Question

An important factor contributing to the onset and maintenance of labour contractions is :
A. Increased contractility of the uterine muscles from increased oxytocin levels.
B. Mechanical pressure on the cervix generated by the emerging baby’s head.
C. Stretching of the uterus during the final stages of fetal growth.
D. All of the above.

Answer 1

Hint: Women who received oxytocin induction had significantly higher uterine contraction pressures than women who received oxytocin augmentation. Oxytocin boosts the strength and frequency of uterine contractions. The uterus will stretch as well as expand as the body prepares for the baby. The ligaments which support the uterus will stretch, causing mild cramping.

Step by step solution:
According to research, the increased estrogen to progesterone ratio that occurs prior to the onset of labour causes an increase in the number of oxytocin receptors in the uterus. The hypothalamus, specifically the paraventricular and supraoptic nuclei, initiates the process. The paraventricular nucleus is the most important contributor to oxytocin production and secretion. The hypothalamic-hypophyseal tract then transports oxytocin to the posterior pituitary. Following its arrival in the posterior pituitary, oxytocin enters the systemic blood supply, where it is delivered to oxytocin receptors on myometrial cells. During pregnancy, the uterine epithelial lining also produces oxytocin, which acts in both an autocrine and paracrine manner. Oxytocin's positive feedback mechanism accelerates the onset of labour.
Mechanical stretch is the tension placed on myometrial cells as the uterus dilates. Physical uterine stretching causes an influx of ions, specifically sodium and calcium, which changes the action potential across myometrial cells. This modification speeds up the onset of uterine contractions. Just before labour, the number of connexins (gap junctions) between myometrial cells increases, allowing coordinated muscle contraction. Earlier in pregnancy, these cell-to-cell contacts are less abundant and tend to favour uterine quiescence.
Cervical dilation occurs when an unborn baby's changing position induces pelvic pressure to become severe enough to cause the cervix to thin and open. These changes are usually brought about by contractions that further increase the amount of pressure on the cervix. The more pressure the mother feels as dilation occurs, the lower the baby is in her mother's pelvis.

So, option D is correct.

Note: The mother could go into labour at any time during the final month. Because space is limited, they may notice less movement. The fetus' position may have changed at this stage to prepare for birth. It should be head down in their uterus. They may feel very uneasy in this final stretch as the fetus settles into its pelvis and prepares for birth.