Post-term Pregnancy

Aka: Post-term Pregnancy, Postterm Pregnancy, Post-dates Pregnancy, Postdates Pregnancy, Late-Term Pregnancy

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II. Epidemiology

  1. True Prevalence of Post-dates Pregnancy: 0.25% (decreased from 2-3% in 2014)
  2. Pseudoprevalence (misdated pregnancy): 10-14%
    1. Inaccurate clinical dating (esp. overestimation) is common if based on Last Menstrual Period (LMP) alone
    2. LMP-based dating assumes accurate recall, regular Menses, no recent OCPs and Ovulation on day 14 of cycle
    3. Most accurate and reliable dates are based on early Ultrasound with Crown-Rump Length

III. Definitions

  1. Late-Term Pregnancy
    1. Gestational age 41 weeks 0 days to 41 weeks 6 days
  2. Post-term Pregnancy
    1. Over 294 days (42 weeks) beyond first day of LMP
  3. Post-Mature Infant
    1. Truly post-term by objective criteria

IV. Risk Factors

  1. Inaccurate Pregnancy Dating
  2. Prior Postdates Pregnancy
  3. Nulliparity
  4. Maternal age >30 years old
  5. Obesity (BMI>30 kg/m2)
  6. Male fetus
  7. Genetic predisposition
    1. Mother who herself was a product of a Postdates Pregnancy has a 50% risk of Postdates Pregnancy >42 weeks
    2. Morken (2011) BJOG 118(13): 1630-5 [PubMed]
  8. Other causes (rare)
    1. Fetal Anencephaly
    2. Placental surface deficiency

V. Signs: Physical Characteristics of a post-dates infant

  1. Underweight due to loss of subcutaneous fat
  2. Long and thin in girth
  3. Skin with patchy areas of Desquamation
  4. Skin, long nails may be covered with meconium
  5. Wrinkled hands and feet on ventral surfaces

VI. Complications: Fetal (after 41 weeks gestation)

  1. Perinatal morbidity and mortality increases after 41 weeks gestation (Late-Term Pregnancy)
    1. Mortality increases 30x fold from 37 week to 41 week delivery
    2. Mortality 0.11 per 1000 births at 37 weeks, and 3.18 per 1000 births at 41 weeks
    3. Mortality is as high as 11.5 per 1000 births at 43 weeks
    4. Muglu (2019) PLoS Med 16(7): e1002838 [PubMed]
    5. Nakling (2006) Acta Obstet Gynecol Scand 85(6): 663-8 [PubMed]
    6. Brukner (2008) Am J Obstet Gynecol 199(4): 421 [PubMed]
  2. Meconium Aspiration Syndrome
  3. Oligohydramnios
  4. NICU admission
  5. Macrosomia related Birth Trauma (e.g. Fractures, Peripheral Nerve Injury)
  6. Neurologic risks (possible)
    1. Seizure Disorder (1.2 Incidence rate ratio)
      1. Ehrenstein (2007) Pediatrics 119(3): e554-61 [PubMed]
    2. Cerebral Palsy (1.36 per 1000 compared with 0.99 per 1000)
      1. Moster (2010) JAMA 304(9): 976-82 [PubMed]

VII. Complications: Maternal (after 42 weeks gestation)

  1. Postpartum Hemorrhage
  2. Labor Dystocia
  3. Chorioamnionitis or Endometritis
  4. Cervical Laceration
  5. Cesarean delivery

VIII. Management: Approach

  1. Confirm dates
    1. First Trimester Ultrasound (Crown-Rump Length) modifies LMP-based dates if difference >5-7 days
      1. Do not adjust dates based on later Ultrasounds (later discrepancies are related to Fetal Growth)
    2. Later Ultrasound (BPD, head circ, abd circ, femur len) modifies LMP based on timing
      1. Gestation 14 to 16 weeks based on LMP: Modify ED+C if U/S dates >7 day discrepancy
      2. Gestation 16 to 21 weeks based on LMP: Modify ED+C if U/S dates >10 day discrepancy
      3. Gestation 22 to 27 weeks based on LMP: Modify ED+C if U/S dates >14 day discrepancy
      4. Gestation >28 weeks based on LMP: Modify ED+C if U/S dates >21 day discrepancy
  2. Consider sweeping membranes after 38 weeks gestation
    1. Number Needed to Treat to prevent one induction: 8
    2. Associated with Relative Risk of pregnancy beyond 41 weeks: 0.59
    3. Associated with Relative Risk of pregnancy beyond 42 weeks: 0.28
    4. Boulvain (2005) Cochrane Database Syst Rev (1): CD000451 [PubMed]
  3. Other measures that may stimulate Cervical Ripening and spontaneous labor after 38 weeks gestation
    1. Breast Massage and nipple stimulation, for 15-20 min, 3 times daily
      1. Stark (2022) Am J Obstet Gynecol MFM 4(2): 100575 [PubMed]
    2. Walking 30 minutes, 3 times weekly at 2.5 miles/hour pace
      1. Pereira (2022) J Matern Fetal Neonatal Med 35(4): 775-9 [PubMed]
  4. Informed Consent to continue expectant management beyond 41 weeks
    1. Discuss risks of late-term and Post-term Pregnancy
    2. Discuss risks of Labor Induction and Cesarean Section
  5. Assess for likelihood of induction success
    1. Bishop Score >5-6
    2. Shorter Cervical Length
    3. Lower Body Mass Index
  6. Antenatal surveillance protocol starting at 41 weeks (otherwise low risk pregnancies)
    1. Perform amniotic fluid and Fetal Assessment twice weekly starting at 41 weeks
    2. Amniotic fluid assessment
      1. Amniotic fluid index (AFI) >5 ml or
      2. Amniotic fluid pocket >2 cm x 2 cm fluid pocket
        1. Lack of pocket associated with fetal asphyxia
        2. Lack of pocket associated with perinatal mortality
    3. Fetal Assessment
      1. Nonstress Test reactive or
      2. Contraction Stress Test reactive or
      3. Biophysical Profile 6 or more
  7. Induction Indications
    1. Oligohydramnois
      1. Amniotic fluid pocket<2 cm single pocket or
      2. Amniotic fluid index < 5ml
    2. Failed Fetal Assessment
      1. Non-reactive Non-Stress Test (with or without confirmatory Biophysical Profile) or
      2. Positive Contraction Stress Test or
      3. Biophysical Profile <6 or
      4. Non-reassuring umbilical artery doppler
    3. Gestational age >41-42 weeks
      1. Fetal mortality reduced for delivery at 41 weeks with NNT 328 to spare one fetal death
        1. Hussain (2011) BMC Public Health 11(suppl 3): S5 [PubMed]
      2. Induction vs expectant management are both reasonable options at 41 weeks
      3. Induction at 41 weeks is associated with a lower Cesarean Section rate
        1. Number Needed to Treat to induce at 41 weeks to prevent one cesarean: 30
        2. Gulmezoglu (2012) Cochrane Dtabase Syst Rev (6): CD004945 [PubMed]
    4. Consider elective Labor Induction for low risk Nulliparous women as early as 39 weeks
      1. Lowers risk of cesarean delivery (NNT 28) and PIH (NNT 20)
      2. Employ Shared Decision Making regarding risks and benefits of early induction
      3. Grobman (2018) N Engl J Med 379(6): 513-23 [PubMed]
      4. (2019) Am J Obstet Gynecol 221(1): B2-4 [PubMed]

IX. References

  1. Gawrys (2024) Am Fam Physician 110(4): 378-84 [PubMed]
  2. Wang (2014) Am Fam Physician 90(3): 160-5 [PubMed]

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