activation of myometrial gap junctions
Abnormal placental implantation/placental abruption/placental ischemic disease (preeclampsia and fetal growth restriction)
Systemic inflammation/infections (e.g., UTI, autoimmune conditions)/immunopathology (e.g., antiphospholipid antibodies)
Local inflammation/infections (e.g., intraamniotic infections, group B Streptococcus [GBS])—colonization with infectious organism noted in 25–40% of all preterm births
Uterine abnormalities/overdistension (multiple gestation, polyhydramnios)/cervical insufficiency/premature dilation
Preterm premature rupture of membranes
Trauma
Fetal abnormalities
Hypertensive disorders of pregnancy

Genetics

Familial predisposition; numerous gene candidates mediating various pathways have been identified, but causality and gene–environment interactions are not well-defined.

RISK FACTORS

Prior preterm delivery (most significant risk factor): >3-fold if previous preterm birth
Demographic factors, including social and economic disadvantages and black race with concern for chronic stress from structural racism
Short interpregnancy interval (<18 months)
Prepregnancy weight <45 kg (100 lb), body mass index <18.5
Substance abuse (e.g., cocaine, tobacco)
Unintended pregnancy
Cervical insufficiency/short cervical length of <25 mm
Abdominal surgery/trauma during pregnancy; history of dilation and curettage
Uterine structural abnormalities, such as large fibroids or müllerian abnormalities
Serious maternal infections/diseases; bacterial vaginosis and genital tract infections; bacteriuria
Multiple gestation; intrauterine growth restriction; polyhydramnios
Placenta previa; vaginal bleeding; premature placental separation (abruption)

GENERAL PREVENTION

Patient education at each visit in 2nd and 3rd trimesters for those at risk and periodically in the last two trimesters for the general population
For women with a singleton pregnancy and no history of spontaneous preterm births, visualization of the cervix on routine fetal anatomical scan between 18 0/7 and 22 6/7 weeks’ gestation should be performed. If found to be shortened, a transvaginal ultrasound should be done for further evaluation (1)[C]. This recommendation is the same for multiple gestation and women with a history of medically induced preterm birth. Twin pregnancies are more likely to have a shortened cervix during the 2nd trimester, and women with a history of medically induced preterm birth are more likely to have a subsequent preterm birth (1).
For women with history of spontaneous preterm birth and singleton pregnancy, a screening transvaginal ultrasound to evaluate cervix length should be done at 16 0/7 weeks’ gestation and repeated every 1 to 4 weeks (protocols vary) until 24 0/7 weeks (1).
Primary prevention involves counseling on: interval contraception to optimize pregnancy spacing and smoking cessation; if previous preterm birth, evaluate if etiology is likely to recur and target intervention to specific condition. Women with history of spontaneous preterm birth and singleton pregnancy should be offered vaginal or intramuscular progesterone to prevent preterm birth with a shared decision model (1).
Secondary prevention:
- For patients with a cervix of <25 mm in length with or without a history of previous spontaneous preterm delivery and singleton pregnancy, it has been recommended to give vaginal progesterone if not already on progesterone. Most protocols give 200 mg daily at diagnosis at 18 0/7 to 25 6/7 to continue until 36 to 37 weeks’ gestation (1). An FDA advisory committee voted in late 2022 to withdraw approval of 17-hydroxyprogesterone and its generic equivalents based on results from postmarket confirmatory trial data in the PROLONG study, released in October 2019, showing lack of efficacy. Data is insufficient to recommend vaginal progesterone for multiple gestation (1).
- Women with a singleton pregnancy and history of spontaneous preterm birth with shortened cervix should be considered for a cerclage (1). Cerclage can be considered for any patient who is noted to have cervical insufficiency on physical exam (1).
Tocolysis

DIAGNOSIS

Diagnosis is generally based on a combination of significant cervical changes (such as dilation, effacement) with regular contractions. However, there is no single test that will reliably diagnose or predict true preterm labor.
ACOG defines preterm labor as regular contractions noted with change in cervical dilation, effacement, or both, or regular contractions and cervical dilation of at least 2 cm at presentation.

HISTORY

Address risk factors
Regular uterine contractions or cramping; dull, low backache or pain; intermittent lower abdominal pain; increased low pelvic pressure
Change in vaginal discharge; vaginal bleeding
Amniotic fluid leakage

PHYSICAL EXAM

Sterile speculum exam for membrane rupture evaluation, cultures, and cervical inspection
Bimanual cervical exam if intact membranes

ALERT

Avoid bimanual examination when possible if rupture of the membranes is suspected.

