Preterm Labor

Aka: Preterm Labor

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II. Definitions

  1. Preterm Labor
    1. Contractions with Uterine Cervical Length change
    2. Gestational age 20-37 weeks
      1. Late preterm: 34 to 37 weeks
      2. Early preterm: 32 to 34 weeks
      3. Very Early Preterm: >32 weeks

III. Epidemiology

  1. Incidence 7-10% of deliveries are preterm (3% are early preterm)

IV. Differential Diagnosis

  1. Gastrointestinal
    1. Appendicitis
    2. Constipation
  2. Uterus
    1. Uterine Fibroids
    2. Placental Abruption
  3. Urinary
    1. Acute Cystitis
    2. Pyelonephritis
    3. Nephrolithiasis
  4. Musculoskeletal
    1. Abdominal wall strain

V. Risk Factors

  1. No associated risk factor in 50% of Preterm Labor
  2. Chemical Use
    1. Tobacco Abuse over 1/2 pack per day Cigarettes
    2. Chemical Dependency (Cocaine, Heroin)
  3. Prior cervical procedure
    1. Prior Cervical Cone Biopsy or LEEP (RR 2)
    2. History of prior dilitation and curettage (RR 1.3, higher if more than one prior D&C)
  4. Preterm cervical changes
    1. Advanced cervical dilatation
    2. Cervical Length decreased on endovaginal Ultrasound
      1. Measure length with empty Bladder, with probe inserted into anterior fornix
      2. Cervical Length <2.5 cm at <28 weeks is associated with a RR 6.19 of preterm birth
      3. Iams (1996) N Engl J Med 334(9): 567-72 [PubMed]
  5. Increased Uterine Size
    1. Multiple Gestation (50% of twin births, 90% of triplets are born before 37 weeks gestation)
    2. Polyhydramnios
  6. Prior pregnancy history
    1. Interval between prior pregnancy <18 months
    2. Prior preterm delivery (RR 1.5 to 2)
    3. History spontaneous second trimester abortion
  7. Demographics
    1. Low socioeconomic status or poor nourishment
    2. Black patients (14.9% preterm delivery vs 8.9% in non-hispanic white patients)
  8. Comorbidity
    1. Low pre-pregnant weight (Body Mass Index <19.8 kg/m2)
    2. Periodontal Disease
    3. Diabetes Mellitus
    4. Thyroid disease
    5. Hypertension
  9. Uterine anomaly
    1. Unicornuate Uterus or bicornuate Uterus
    2. Uterine Fibroids
    3. Diethylstilbestrol (DES) exposure in utero
  10. Genitourinary Infection (40% of preterm births)
    1. Urinary Tract Infection
      1. Pyelonephritis
      2. Asymptomatic Bacteriuria in Pregnancy
    2. Vaginal infections
      1. Group B Streptococcus (PPROM)
      2. Bacterial Vaginosis (associated with a two fold increased risk of preterm birth)
        1. However, Bacterial Vaginosis treatment does not appear to modify risk
        2. Routine asymptomatic screening is not recommended
        3. (1995) N Engl J Med 333:1732-42 [PubMed]
    3. Sexually Transmitted Disease
      1. Neisseria gonorrhoeae
      2. Chlamydia trachomatis
      3. Trichomonas vaginalis
      4. Syphilis
    4. Infections with possible risk
      1. Ureaplasma Urealyticum
      2. Mycoplasma hominis
  11. References
    1. Goldenberg (2008) Lancet 371(9606): 75-84 [PubMed]

VI. Symptoms

  1. Keep a high index of suspicion
    1. Contraction frequency does not predict risk
    2. Symptoms do not predict risk
  2. Pelvic pressure
  3. Vaginal pain
  4. Menstrual-like cramps
  5. Backache
  6. Vaginal Discharge or fluid leakage (see PPROM)
  7. Vaginal Bleeding

VII. Signs

  1. Examine Cervix as soon as possible
    1. Assess Uterine Cervical Length and dilation
    2. Avoid cervical exam until labor if PPROM confirmed
  2. Examine Uterus
    1. Assess for Abruptio Placentae
    2. Check for firm, tender Uterus with minimal relaxation

VIII. Evaluation: Five key concerns

  1. Precautions
    1. Preterm Labor patients who deliver within 6 days of presentation: <10%
      1. Assessment goal is to differentiate the patients at high risk of delivery
    2. Avoid digital vaginal exam UNLESS delivery is imminent (risk of infection)
      1. Perform sterile speculum exam and cervical Ultrasound instead
  2. Is patient preterm (<37 weeks)?
    1. Review Last Menstrual Period and prior Ultrasounds (especially earliest Ultrasounds)
    2. Late preterm: 34 to 37 weeks
    3. Early preterm: 32 to 34 weeks
    4. Very Early Preterm: >32 weeks
  3. Determine if patient is in labor
    1. Evaluate abdominal or Pelvic Pain
    2. Distinguish Preterm Labor from preterm contractions
    3. Findings most suggestive of Preterm Labor
      1. Contractions >6/hour
      2. Cervical dilatation >3 cm
      3. Cervical effacement >80%
      4. Preterm Rupture of Membranes
      5. Vaginal Bleeding
  4. Determine Uterine Cervical Length
    1. Avoid digital cervical exam due to infection risk (unless imminent delivery)
    2. Sterile speculum exam
    3. Ultrasound Exam of Uterine Cervical Length
  5. Determine if membranes are ruptured
    1. See Premature Rupture of Membranes
    2. See Labs below
  6. Establish clear Gestational age
    1. Review Last Menstrual Period
    2. Review Estimated Due Date
    3. Review prior Ultrasound dating
    4. Does fundal height correlate with Gestational age
  7. Evaluate maternal and fetal health
    1. Consider underlying injury or infection
      1. Recent Trauma (Placental Abruption may present with Vaginal Bleeding)
      2. Vaginal infection
      3. Urinary Tract Infection in Pregnancy
    2. Consider comorbidity
      1. Gestational Diabetes
      2. Pregnancy Induced Hypertension
      3. Intrauterine Growth Restriction
      4. Oligohydramnios or Polyhydramnios
    3. Evaluate fetal activity and fetal health
      1. External Fetal Monitoring

