What is a Birth Canal?
The muscular canal connects the uterus to the rest of the body. The baby passes through the birth canal during delivery which is also often known as a vaginal canal or born canal. The uterus, ovaries, fallopian tubes, cervix, and vagina are all the parts that contribute to the birth canal and are the primary organs of the female reproductive system. The act of bringing a child forth from the uterus, or womb, through the birth canal is known as birth, also known as childbirth or parturition. The mechanism and sequence of changes that occur in a woman's organs and tissues as a result of the growing foetus are constantly evolving. The gestation period of the labour has three important stages: the dilation stage, expulsion stage and placental stage. It is in the second stage which is the expulsion where there are various fetal presentations and passage through the birth canal. And in this article, we will discuss all the possible fetal head positions in the birth canal once the stage of expulsion terminates.
Different Fetal Presentation and Passage Through the Birth Canal
The location of the foetus and the form of the mother's pelvis determine how the child moves through the birth canal during the second stage of labour. When the mother's pelvis is of the normal form the following sequence is more common wherein the child is lying with the top of its head lowermost and transversely positioned and the back of its head (occiput) oriented toward the left side of the mother. As a result, the top of the head is in front, and its long axis is transverse.
[Image will be uploaded soon]
Entry of the Fetal Head Into the Birth Canal
The pressure exerted on the child's buttocks by uterine contractions and bearing-down efforts is transmitted along the vertebral column, driving the head into and through the pelvis. Because the spine is attached to the base of the skull, the rear of the head progresses faster than the brow, causing the head to flex until the chin comes to rest and lie against the breastbone. As a result of this flexing mechanism, the top of the head becomes the leading pole, and the ovoid head circumference that entered the birth canal is replaced by a smaller, virtually round circumference with a long diameter of about 2 cm (0.75 inches) smaller than the previous circumference.
As the head moves farther into the delivery canal, the birth canal becomes more narrow. The bony pelvis and the sling-like pelvic floor, or diaphragm, which slopes downward, forward, and inward, provide resistance. When the back of the child's head, which is the child's leading section, is driven against the slanting wall on the left side, it is naturally pushed forward and to the right, as it moves forward. This internal rotation of the head aligns its largest diameter with the largest diameter of the pelvic outflow, substantially aiding the adaptation of the advancing head to the shape of the cavity through which it must pass. As soon as the head is delivered, the neck, which was twisted during internal rotation, untwists. As a result, the top of the head is tilted to the left and backwards almost soon after birth.
Baby Rotation During Birth in the Birth Canal
With a curled spine, head down, and arms and legs are drawn tight to the body, the foetal position resembles a C-shape. While the kid will stretch, kick, and move around a lot in the womb, here is usually where they will spend most of their time. The most pleasant posture for a newborn in the womb and after delivery is the foetal position. This position becomes more crucial as the due date approaches because it helps the baby move into the ideal position for birth and reduces the risk of difficulties during delivery. The foetal position in the uterus usually indicates the baby is hanging upside down. Babies have an uncanny ability to sense when the delivery is approaching. Most newborns are able to arrange themselves into a head-down position by 36 weeks, allowing them to escape as quickly as possible. Others determine that they are perfectly content and have no intention of leaving.
Fetal Head Positions and Movements of the Birth Canal
Your doctor will check on your baby's location in the womb in the weeks and days running up to your due date. Your doctor may employ a few strategies to gently move your baby into the ideal position for birth if it hasn't already done so. External cephalic version (ECV) is the medical term for the procedure of coaxing a baby into place inside the womb- because the coaxing comes largely from the outside of the stomach, it's called external. Turning the baby cephalic actually means turning it into the head-down position by gently massaging it. And Version in ECV is just another term for turning the baby.
ECV is Only Done When These Following Conditions Hold-
When there is only one baby in the womb and not in the case of twins, triplets or more.
If one is scheduled for delivery in the hospital ECV can be applied because in an emergency case they can be immediately sent for C-section.
When the mother is between 36 and 42 weeks along.
Labor has not yet begun.
The water hasn't broken yet, but one is in labour.
When the baby is surrounded by a large amount of amniotic fluid.
When the infant isn't contacting the birth canal's opening and no part of their body is in the birth canal.
Stomach muscles are understandably less tight when one has previously had a baby and in the case of a first-time mother, ECV will not probably be successful or will be a delayed process due to the tight muscles.
During the External Cephalic Version Procedure Following Steps are Followed-
To locate the exact location of the fetal head a fetal ultrasound is done and then the nudging process to shift the position if not ideal starts.
An injection is given that relaxes the womb.
The doctor massages by pressing on the stomach using both hands for an evenness.
To keep you comfortable, one may be given a numbing (epidural) medicine.
The vital signs, such as heart rate and blood pressure, are thoroughly monitored.
To make it easier for your baby to move, your doctor will make sure there is enough fluid around them.
During the procedure, the provider will employ baby heart monitoring to ensure that your baby's heart rate is normal.
Birth Canal Issues
The following issues can delay the delivery of the baby and that can lead to damaging consequences.