DIFFERENTIAL DIAGNOSIS

Braxton-Hicks contractions/false labor
Round ligament pain
Lumbosacral muscular back pain
UTI or vaginal infections
Adnexal torsion
Appendicitis
Nephrolithiasis

DIAGNOSTIC TESTS & INTERPRETATION

Initial Tests (lab, imaging)

There are no specific tests that completely or accurately predict preterm birth, although they help with risk stratification.
In symptomatic women from 22 to 34 weeks’ gestation with intact membranes and no intercourse or bleeding in past 24 hours, obtain a fetal fibronectin (fFN) swab from the posterior vaginal fornix.
- If results are positive (≥50 ng/mL), patient is at a modest increased risk of preterm birth (positive predictive value [PPV] 13–30% for delivery within 2 weeks). However, if results are negative, >97% of patients will not deliver in 14 days, so can consider avoiding complicated or high-risk interventions.
- fFN and shortened cervix alone has low predictive value, so other data should be used as well in acute management of patient (2).
Urinalysis and urine culture; cultures for gonorrhea, chlamydia, and wet prep
Vaginal introitus and rectal culture for GBS if indicated
pH and fern test of vaginal fluid to evaluate for rupture of membranes
CBC with differential and Kleihauer-Betke test if abruption suspected
US to identify number of fetuses and fetal position, confirm gestational age, estimate fetal weight, quantify amniotic fluid, and look for conditions that contraindicate tocolysis
Transvaginal US to evaluate cervix length, funneling, and dynamic changes after obtaining fFN (if clinical assessment of the cervix is uncertain or if the cervix is closed on digital exam); ACOG and Society for Maternal-Fetal Medicine (SMFM) recommend transvaginal cervical length screening in patients with history of previous spontaneous preterm birth.

Diagnostic Procedures/Other

Monitor contractions with external tocodynamometer.

TREATMENT

GENERAL MEASURES

Treat underlying risk factors. Hospitalization is necessary if the patient needs IV tocolysis.

MEDICATION

Tocolysis may allow time for interventions such as transfer to tertiary care facility and administration of corticosteroids but may not prolong pregnancy significantly (2).

First Line

Corticosteroids (2):
- Corticosteroids decrease neonatal respiratory distress, intraventricular hemorrhage, necrotizing enterocolitis, and overall perinatal mortality.
- Give if the mother is at 24 to 34 weeks’ gestation and is at risk for delivery in the next 7 days (2); can consider if mother is 23 0/7 to 23 6/7 weeks’ gestation and at risk for delivery in the next 7 days
- Rescue course is considered if <34 weeks’ gestation, at risk of delivery in next 7 days, and previous course of corticosteroids was administered 14 days prior (2).
- Betamethasone 12 mg IM × 2 doses 24 hours apart or dexamethasone 6 mg IM q12h for 4 doses (2)
- Steroids may reduce the risk of respiratory morbidities in singleton infants born to nondiabetic mothers in the late preterm period (34 0/7 to 36 6/7 weeks’ gestation). If delivery is likely during this time period, administration of steroids may be considered as above. Tocolysis is not recommended.
Tocolysis: reserved for women who benefit from a 48-hour delay to receive corticosteroids as there is no evidence showing tocolysis causes improvement in neonatal outcomes up to 34 weeks’ gestation (2)
- Nifedipine is a calcium channel blocker (CCB) that can be used for tocolysis: 20 mg PO loading dose and then 10 to 20 mg q4–6h for 48 hours (do not use sublingual route); check BP often and avoid hypotension; concurrent use with magnesium sulfate is discouraged due to the theoretical risk of neuromuscular blockade (3); contraindications: hypotension, preload dependent cardiac abnormalities such as aortic insufficiency (2)
- Indomethacin is a nonsteroidal anti-inflammatory drug used for tocolysis: 50 to 100 mg PO initial dose and then 25 to 50 mg q6–8h; best to use in combination with magnesium if it is being used for neuroprotection (2),(3); consider for use up to 32 weeks (2); contraindications: platelet dysfunction, bleeding disorder, gastrointestinal ulcerative disease, and asthma (if hypersensitive to aspirin) (2)
- Terbutaline is a; β-adrenergic receptor agonist given for tocolysis; given via SC administration; if contractions persist or pulse >120 beats/min, change to another tocolytic agent (3). Due to reports of serious cardiovascular events and maternal deaths, PO or long-term SC administration of terbutaline should not be given (3); contraindications: tachycardia-sensitive maternal cardiac disease and poorly controlled diabetes mellitus (2)
- Contraindications to tocolysis: severe preeclampsia, eclampsia, hemorrhage with hemodynamic instability, preterm premature rupture of membranes, chorioamnionitis, advanced labor, intrauterine growth restriction, fetal distress, or lethal fetal abnormalities
Magnesium sulfate: recommended for imminent delivery prior to 32 weeks for neuroprotection as it reduces the risk and severity of cerebral palsy
Antibiotics for GBS prophylaxis if culture is indicated

