Preterm Labor

Definitions

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

Epidemiology

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

Differential Diagnosis

Gastrointestinal
1. Appendicitis
2. Constipation
Uterus
1. Uterine Fibroids
2. Placental Abruption
Urinary
Musculoskeletal
1. Abdominal wall strain

Risk Factors

No associated risk factor in 50% of Preterm Labor
Chemical Use
1. Tobacco Abuse over 1/2 pack per day Cigarettes
2. Chemical Dependency (Cocaine, Heroin)
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)
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]
Increased Uterine Size
1. Multiple Gestation (50% of twin births, 90% of triplets are born before 37 weeks gestation)
2. Polyhydramnios
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
Demographics
1. Low socioeconomic status or poor nourishment
2. Black patients (14.9% preterm delivery vs 8.9% in non-hispanic white patients)
Comorbidity
1. Low pre-pregnant weight (Body Mass Index <19.8 kg/m2)
2. Periodontal Disease
3. Diabetes Mellitus
4. Thyroid disease
5. Hypertension
Uterine anomaly
1. Unicornuate Uterus or bicornuate Uterus
2. Uterine Fibroids
3. Diethylstilbestrol (DES) exposure in utero
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
References
1. Goldenberg (2008) Lancet 371(9606): 75-84 [PubMed]

Symptoms

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

Signs

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

Evaluation
Five key concerns

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
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
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
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
Determine if membranes are ruptured
1. See Premature Rupture of Membranes
2. See Labs below
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
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

Labs

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)
Microscopy to evaluate Vaginitis
1. Saline Wet Preparation (Trichomonas, Bacterial Vaginosis)
2. KOH Preparation
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
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
Fetal Lung Maturity Assessment
1. Indicated for 34 week gestation or greater

Imaging
Obstetric Ultrasound

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

Efficacy
Evaluation criteria

Evaluation criteria do not predict preterm delivery
1. Fetal Fibronectin
2. Uterine contraction frequency
3. Cervical Length assessment
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
References
1. Iams (2002) N Engl J Med 346:250-5 [PubMed]

Evaluation

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

Management

See Preterm Labor Management

Prevention

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]
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]
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]

References

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