Interpretation
Alterations in the functionality of the abdominal muscles resulting from pregnancy-induced morphological changes have long been known25, and it has been proposed that women with stronger abdominal muscles might push more effectively, shortening the second stage of labor26. A prolonged second stage has been associated with adverse birth outcomes, including a greater risk of perinatal asphyxia27. However, in the present work, no association was seen between any abdominal muscle thickness and the duration of labour. This suggests that the improved CB gas values observed in offspring born to women with greater abdominal wall muscle thicknesses is due to metabolic rather than structural mechanisms. Uterine contractions during labour induce metabolic stress in the foetus, which is reflected in CB gas values at birth28.
In the present work, the subjects with a thicker EO at rest had newborns with less acidic cord arterial blood, which agrees with the association seen between a greater EO thickness during activation and a better 1-minute Apgar test score. Reductions in the pH of the arterial CB are generally related to acute labour difficulties, such as an acute occlusion of the umbilical cord.29
Acidosis in the umbilical artery can lead to vasoconstriction in the peripheral tissues (mostly) of the foetus30. Importantly, the present results suggest that babies born to mothers with greater abdominal muscle thicknesses are better oxygenated, as shown by the venous CB in particular; gas values measured in the venous CB reflect the concentration of oxygen originally provided to the fetus29. The mother’s skeletal muscle might play a role in the oxygenation of the cord blood via the release of myokines, which could cause vasodilation in the placental blood vessels31. Certainly, the myokines released by muscle tissue are known to protect against ischemia in angina32. Although no study has specifically examined the association between maternal myokines and birth outcomes, a systematic review33 suggests that myokines may act as signalling molecules between maternal skeletal muscle and the placenta, regulating maternal–foetal blood flow. Further studies on myokine levels and their association with abdominal muscle thickness in pregnant women are needed to better understand the mechanisms behind the present findings.
A greater IRD, both at rest and during crunches, was related to a higher PCO2, a lower PO2, and lower oxygen saturation in the arterial CB, and a more acidic pH for the venous CB. In addition, neonates born to mothers with a higher IRD during crunches had a lower venous CB oxygen saturation. These results confirm that a smaller IRD is associated with greater oxygenation of the foetus at birth. However, no association was seen between the IRD and any other birth outcomes, such as the Apgar test score or the duration of labour. This agrees with that described by Oliveira et al.12In contrast to the present findings, the latter authors found no association between IRD and CB pH or PCO212. These differences might be attributable to the fact that the latter study involved a smaller sample (n=24).
No cut-off has been established for diagnosing rectus diastasis in pregnant women. Indeed, the literature quotes the use of cut-off points at 2 cm above the umbilicus ranging from 20 to 86 mm19,34,35. For this reason, and following the criterion used in a previous study35, the IRD 50th percentile was used in the present work as a cut-off to explore whether there was any relationship between IRD and birth outcomes. Most of the previously discussed associations with IRD were maintained only for women with an IRD above the median during crunches. Being above the median was also associated with poorer CB gas values, but not with the rest of the studied birth outcomes. Although the literature contains no studies with which to reliably compare the present results, the finding of the above associations during crunches could be clinically important. Maternal pushing during the second stage of labour usually involves crunching to assist the baby’s descent through the birth canal. Therefore, the ability to reduce the IRD at this stage might be decisive in improving the oxygenation of the foetus. Further studies exploring the influence of core exercises during pregnancy on the above-described outcomes are needed.