Second Line

Magnesium sulfate by IV infusion has not been shown to be superior to placebo in prolonging pregnancy >48 hours. The side effects are generally greater than those with CCBs or NSAIDs. Therefore, this agent should be used cautiously; contraindication: myasthenia gravis
Antibiotics should not be used as tocolysis or to improve neonatal outcomes for preterm labor with intact membranes and may be associated with harm.

ISSUES FOR REFERRAL

If delivery is inevitable but not immediate, consider transport to a tertiary care center or hospital equipped with a neonatal ICU.
Consider consultation with maternal–fetal medicine specialist.

ADDITIONAL THERAPIES

Data to prove efficacy of pelvic rest (e.g., no douching or intercourse) is lacking. Bed rest and hydration have not been shown to be effective in the prevention of preterm delivery and are not recommended (2).
If there is preterm prelabor rupture of membranes, latency antibiotics are indicated.

SURGERY/OTHER PROCEDURES

Consider cerclage with history of recurrent 2nd trimester losses and diagnosis of cervical insufficiency, ultrasound finding of shortened cervix (<25 mm) and history of preterm birth, and rescue cerclage can be done emergently when cervical insufficiency is noted in patient with threatened preterm labor

ADMISSION, INPATIENT, AND NURSING CONSIDERATIONS

Suspected/threatened preterm labor management: IV access with IV hydration, continuous fetal and contraction monitoring, assessing cervical dilatation and effacement, and monitoring for fluid overload

ONGOING CARE

FOLLOW-UP RECOMMENDATIONS

Patient Monitoring

Weekly office visits with contraction monitoring, cervical checks, or cervical US if at high risk for recurrence
Routine use of maintenance tocolysis is ineffective in preventing recurrent preterm labor or preterm birth.

PATIENT EDUCATION

Call physician or proceed to hospital whenever regular contractions last >1 hour, bleeding, increased vaginal discharge or fluid, and decreased fetal movement.

PROGNOSIS

If membranes are ruptured and no infection is confirmed, delivery often occurs within 3 to 7 days.
If membranes are intact, 20–50% deliver preterm.

COMPLICATIONS

Labor resistant to tocolysis, pulmonary edema, intraamniotic infection

Authors

Bindusri Paruchuri, MD

REFERENCES

American College of Obstetricians and Gynecologists. Prediction and prevention of spontaneous preterm birth: ACOG Practice Bulletin, Number 234. Obstet Gynecol. 2021;138(2):e65–e90. [PMID:34293771]
American College of Obstetricians and Gynecologists’ Committee on Practice Bulletins—Obstetrics. Practice Bulletin No. 171: management of preterm labor. Obstet Gynecol. 2016;128(4):e155–e164. [PMID:27661654]
Rundell K, Panchal B. Preterm labor: prevention and management. Am Fam Physician. 2017;95(6):366–372. [PMID:28318214]

CODES

ICD10

O60.03 Preterm labor without delivery, third trimester
O60.0 Preterm labor without delivery
O60.02 Preterm labor without delivery, second trimester
Z87.51 Personal history of pre-term labor
O60 Preterm labor
O60.00 Preterm labor without delivery, unspecified trimester

SNOMED

6383007 Premature labor
698717006 Preterm spontaneous labor with term delivery
10761141000119107 Preterm labor in second trimester with preterm delivery in second trimester
10761191000119104 Preterm labor in third trimester with preterm delivery in third trimester
10761241000119104 Preterm labor with preterm delivery
10761341000119105 Preterm labor without delivery
698716002 Preterm spontaneous labor with preterm delivery

CLINICAL PEARLS

Treatment of preterm labor/tocolysis may delay delivery to facilitate short-term interventions such as administration of corticosteroids, which improve neonatal outcomes.
Magnesium sulfate is recommended for neuroprotection for imminent deliveries prior to 32 weeks.

Last Updated: 2026