Differences in Abdominal and Lumbar Muscle Thickness and Contractile Function Between Nulliparous, Primiparous, and Multiparous Women 6 Months Postpartum.

Objective: The purpose of this study was to evaluate the differences between primiparous and multiparous women at 6 months postpartum and nulliparous women regarding the thickness (including thickness pattern) and contractile function of the abdominal and lumbar muscles.

Methods: A cross-sectional observational study of 80 women, both primiparous (n = 29) and multiparous (n = 25), as well as including nulliparous as a comparison group (n = 26), was conducted. B-mode ultrasound imaging was used for the measurement of abdominal and lumbar muscle thickness in 2 conditions (resting and activation). Moreover, the contractile function of these muscles was determined by the thickness ratio.

Results: Significant differences between groups were observed in abdominal muscle thickness. During activation, the obliques showed a lower thickness in both groups of postpartum women compared to nulliparous women, while lower values for the thickness for the transverse abdominis (TrA) were only observed in primiparous women. The thickness pattern observed in nulliparous women both in resting and in activation (internal oblique [IO] > external oblique [EO] > TrA) was also achieved among postpartum women, despite no statistical differences observed between IO and EO in these groups. No significant differences between groups were observed for contractile function, showing the TrA greater values, followed by IO and EO in both nulliparous and postpartum women.

Conclusions: This study suggests that the thickness (and the thickness pattern) of the abdominal muscles in women at 6 months postpartum differs from nulliparous, while for the lumbar muscles, it is comparable to nulliparous. However, despite the obliques being thinner in postpartum women, the abdominal muscles are functionally competent.

Impact: This is believed to be the first study to compare abdominal and lumbar muscles thickness and contractile function specifically between primiparous and multiparous women. Although at 6 months' postpartum abdominal thickness differs from nulliparous women, these muscles are functionally competent. These findings provide valuable insights for use in the development of postpartum interventions.