Low-Lying Placenta

Definition

Key cutoff: If the placental edge is >20 mm from the internal os, it is reported as "normal." If it is within 20 mm but not covering the os, it is labeled "low-lying."

Incidence and Epidemiology

• Placenta previa (the more severe end of the spectrum) affects approximately 0.5% of all third-trimester pregnancies.
• Up to 10% of midtrimester sonograms show a low-lying or previa placenta.
• At least 90% of placentas identified as low-lying or previa in early pregnancy resolve by the third trimester via placental migration (trophotropism) - the placenta grows toward the better-vascularized fundus, while the lower segment tissue atrophies. - Creasy & Resnik's, p. 1018

Risk Factors

Pathogenesis

• The lower uterine wall elongates in the third trimester
• Cervical dilation occurs at or near labor onset
• Prelabor contractions, intercourse, or cervical examination disturbs the placental margin
• Creasy & Resnik's, p. 1018; Rosen's Emergency Medicine, p. 3355

Clinical Features

• Painless, bright-red vaginal bleeding is the hallmark - typically in the second half of pregnancy.
• Small "herald bleeds" may precede major hemorrhage.
• In up to 10% of cases, there is no bleeding until labor begins.
• Uterine irritability may be present in ~20% of cases but is generally mild.
• No abdominal pain - this distinguishes it from placental abruption.

Warning: Digital or instrument examination of the cervix is CONTRAINDICATED until placenta previa/low-lying placenta is excluded by ultrasound - it can precipitate catastrophic hemorrhage.

Diagnosis

Ultrasound (Method of Choice)

• Transvaginal ultrasound (TVUS) is the gold standard - more accurate than transabdominal ultrasound for measuring the os-to-placental edge distance.
• The bladder must be emptied before scanning to avoid falsely diagnosing previa.
• Precise measurement of the distance from the placental edge to the internal os is essential.
• A low-lying placenta found in the second trimester may resolve with continued uterine growth; one persisting late in gestation is less likely to improve.

MRI

• Used in equivocal cases or when placenta accreta syndrome is suspected (e.g., if previous cesarean scar is present near the low-lying placenta).

Differential Diagnosis

• Placenta previa (covers the os)
• Localized uterine contraction mimicking a low-lying edge
• Placenta accreta spectrum (invasive placentation - must always be considered)
• Succenturiate lobe
• Vasa previa (cord vessels crossing the os)

Associated Conditions

• Placenta accreta spectrum - particularly with prior cesarean scar
• Vasa previa - fetal vessels running through membranes near the os, risk of fetal exsanguination at rupture of membranes

Management

Antepartum (Stable Patient)

• Serial TVUS to monitor for spontaneous resolution (especially if diagnosed before 28 weeks)
• Pelvic rest (no intercourse, no vaginal exams)
• Corticosteroids for fetal lung maturity if preterm delivery is anticipated
• Rh immune globulin (300 mcg IM within 72 hours) if mother is Rh-negative

If Bleeding Occurs (Emergency)

1. Two large-bore IV lines + fluid resuscitation
2. Continuous fetal monitoring
3. CBC, type and crossmatch
4. Coagulation panel (PT, PTT, platelets, fibrinogen - normal in pregnancy: 400-450 mg/dL; levels <300 mg/dL indicate significant coagulopathy)
5. Fresh-frozen plasma or fresh whole blood if coagulopathy develops
6. Urgent obstetric consultation

Delivery

• Cesarean section is recommended if the placental edge is ≤1 cm from the os or if it is covering the os at the time of delivery planning.
• If placental edge is 1-2 cm from the os, vaginal delivery may be considered on a case-by-case basis with careful monitoring.
• Grainger & Allison's, p. 915

Key Points Summary

Recent Evidence

Oligohydramnios

Definition

• AFI (Amniotic Fluid Index) method: AFI <5 cm (significant oligohydramnios)
• Single Deepest Pocket (SDP) method: <2 cm in both vertical and horizontal planes (score = 0 on biophysical profile)
• Normal AFI: 8-24 cm from 16 weeks onward
• "Borderline" oligohydramnios: AFI 5-8 cm
• Pfenninger & Fowler's Procedures for Primary Care; Creasy & Resnik's Maternal-Fetal Medicine

How Amniotic Fluid is Formed

Causes (Mnemonic: DRIPPC)

Excluding PROM, approximately 83% of patients with oligohydramnios will have fetal IUGR; however, only 16% of patients with IUGR will have oligohydramnios. - Pfenninger & Fowler's

Pathogenesis Diagram (Robbins Pathologic Basis of Disease)

Complications - The Potter Sequence

1. Pulmonary Hypoplasia (most serious)

• Restricted thoracic expansion prevents normal lung growth
• Reduced hydraulic pressure on lungs affects stretch receptors and Ca²⁺ regulation, impairing lung growth
• Risk increases significantly if oligohydramnios occurs before 26 weeks (canalicular phase of lung development: 16-24 weeks)
• Leads to potentially lethal pulmonary hypoplasia (small, bell-shaped chest)
• The Developing Human - Moore's Embryology; Creasy & Resnik's

2. Facial Compression (Potter's Facies)

• Flattened facies
• Prominent infraorbital (epicanthal) folds
• Beaked/parrot nose
• Low-set ears

3. Musculoskeletal Deformities

• Positional abnormalities of hands and feet (clubfoot / talipes equinovarus)
• Hip dislocation
• Flexion contractures

4. Amnion Nodosum

• Nodules on the amnion surface (squamous cell debris from fetal skin)
• Pathognomonic of prolonged oligohydramnios

5. Umbilical Cord Compression

• Absent fluid cushioning around the cord
• Causes variable decelerations in fetal heart rate during labor

6. Breech Presentation

• Reduced fluid limits fetal movement and rotation

Diagnosis

Ultrasound (Method of Choice)

• Divide uterus into 4 quadrants
• Measure the deepest vertical fluid pocket in each quadrant (avoiding cord, using color Doppler)
• Sum the 4 values = AFI
• Repeat 3 times and average if AFI <8 cm

• Fetal biometry (estimated fetal weight for IUGR)
• Renal anatomy (bilateral renal agenesis, cystic kidneys, hydronephrosis)
• Bladder filling (small/absent bladder suggests renal cause)
• Amniotic membrane integrity

Additional investigations

• Speculum exam: for pooling of amniotic fluid (PROM)
• Fetal MRI: indicated in severe oligohydramnios when sonographic visualization is limited (e.g., to detect associated anomalies or assess renal structure)
• Fetal urine electrolytes, osmolality, β₂-microglobulin (if obstructive uropathy is suspected - to assess renal function prognosis)
• Fetal karyotype if structural anomalies found

Clinical Features and Intrapartum Complications

• Fetal heart rate abnormalities (variable decelerations from cord compression)
• Depressed Apgar scores
• Meconium passage (fetal distress)
• Higher rates of operative/cesarean delivery

Management

Based on Gestational Age and Etiology:

Amnioinfusion

• Transcervical infusion of normal saline into the amniotic cavity during labor
• Reduces cord compression, decreases variable decelerations
• Does not improve perinatal survival when used for PROM-associated oligohydramnios

Key rule: In most cases, oligohydramnios at term is an indication for delivery. - Pfenninger & Fowler's

Prognosis (Obstructive Uropathy / LUTO)

• Poor prognosis associated with: earlier gestational age at onset, more severe oligohydramnios, associated structural anomalies, elevated fetal urine β₂-microglobulin/osmolality
• Even with postnatal correction: 25-30% of surviving neonates require dialysis by age 5 - Miller's Anesthesia

Recent Evidence