Parturition is the act of giving birth to a baby. It consists of many different stages which interact to provide delivery of child and the placenta. Childbirth usually occurs after 38 weeks of gestation (in clinical terms 40 weeks since amenorrhea). Parturition tends to last between 8 hours (in multiparti mothers) to 14 hours (in primipari mothers), although longer labours are not unusual.
The primary organ involved is the uterus. This contains a layer known as the myometrium which contains bundles of non-striated (smooth) muscle as well as connective tissue (collagen), nerves and blood vessels. The muscles of the inner wall of the myometrium are circular, whereas the muscles on the outer wall are longitudinal. During pregnancy this layer increases in mass approximately 20 fold, with hypertrophy increasing from 50 um to 500 um, hyperplasia increasing and glycogen deposits increasing.
The muscle cells act as a syncytium, in which spontaneous depolarising pacemaker potentials occur. If these are above a certain threshold (known as the action potential) then a wave of calcium is released and a contraction occurs.
During labour the uterus can be divided into two regions, the upper, muscular region and the lower region which generally acts to provide a passive fibromuscular birth canal.
The cervix consists largely of fibrous collagen in a proteoglycan matrix. It is normally hard, and needs to be softened to allow passage of the baby. The presence of keratan sulfate decreases the amount of collagen, whereas dermatan sulfate increases the amount of collagen.
There are several chemicals which allow the softening of the cervix, including:
- Proinflammatories such as cytokines which induce an inflammatory response
- Nitrous oxide (although not directly responsible it is the product following the catabolism of inhibiting NOS)
Contractions are high amplitude shortening and relaxing of muscles which occur with increasing frequency throughout labour. These contractions are responsible first for dilating the cervix, moving the foetus through the pelvis and eventually expelling the baby. The primary hormone involved is oxytocin, a protein (9 amino acids long) which is released following a nervous response which results from the weight of the baby on the cervix, among other things.
Oxytocin stimulates the decidual layer to release prostaglandins (which in turn enhance the effect of oxytocins). These act on the myometrium to produce a contraction by initiating the release of contraction associated proteins (CAP's) which:
- increasing gap junctions between cells to increase the effectiveness of the synctium and increase the likelihood that an action potential will occur
- increasing ion channels for calcium to increase the effect of an action potential
- increasing oxytocin and prostaglandin receptors to enhance the above two effects
Calcium activates myosin light chain kinase, which in turn induce contractions.
A high estrogen:progesterone ratio is required for contractions, as this results in an increase in the release of prostaglandins (by stimulating the release of phosphorlipase from lysosomes) and increasing the number of oxytocin receptors.
Labour is the clinical process of parturition. A mother tends to become aware of labour with either 2-4 rapid contractions within a 10-minute period or the breaking of the amniotic membranes (the "water breaking").
Although the process is not entirely understood, scientists believe the hormone CRH and CRH binding proteins are responsible for regulating labour, as they induce placental release of estrogen and localised release of prostaglandins. Levels increase exponentially throughout pregnancy, and tend to be higher in mothers who give birth preterm.
Stage One is associated with the start of painful contractions and the dilating of the cervix. Contractions are initially slow until the cervix reaches a diametre of 3 cm, then the increase in speed until the end of stage one, at which point the cervix is at a diametre of approximately 10 cm. Each muscle cell does not completely relax, and therefore shortens with each contraction. The retraction ring gradually moves upwards.
Stage Two is associated with the delivery of the foetus. As the foetus moves down it is rotated due to the dimensions of the pelvis (ellipsoid laterally at the pelvic inlet, circular in the middle and ellipsoid medially at the pelvic outlet). This generally results in the baby's head facing the mother's rectum. Abdominal contractions assist with the expulsion of the child. A breach birth occurs if the baby is delivered feet first.
Stage Three is associated with the delivery of the placenta. Because of the importance of completely removing the entire placenta (abandoned villi can result in maternal haemorrhage) this stage is usually assisted with steady traction on the umbilical cord and doses of oxytocin.
Procedures Associated with Parturition
This section does not cover Cesarean sections
Episiotomy - a surgical incision to assist the passage of the baby
Preterm and Post-Term Parturition
Children born before 35 weeks gestation are known as preterm, and children born after 40 gestational weeks are known as post-term. The former are susceptible to higher rates of mortality and morbidity (due to underdeveloped organs such as the lungs) whereas the latter can damage mothers during birth (due to their increased size).
Risk factors associated with preterm birth include:
- age (mothers younger than 17 and older than 35)
- history of preterm births
- multiple pregnancies
- foetal abnormality
- growth restriction
- male foetus
- maternal factors (drug and alcohol use, smoking, extreme stress, vigorous exercise, low BMI)
Management of preterm labour includes providing corticosteroids (to increase surfacant in the lungs, hence increasing the ease at which the newborn can survive) and relaxin.