Maternal parity and functional contractility of human myometrium in vitro in the third trimester of pregnancy

Forces Involved with Labor and Delivery-A Biomechanical Perspective

Childbirth is a primarily biomechanical process of physiology, and one that engineers have recently begun to address in a broader fashion. Computational models are being developed to address the biomechanical effects of parturition on both maternal and fetal tissues. Experimental research is being conducted to understand how maternal tissues adapt to intrauterine forces near the onset of labor. All of this research requires an understanding of the forces that are developed through maternal efforts-both uterine contractions and semi-voluntary pushing-and that can be applied by the clinician to assist with the delivery. This work reviews the current state of knowledge regarding forces of labor and delivery, with a focus on macro-level biomechanics.

Spontaneous Human Myometrial Contractility in the Third Trimester of Pregnancy in Relation to Past Mode of Delivery

OBJECTIVE

It is well established that women with a previous vaginal delivery have higher success rates in relation to vaginal birth after cesarean than those without. The aim of this study was to examine the effect of past mode of delivery on contractile parameters of human myometrium in vitro.

STUDY DESIGN

Myometrial strips were excised from 64 women at cesarean delivery (CD) and recordings of spontaneous contractile activity analyzed and compared across three clinical groups: (1) women with no previous delivery (Group 1); (2) women with CD only (Group 2); and (3) women with a history of vaginal delivery and CD (Group 3).

RESULTS

Myometrial samples from women in Group 3, women who had a previous vaginal delivery, had a significantly greater maximum amplitude of contractions (p < 0.05), a greater force (mean contractile force) of contractions (p < 0.01), and a faster rate of rise (p < 0.01) and relaxation of contractions (p < 0.05) than those in Groups 1 and 2.

CONCLUSION

Many of the functional parameters of human uterine contractions are altered, or enhanced, in the women who have had a previous vaginal delivery, when compared with those without. This may partly explain the clinical differences observed in labor.

Effects of maternal parity on response of human myometrium to oxytocin and ergometrine in vitro

OBJECTIVE

The aim of this study was to compare the effects of oxytocin and ergometrine on the intrinsic contractile parameters of human uterine smooth muscle at term between primiparous and multiparous women.

STUDY DESIGN

Myometrial biopsies were obtained from women undergoing planned caesarean section at term. The biopsies were dissected into eight uniform strips and mounted in tissue baths for isometric recording. The strips were challenged with increasing concentrations of oxytocin and ergometrine. Parameters of contractile activity, including mean contractile force (MCF) and maximum amplitude of contractions (MAMP) were recorded and analysed. Results were compared between primiparous (Group 1) and multiparous (Group 2) women.

RESULTS

Myometrial biopsies were obtained from n = 11 donors (88 tissue strips), of which n = 5 were Group 1 and n = 6 were Group 2. In relation to oxytocin, the MAMP value observed was significantly greater in Group 2 than in Group 1 (151 ± 18mN vs 67 ± 14mN, P < 0.01). Regarding ergometrine, the MCF response was greater in Group 2 samples (24 ± 10 mN) than that in Group 1 (18 ± 2mN) (P < 0.05).

CONCLUSION

Our findings highlight that women in a first pregnancy have a decreased response to both oxytocin and ergometrine in an in vitro setting when compared with women in a subsequent pregnancy, and this may have clinical implications regarding the management of postpartum haemorrhage in this cohort.