The average duration of human pregnancy is about 38 weeks, which is known as the gestation period. Nearing the end of pregnancy, vigorous contraction of uterine muscles causes expulsion of the fetus, or delivery of the baby. The process of delivery of the fetus (childbirth) is called parturition. Since it is regarding birth, we decided to start of the journey with a discussion on the process and gave birth to ScienceBits.
Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully-developed fetus and the placenta to cause mild uterine contractions. This signal is sent to the posterior pituitary of the mother through the servous system and triggers the release of Oxytocin. This is the fetal ejection reflex. Facilitated by estrogen, oxytocin binds to the receptors on myometrium of the uterus and results in stronger and stronger contractions. This leads to the expulsion of the baby out of the uterus through the birth canal. Soon after the infant is delivered, the placenta is also expelled out of the uterus.
The Three Stages of Parturition
I. Dilation Stage
The uterine contractions start from top and occur after long intervals (once every thirty minutes). This forces the fetus outward, pushing its head against the cervix. As a result, the cervix gets dilated with vagina also showing similar dilation. This increases the stimulus to secrete oxytocin, further increasing the strength and frequency of contractions (one to three every minute). With continued powerful contractions, the amnion which acted as a shock absorber, ruptures and amniotic fluid flows out through vagina. This is when water is said to be broke.
II. Expulsion Stage
With further increase in the intensity of uterine and abdominal contractions, the fetus comes out through cervix and vagina, with head coming out first. If the fetus comes out feet or buttock first, the birth is termed as a ‘breech birth‘. Expulsion may take twenty minutes to over an hour. The umbilical cord is cut and major changes take place in the physiology of the baby. Blood flow through the umbilical cord, foramen ovale and ductus arteriosus ceases and adult pattern of blood flow through the heart, aorta and pulmonary arteries begins. Ligamentum arteriosum and fossa ovalis are the remnants of ductus arteriosus and foramen ovale respectively. This is supported by nitric oxide (NO). The infant’s lungs expand and it begins breathing. The cry of the baby signifies the transition of the circulatory system.
III. Stage of After-birth
Within quarter an hour of delivery, the placenta and remains of the umbilical cord are expelled out. The time period of up to six weeks after this event, is the postnatal period in which the hormone levels and uterus of the mother regains its normalcy and the infant adjusts to life outside the womb.
In case of twin pregnancy, high blood pressure and other complications, a Cesarean delivery is undertaken. In a typical Cesarean delivery, a surgical incision is made in the maternal lower abdomen. A second is made to expose the uterus and the baby is taken out of the uterus. The incisions are then closed and the process of childbirth is complete.
Childbirth is truly an important event in the life of a mother and is also termed sometimes as the ‘rebirth of the mother‘.
Image courtesy: Evolving Sciences.