Does Gestational Weight Gain Influence Perineal Trauma in Underweight First-Time Mothers?

OBGYN Clinical Update

New Evidence Suggests Weight Gain May Not Be the Key Risk Factor

Perineal trauma remains one of the most common complications of vaginal birth, with obstetric anal sphincter injury (OASI) carrying significant long-term implications for pelvic floor function, continence, and quality of life. While maternal BMI has been extensively studied as a risk factor, the impact of gestational weight gain (GWG) among underweight women has remained unclear.

A recent retrospective cohort study from a tertiary academic medical center evaluated whether GWG influences the risk of perineal injury among underweight primiparous women—and the results may challenge common assumptions.

Study at a Glance

Publication: International Journal of Gynecology & Obstetrics, 2025

Design: Retrospective cohort study (2012–2023)

Population: 4,034 underweight (BMI <18.5 kg/m²) primiparous women with singleton vertex vaginal deliveries

Groups Based on GWG:

• Below recommendations: <12.5 kg (49%)

• Within recommendations: 12.5–18 kg (39%)

• Above recommendations: >18 kg (12%)

  
  

Primary Outcome: Overall perineal injury

Secondary Outcome: Obstetric anal sphincter injury (OASI)

Key Findings

Gestational Weight Gain Was Not Associated With Perineal Trauma

Despite substantial differences in maternal weight gain, rates of perineal injury were remarkably similar across all groups.

After multivariable adjustment:

• GWG below recommendations: OR 1.18 (not significant)

• GWG above recommendations: OR 1.06 (not significant)

The authors concluded that gestational weight gain itself does not independently influence the risk of perineal trauma or OASI in underweight nulliparous women. (Obstetrics & Gynecology)

What Actually Increased Risk?

Several established obstetric factors emerged as significant predictors of perineal injury:

Vacuum-Assisted Delivery

• OR 3.88 for overall perineal injury

• OR 2.19 for OASI

Higher Birth Weight

• OR 2.12 per kilogram increase for overall trauma

• OR 3.24 per kilogram increase for OASI

Epidural Analgesia

• OR 1.52 for overall perineal injury

These findings reinforce the importance of intrapartum factors over antenatal weight gain in determining perineal outcomes. (Obstetrics & Gynecology)

An Interesting Observation: BMI Still Matters

Although all women in the cohort were classified as underweight, a higher BMI within the underweight range was associated with a lower risk of perineal trauma.

This suggests that even small differences in baseline maternal body habitus may influence tissue elasticity, pelvic floor resilience, or labor mechanics. Further research is needed to clarify these mechanisms. (Obstetrics & Gynecology)

Clinical Implications

This study provides reassuring data for counseling underweight pregnant women:

✅ Excessive gestational weight gain does not appear to protect against perineal trauma.

✅ Inadequate weight gain does not appear to increase the risk of perineal injury.

✅ Birth weight and operative vaginal delivery remain stronger determinants of perineal outcomes than maternal pregnancy weight gain.

✅ Efforts to reduce severe perineal trauma should continue to focus on intrapartum management strategies rather than gestational weight gain alone.

Clinical Pearls

🔹 Underweight women should continue to be counseled regarding recommended pregnancy weight gain for fetal and maternal health outcomes, but not specifically for prevention of perineal trauma.

🔹 Vacuum-assisted delivery remains one of the strongest modifiable risk factors for both perineal injury and OASI.

🔹 Fetal birth weight continues to be a key predictor of severe perineal trauma.

🔹 Risk assessment for OASI should prioritize obstetric and intrapartum factors over gestational weight gain patterns.

Bottom Line

Among more than 4,000 underweight primiparous women, gestational weight gain—whether below, within, or above recommended guidelines—did not affect rates of perineal injury or obstetric anal sphincter injury. Instead, vacuum-assisted delivery, higher neonatal birth weight, and epidural use were the principal drivers of perineal trauma. These findings suggest that optimizing intrapartum care may have a greater impact on preventing perineal injury than focusing on gestational weight gain alone.

Reference

Gilboa I, Gabbai D, Attali E, et al. Impact of gestational weight gain on perineal injury and obstetric anal sphincter injury among underweight primiparous women. International Journal of Gynecology & Obstetrics, 2025.