What is the difference between a dorso‑anterior and dorso‑posterior transverse lie, and how should each be managed in the third trimester?

Transverse Lie: Dorsoanterior vs Dorsoposterior

Anatomical Definitions

In a transverse lie, the distinction between dorsoanterior and dorsoposterior refers to the position of the fetal spine (dorsum) relative to the maternal abdomen. In a dorsoanterior position, the fetal back is oriented toward the mother's anterior abdominal wall, while in a dorsoposterior position, the fetal back is oriented toward the mother's spine 1.

• The fetal spine position tends to correspond with maternal positioning during rest—when mothers lie supine, fetal posterior or transverse spinal positions are more likely to occur 2.
• The fetal cephalic presentation does not always correspond with the fetal spinal position, meaning head position and back position can vary independently 2.

Clinical Significance and Natural History

Most transverse lies discovered incidentally on ultrasound before term are transient and resolve spontaneously. The persistence rate varies dramatically by gestational age 3:

• At 20-25 weeks: 2.6% persistence to term 3
• At 36-40 weeks: 11.8% persistence to term 3
• Overall persistence rate across all gestational ages: 6.0% 3

Low-lying placenta or placenta previa is present in approximately one-third of persistent transverse lies (35.7% of cases persisting to term), making placental location assessment critical 3.

Third Trimester Management Algorithm

Initial Assessment (Before 36 Weeks)

• Confirm fetal position with ultrasound, documenting whether the spine is dorsoanterior or dorsoposterior 1.
• Evaluate placental location—if low-lying or previa is present, external cephalic version is contraindicated and cesarean delivery is indicated 3.
• Assess amniotic fluid volume, as oligohydramnios reduces the success of positional correction 2.
• Determine whether the fetal head is engaged, as engagement significantly reduces correction success 2.

Conservative Positional Management (32-36 Weeks)

Maternal postural techniques can be attempted to encourage spontaneous version, particularly the hand-knee (all-fours) position, which shows better results than lateral positioning 2:

• Hand-knee posture: Mother assumes all-fours position for 10-15 minutes, 2-3 times daily 2.
• Sims' position (lateral recumbent): Less effective alternative 2.
• Success is higher when fetal head is not engaged and amniotic fluid volume is adequate 2.

External Cephalic Version (After 36-37 Weeks)

If transverse lie persists beyond 36-37 weeks and membranes are intact, external cephalic version under tocolysis should be offered as the primary intervention to avoid cesarean delivery 4, 5:

• Success rate for converting transverse to longitudinal lie: 83% 4.
• Of successful conversions, 60% result in vaginal delivery 4.
• External version reduces cesarean section rate by approximately 50% 4.
• The procedure should be performed with ultrasound confirmation, informed consent, and tocolytic support 4.

Intrapartum Management

If transverse lie is present in labor with intact membranes, external version under tocolysis can still be attempted before proceeding to cesarean delivery 4:

• Membranes must remain intact for safe version attempt 4.
• If version is unsuccessful or membranes rupture, immediate cesarean delivery is indicated 5.
• Do not attempt version if membranes are ruptured, as this significantly increases risk 5.

Cesarean Delivery Indications

Proceed directly to cesarean delivery without attempting version if 4, 5:

• Placenta previa or low-lying placenta is present 3.
• Membranes are ruptured 4, 5.
• External version fails after appropriate attempts 4.
• Labor is advanced or fetal distress is present 5.

Key Differences in Management: Dorsoanterior vs Dorsoposterior

The dorsoanterior versus dorsoposterior distinction does not fundamentally change management strategy, but dorsoanterior positions may be slightly more favorable for spontaneous version with maternal positioning 2:

• Dorsoanterior: Fetal back against maternal anterior abdominal wall—may respond to hand-knee positioning by encouraging rotation 2.
• Dorsoposterior: Fetal back against maternal spine—maternal supine positioning should be avoided as it perpetuates this position 2.

Common Pitfalls

• Failing to assess placental location before attempting external version—placenta previa is present in over one-third of persistent transverse lies and is an absolute contraindication to version 3.
• Attempting external version after membrane rupture—this dramatically increases risk and should prompt immediate cesarean delivery 4, 5.
• Assuming all transverse lies require intervention before 36 weeks—most resolve spontaneously, particularly those discovered at 20-25 weeks (97.4% resolution rate) 3.
• Abandoning external version as an option in labor—if membranes are intact, version under tocolysis can still achieve 83% success and reduce cesarean rates by 50% 4.