IX. Labs

  1. Evaluate for Rupture of Membranes
    1. AmniSure ROM Test (Placental alpha microglobulin 1 or PAMG-1 Protein Marker Test)
    2. Fluid seen pooling from cervical os
    3. Nitrazine Testing (pH 7.1 to 7.3)
    4. Ferning (arborization)
  2. Microscopy to evaluate Vaginitis
    1. Saline Wet Preparation (Trichomonas, Bacterial Vaginosis)
    2. KOH Preparation
  3. Culture
    1. Gonorrhea PCR
    2. Chlamydia PCR
    3. Group B Streptococcus Culture (Todd Hewitt media)
      1. Periurethral or outer-third of vaginal swab
      2. Rectal swab
    4. Urinalysis and Urine Culture
    5. Consider non-genitourinary sources of infection
  4. Other Testing
    1. Fetal Fibronectin
      1. Reassuring if negative
      2. Poor Positive Predictive Value
      3. Not useful in screening asymptomatic patients for Preterm Labor risk
      4. Consider for symptomatic patients 24 to <35 weeks if positve result would direct transfer to higher level care
    2. Urine testing
      1. Urinalysis and Urine Culture
      2. Urine Drug Screening
  5. Fetal Lung Maturity Assessment
    1. Indicated for 34 week gestation or greater

X. Imaging: Obstetric Ultrasound

  1. Fetal evaluation
    1. Biophysical Profile
    2. Amniotic fluid index
  2. Placental location
  3. Fetal Presentation
  4. Estimated Fetal Weight (EFW)
  5. Ultrasound Exam of Uterine Cervical Length
  6. Oligohydramnios (may suggest Rupture of Membranes)

XI. Efficacy: Evaluation criteria

  1. Evaluation criteria do not predict preterm delivery
    1. Fetal Fibronectin
    2. Uterine contraction frequency
    3. Cervical Length assessment
  2. These criteria however have Negative Predictive Value
    1. No Cervical Length change (3 cm or more) and negative fibronectin
    2. Suggests <10% chance of preterm delivery within 14 days
  3. References
    1. Iams (2002) N Engl J Med 346:250-5 [PubMed]

XII. Evaluation

  1. Universal Cervical Length screening is not currently part of guidelines and is controversial

XIII. Management

  1. See Preterm Labor Management

XIV. Prevention

  1. Progesterone Therapy
    1. Indications
      1. Single gestation pregnancy (not useful in Multiple Gestation pregancies) AND
      2. Prior spontaneous preterm delivery OR Cervical Length <=2 cm before 24 weeks gestation
    2. Protocol: Prior spontaneous preterm delivery
      1. Hydroxyprogesterone caproate (Makena) 250 mg IM weekly from 16 to 24 weeks OR
    3. Protocol: Cervical Length <=2 cm before 25 weeks gestation
      1. Vaginal micronized Progesterone 200 mcg daily
    4. References
      1. Iams (2014) N Engl J Med 370(3): 254-61 [PubMed]
  2. Cervical Cerclage
    1. Indicated in Prior spontaneous preterm delivery AND Cervical Length <=2.5 cm before 24 weeks gestation
    2. Contraindicated in Multiple Gestation pregnancy (associated with increased preterm delivery risk)
    3. Owen (2009) Am J Obstet Gynecol 201(4): 375 [PubMed]
  3. Other measures that are NOT recommended
    1. Activity Restriction
      1. Activity restriction may paradoxically increase the risk of preterm birth
      2. Avoid activity restriction in the prevention of preterm birth
    2. Cervical Pessary (not recommended)
      1. As of 2022, not recommended
        1. Not effective in preventing Preterm Labor for Cervix <25 mm, Multiple Gestation or prior preterm birth
      2. Initial studies found significantly decreased risk of preterm delivery
        1. Goya (2012) Lancet 379(9828): 1800-6 [PubMed]

XV. References

  1. Iams in Gabbe (2002) Obstetrics p.755
  2. (2021) Obstet Gynecol 138(2):e65-e90 +PMID: 34293771 [PubMed]
  3. Arnold (2022) Am Fam Physician 106(3): 337-9 [PubMed]
  4. Rundell (2017) Am Fam Physician 95(6): 366-72 [PubMed]

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