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Vaginal delivery involves four stages of labour: the shortening and opening of the cervix during the first stage, descent and birth of the baby during the second, the delivery of the placenta during the third, and the fourth stage of recovery which lasts until two hours after the delivery.
Second stage of labor starts when the cervix is dilated to 10 cm and finishes with the birth of the baby. This stage is characterized by strong contractions and active pushing by the mother. It can last from 20 minutes to 2 hours. [16] Third stage of labor starts after the birth of the baby and is finished when the placenta is delivered. [15]
After birth, the fundus contracts downward into the pelvis one centimeter each day. After two weeks the uterus will have contracted and return into the pelvis. [9] The sensation and strength of postpartum uterine contractions can be stronger in women who have previously delivered a child or children. [10]
[36] [37] The second phase of labor begins when the cervix has dilated to 10 cm (4 in), which is regarded as its fullest dilation, [32] and is when active pushing and contractions push the baby along the birth canal leading to the birth of the baby. [35]
From that point, pressure from the presenting part (head in vertex births or bottom in breech births), along with uterine contractions, will dilate the cervix to 10 centimeters, which is "complete." Cervical dilation is accompanied by effacement, the thinning of the cervix. General guidelines for cervical dilation: Latent phase: 0–3 centimeters
These labour contractions are characterized by their rhythmic tightening and relaxation of the myometrium, the most prominent uterine muscle. Labour contractions primarily serve the purpose of opening and dilating the cervix, [7] which leads to the assisting of the passage of the baby through the vaginal canal during the first stage of labour.
Histologically, the cervix undergoes significant changes towards the end of gestation, allowing the ripening of cervix for the passage of birth delivery. First, there is an increase in the cervical synthesis of glycosaminoglycan hyaluronan (HA), which increases tissue hydration, thereby catalyzing the degradation of collagen and elastin-fibers.
The Ferguson reflex (also called the fetal ejection reflex) is the neuroendocrine reflex comprising the self-sustaining cycle of uterine contractions initiated by pressure at the cervix, more precisely, the internal end of cervix, or vaginal walls. It is an example of positive feedback in biology. The Ferguson reflex occurs in mammals.