INTRODUCTION
The abdominal musculature undergoes morphological and functional changes during pregnancy1. For instance, the inter-recti distance (IRD) increases, especially in the third trimester2, while other stabilizing muscles of the lumbar spine, such as the transverse abdominis (TrA) and the internal oblique (IO) and external oblique (EO)3 muscles, tend to become thinner as pregnancy progresses4. A number of studies have explored the associations shown between such changes in the abdominal muscles during pregnancy, and lower back pain4, pelvic symptoms5, and quality of life6, and with pelvic floor status7 in the postpartum period. Others have focused on the role of pelvic floor muscles on birth outcomes8. However, despite the fact that the abdominal muscles play an important role during labor9, little is known about their influence on birth-related outcomes.
The second stage of labour begins with the full dilation of the cervix and ends with the birth of the baby. It includes a passive phase, in which the descent of the foetus takes place, and an active phase, during which the mother voluntarily pushes the foetus through the birth canal10. The successful delivery of the baby therefore depends on both uterine contractions and maternal abdominal effort9. Excessive stretching of the abdominal muscles during pregnancy has been previously associated with a reduction in contractile strength11, and interestingly, a larger diastasis recti has been associated with poorer electromyographic readings for the rectus abdomini muscles during labour12.
There are other pathways, however, via which maternal muscle status might influence birth outcomes. For example, muscles also have an important endocrine function via the secretion of myokines that can improve vascular endothelial function13, perhaps leading to better placental and umbilical cord blood (CB) flow during birth. Thicker abdominal muscles might, therefore produce more myokines, further improving these variables.
The aim of the present work was to explore the associations shown by the thickness of abdominal muscles and the IRD at the 34thweek of gestation (w.g.), with the duration of the first and second stages of labour, the neonatal Apgar test score, and umbilical CB gas values at birth.