Podcast Summary

Podcast: Dr. Chapa’s OBGYN Clinical Pearls
Episode: The 2025 Big Baby Trial
Date: January 5, 2026
Host: Dr. Chapa

Episode Overview

This episode dives deep into the findings and clinical implications of the 2025 "Big Baby Trial," a landmark multicenter study published in The Lancet that examined whether early induction (38–38+4 weeks) for fetuses suspected of being large for gestational age (LGA) could reduce the occurrence of shoulder dystocia compared to expectant management. Dr. Chapa provides a critical, evidence-based analysis, clarifies differences between intent-to-treat (ITT) and per protocol analyses, and reinforces why current ACOG/SMFM guidelines have not shifted in response to this study.

Key Discussion Points and Insights

1. Setting the Stage: Current Guidance on Suspected Macrosomia

• Current Recommendations:
  • Neither ACOG nor SMFM supports routine early induction of labor for suspected fetal macrosomia (big baby) [00:11–01:40].
  • Cesarean section is offered at specific weight thresholds:
    • ≥5,000g (without diabetes)
    • ≥4,500g (with diabetes)
• Quote:

  “As of right now, ACOG's current guidance and SMFM does not support routine early induction for LGA macrosomia. Okay, that's not a thing.” — Dr. Chapa [01:36]

2. What Was the Big Baby Trial?

• Trial Design:
  • Large, multicenter, open-label randomized trial conducted in NHS hospitals in England, Scotland, and Wales.
  • Compared elective induction of labor at 38–38+4 weeks vs. expectant management for suspected LGA.
• Primary Outcome:
  • Reduction in shoulder dystocia.
• Key Finding (Per Protocol):
  • 40% reduction in shoulder dystocia risk in early induction group [03:00].
• Key Finding (Intent-to-Treat):
  • No statistically significant reduction in shoulder dystocia [03:55].
• Quote:

  “The per protocol analysis demonstrated a significant reduction, 40% in fact, in shoulder dystocia… However, the key there is… the per protocol analysis. In the intent to treat, there was no reduction in shoulder dystocia.” — Dr. Chapa [03:00–04:15]

3. Understanding “Per Protocol” vs. “Intent to Treat” Analyses

• Definitions and Importance:
  • Per Protocol: Includes only participants who strictly adhered to their assigned intervention, thus vulnerable to selection bias.
  • Intent to Treat (ITT): Preserves randomization and provides a more scientifically valid result by including everyone as randomized [05:10–06:30].
• Selection Bias:
  • Crossover between groups undermined the randomization process in the Big Baby trial.
  • Quote:

    “The whole point of randomization is that you try to limit chance... So when you look at the per group... it’s not clear data, because that has the biggest issue here is selection bias.” — Dr. Chapa [13:40]

• Clinical Implication:
  • ITT findings are more reliable and guide practice.

4. Distinction Between LGA and True Macrosomia

• Big Baby Trial Population:
  • Recruited on LGA (estimated fetal weight >90th percentile), not strictly macrosomia (≥4,000g).
  • Many included babies did not meet true macrosomia criteria at birth [10:15–11:40].
• Quote:

  “All babies that are macrosomic are LGA, but not all LGA babies are Macrosomic, right?” — Dr. Chapa [09:19]

5. Current Guideline Details & Reaffirmation

• Practice Bulletin 178:
  • Reaffirmed in 2024, recommends against induction before 39 weeks unless medically indicated.
  • Uncertainty remains regarding best timing for LGA and unclear if early induction reduces shoulder dystocia [11:30–14:45].
• Meta-analyses and Challenges:
  • Mixed data, need for further study [14:15].
• Quote:

  “Until the results of additional studies are reported, the ACOG continues to discourage induction of labor solely for suspected macrosomia at any gestational age.” — Dr. Chapa [13:28]

6. Critical Appraisal of the Big Baby Trial

• Absolute Reduction:
  • Per protocol: 2.3% (induction) vs. 3.7% (standard care) — a 40% relative reduction, but small absolute numbers [16:10].
• Critiques from Correspondence in The Lancet (Oct 2025):
  • Premature termination & crossover reduced power; confidence interval (0.5–1.09) crossed 1, p-value not significant.
  • LGA definition too broad; most benefit shown with true macrosomia.
  • Poor predictive value of ultrasound for fetal weight—only 40% classified as LGA prenatally were LGA at birth [19:55–23:20].
• Quote:

  “Only 40% of the participants deemed at high risk actually had the infant who was LGA at birth...”
  — Dr. Chapa [22:29]
  “The biggest risk for shoulder dystocia, while big baby is one, it’s a previous history of shoulder dystocia that complicates matters.” — Dr. Chapa [24:40]

7. Take-Home Messages for Practice

• No Need for Routine Early Induction Solely for LGA:
  • Continue to individualize based on patient’s broader risk factors and other indications.
• Shared Decision-Making:
  • Offer elective induction at 39 weeks if desired and appropriate; history of shoulder dystocia is a key consideration.
• Quotes:

  “If you suspect baby is LGA, but below the cutoffs for offering a primary section, why would you go past 39 weeks? At 39 weeks with good dating, you don’t even need a reason.” — Dr. Chapa [24:00]
  “Routine induction just for this doesn’t seem to be the answer either.” — Dr. Chapa [25:19]
  “Maybe as of now, early induction at 38 weeks doesn’t seem to be the answer.” — Dr. Chapa [26:18]

Notable Quotes & Memorable Moments

• Recurring Humor:
  The host uses a quote from “The Office” repeatedly for comedic effect:

  “You didn’t prepare me for a big, giant fat baby.” — Repeated by Dr. Chapa at several points ([00:08], [05:08], [07:09], [25:16], [26:15])

• Guideline Commentary:

  “Guideline committees should now consider whether to spend substantial resources… for modest benefit given uncertain case findings… nuanced counseling is the most promising strategy.” — The Lancet editorial quoted by Dr. Chapa [23:50]

Timestamps for Important Segments

• 00:11–03:40 — Overview of current ACOG/SMFM guidelines and weight thresholds
• 03:40–05:08 — Introduction to the Big Baby trial, ITT vs. per protocol distinction
• 07:13–10:00 — LGA vs. macrosomia, Practice Bulletin 178 details
• 16:10–19:55 — Absolute risks and issues in the trial’s data interpretation
• 19:55–23:20 — Critical correspondence and commentary on limitations of the trial
• 23:50–24:40 — Practical clinical counseling, shared-decision making
• 26:15–26:55 — Final practical takeaways and summary

Practical Pearls

• Don’t ignore large babies, but don’t reflexively induce early based solely on LGA.
• ITT analysis should guide changes in clinical practice, not per protocol (which can be confounded by selection bias).
• Current ACOG/SMFM guidelines, reaffirmed in 2024, remain valid and should inform practice until further decisive evidence emerges.

Bottom Line

Routine early induction at 38–38+4 weeks for LGA, based solely on suspected increased fetal weight, is not supported by current evidence or guidelines. The Big Baby trial, though provocative, does not meet the threshold to change practice due to issues of selection bias, insufficient statistical significance in ITT analysis, and limitations in predictive ability of late pregnancy ultrasound. Careful, nuanced counseling and shared decision-making remain central to managing suspected big babies at term.

Host’s Final Message:

“As of now, early induction at 38 weeks doesn’t seem to be the answer. Short answer is selection bias per protocol versus ITT. Current guidance … does not show any reduction in shoulder dystocia with early induction.” — Dr. Chapa [26:18]