Late-term and postterm pregnancy

Epidemiology & Risk Factors

• Definitions:
  • Late-term: $\ge 41\text{ 0/7}$ to $41\text{ 6/7}$ weeks Gestational Age (GA).
  • Postterm: $\ge 42\text{ 0/7}$ weeks GA.
• Risk Factors:
  • Inaccurate gestational dating (most common cause of apparent postterm pregnancy).
  • Prior postterm pregnancy (highest non-dating risk factor).
  • Nulliparity.
  • Maternal obesity ($\text{BMI}\ge 30{\text{ kg/m}}^2$).
  • Male fetus.
  • Rare congenital anomalies: Placental sulfatase deficiency, fetal anencephaly.

---

Clinical Features

• Maternal:
  • GA $\ge 41\text{ 0/7}$ weeks by verified dating.
  • Discrepancy in fundal height (decreased secondary to oligohydramnios, or increased secondary to fetal macrosomia).
  • Decreased fetal movements.
• Fetal / Neonatal (Dysmaturity / Postmaturity Syndrome):
  • Normal aging placenta undergoes infarction and calcification $\to$ uteroplacental insufficiency.
  • Physical exam findings at birth:
    • Loss of subcutaneous fat (“old man” wrinkled appearance).
    • Desquamated, peeling, parchment-like skin with loss of vernix caseosa and lanugo.
    • Long fingernails and toenails.
    • Green/yellow meconium staining of the skin, umbilical cord, and nails.

---

Diagnosis

• Initial / Dating Confirmation:
  • Review early ultrasound: 1st-trimester Crown-Rump Length (CRL) is the gold standard for accurate GA dating.
• Antenatal Fetal Surveillance (Initiated at $\ge 41\text{ 0/7}$ weeks):
  • Twice-weekly testing using either:
    • Biophysical Profile (BPP): Score $\le 4/10$ indicates fetal compromise.
    • Modified BPP: Nonstress Test (NST) + Amniotic Fluid Volume assessment.
• Key Diagnostic Findings:
  • Oligohydramnios: Single Deepest Pocket (SDP) / Maximum Vertical Pocket (MVP) $<2\text{ cm}$ or Amniotic Fluid Index (AFI) $\le 5\text{ cm}$ (indicates chronic uteroplacental insufficiency $\to$ shunting away from fetal kidneys $\to$ decreased urine output). c
    • Uteroplacental insufficiency and chronic fetal hypoxemia → blood is preferentially distributed to the brain rather than peripheral tissue
  • Fetal Growth Ultrasound: Evaluates for macrosomia (Estimated Fetal Weight $[\text{EFW}]\ge 4500\text{ g}$) vs Fetal Growth Restriction (FGR).

---

Differential Diagnostics

• Inaccurate Dating / Miscalculated GA:
  • Diff by discrepancy between Last Menstrual Period (LMP) and 1st-trimester CRL ($>5\text{–}7\text{ days}$ discrepancy favors early US dating).
• Fetal Growth Restriction (FGR):
  • Diff by $\text{EFW}<10\text{th}$ percentile for GA and abnormal umbilical artery Doppler velocimetry (increased resistance, absent/reversed end-diastolic flow).
• Intrauterine Fetal Demise (IUFD):
  • Diff by real-time ultrasonography demonstrating complete absence of fetal cardiac activity.
• Gestational Diabetes Mellitus (GDM) / Polyhydramnios:
  • Diff by normal/increased amniotic fluid ($\text{AFI}\ge 24\text{ cm}$ or $\text{SDP}\ge 8\text{ cm}$), elevated maternal glucose screening, and absence of placental senescence features.

---

Management

• First-line (Induction of Labor [IOL]):
  • Timing: Routine delivery indicated between $41\text{ 0/7}$ and $42\text{ 0/7}$ weeks; mandatory delivery by $42\text{ 0/7}$ weeks to reduce perinatal mortality and stillbirth.
  • Cervical Assessment (Bishop Score):
    • Unfavorable cervix (Bishop score $<6$): Cervical ripening with Prostaglandin E1/E2 (Misoprostol, Dinoprostone) or mechanical dilators (Foley balloon).
      • Contraindication: Prostaglandins are strictly contraindicated in patients with prior Cesarean delivery or prior transmural uterine surgery (high risk of uterine rupture).
    • Favorable cervix (Bishop score $\ge 6$): IV Oxytocin administration $\pm$ Artificial Rupture of Membranes (AROM / Amniotomy).
• Immediate Delivery Indications (Regardless of GA $\ge 41\text{ 0/7}$):
  • Oligohydramnios ($\text{SDP}<2\text{ cm}$ or $\text{AFI}\le 5\text{ cm}$).
  • Nonreassuring fetal status: Nonreactive NST with persistent variable/late decelerations, or failed BPP ($\le 4/10$).
• Cesarean Delivery:
  • Standard obstetric indications (e.g., cephalopelvic disproportion, category III fetal heart tracing refractory to intrauterine resuscitation, placenta previa, breech presentation).
  • Consider planned C-section for suspected fetal macrosomia ($\text{EFW}\ge 5000\text{ g}$ in non-diabetic; $\ge 4500\text{ g}$ in diabetic).

---

Complications

• Fetal & Neonatal:
  • Oligohydramnios $\to$ Umbilical cord compression $\to$ recurrent variable decelerations and acute fetal acidosis.
  • Meconium Aspiration Syndrome (MAS) (vagal stimulation from cord compression + mature GI tract $\to$ meconium passage in utero).
  • Shoulder dystocia, clavicular fracture, and brachial plexus injuries (Erb palsy) due to fetal macrosomia.
  • Neonatal hypoglycemia, polycythemia, and hypocalcemia.
  • Stillbirth / Intrauterine Fetal Demise (IUFD) (exponentially increases $>42$ weeks).
• Maternal:
  • Postpartum Hemorrhage (PPH) (uterine atony secondary to macrosomia, polyhydramnios, or prolonged labor).
  • Severe perineal lacerations (3rd- and 4th-degree).
  • Chorioamnionitis and endomyometritis.
  • Increased rate of operative vaginal delivery (forceps/vacuum) and Cesarean delivery.