Failure to progress- The fetal head does not move ahead that prolongs the delivery which can be fatal. When this happens in the later active stages during pregnancy it is mostly due to cervical dilations that are sluggish, caused by emotional issues such as worry, stress, and fear of sluggish effacement of a huge baby, a small birth canal or pelvis delivery of multiple babies.
Fetal distress- this occurs when oxygen levels are insufficient, an anaemic condition in the mother, hypertension in the mother as a result of pregnancy, intrauterine growth retardation (IGR) is a condition in which a baby (IUGR) and Amniotic fluid with a meconium stain.
Perinatal asphyxia - “Failure to initiate and sustain breathing at birth” is what perinatal asphyxia is characterised as. It can happen before, during, or right after delivery as a result of a lack of oxygen. It's an amorphous term. A reliable source that deals with a wide range of issues. It leads to hypoxemia, or low oxygen levels, high levels of carbon dioxide and acidosis, or too much acid in the blood.
Shoulder dystocia- When the head is delivered vaginally but the shoulders stay inside the mother, this is known as shoulder dystocia. It is uncommon, but it is more common in women who have never given birth before, accounting for half of all caesarean deliveries in this category.
Excessive bleeding- During the pain in the pelvic or back when accompanied by bleeding the birth canal is at a risk.
Malposition- This occurs when the baby's neck is hyperextended in this face presentation. The baby's bottom is first in a breech presentation. The infant is curled against the mother's pelvis in a shoulder presentation. All the aforementioned reasons can be called the malposition of the fetus and the birth canal becomes incapable of delivering normally and surgery needs to be performed even after the baby does not return to a normal position after ECV.
Placenta previa- When a baby's placenta partially or completely covers the mother's cervix — the uterus's outflow is known as placenta previa. During pregnancy and delivery, placenta previa can cause serious bleeding. One may bleed throughout pregnancy and during delivery if you have placenta previa which can put the mother’s life in extreme danger.
Cephalopelvic disproportion- Cephalopelvic disproportion (CPD) is a pregnancy problem in which the mother's pelvis and the baby’s head are not the same sizes. The baby's head is proportionately too large, or the mother's pelvis is too tiny, for the infant to pass through the pelvic opening comfortably.
Other Ways to Induce Labor
Get moving. Movement may help start labour.
Having Sex is often recommended for getting labour started.
Try to relax in any way that suits the mother.
Having something spicy.
Scheduling an acupuncture session can also prove to be effective as it is believed it will release oxytocin that favours birthing.
Going herbal has been gaining momentum and many get to share their success stories through this method. Inducing labour with red raspberry leaf tea is a popular practice. The tea is said to tonify the uterus. Evening primrose oil is another herbal remedy. It is unlikely to initiate labour contractions, but it may soften the cervix.
Conclusion
As the due date approaches, the location of your baby in the womb becomes increasingly critical. A few weeks before delivery, most newborns achieve the optimum foetal posture. The obstetrician may try to shift the baby into the head down cephalic position if they haven't moved into it by week 36. One may need a caesarean delivery if you're in an unusual foetal position, such as breech, face-to-mama's-belly, sideways, or diagonal. Other foetal positions can make it more difficult for the baby to glide out and raise the risk of difficulties during labour and delivery. While one has no control over the baby's position while in the womb it is very crucial to have a healthy pregnancy for a birth canal delivery and even the slight error and recklessness can lead to a huge loss that can be draining physically, emotionally and mentally on the mother.
FAQs on Birth Canal
1. What is the birth canal, as per the Class 12 Biology syllabus?
The birth canal is the passageway in the female reproductive system through which a baby passes during natural childbirth (parturition). It is not a single, distinct organ but is formed by the combination of two structures: the fully dilated uterine cervix and the vagina, which together create a continuous channel from the uterus to the outside.
2. Which two parts of the female reproductive system combine to form the birth canal?
The birth canal is a composite channel formed by two main structures working in unison during labour. These are:
- The Cervix: The lower, narrow part of the uterus that opens into the vagina. During labour, it thins out (effaces) and widens (dilates) to about 10 cm.
- The Vagina: A muscular, elastic tube that connects the cervix to the exterior of the body. It stretches significantly to allow the baby's passage.
3. What is the significance of 'engagement' in relation to the birth canal before childbirth?
Engagement, also known as 'lightening', is a key event in late pregnancy where the foetus's presenting part, usually the head, descends into the mother's pelvic brim. This movement positions the baby at the top of the birth canal, signifying that the body is preparing for labour and is a primary indicator that childbirth is approaching.
4. How do hormones like relaxin and oxytocin prepare the birth canal for parturition?
Hormones are crucial for preparing the birth canal for the strenuous process of childbirth. Relaxin, secreted by the placenta, softens the ligaments of the pubic symphysis, increasing the flexibility and diameter of the pelvic outlet. Oxytocin, released from the pituitary gland, stimulates powerful uterine contractions and also promotes the release of prostaglandins, which help to soften and dilate the cervix, the gateway to the birth canal.
5. What is the most common and ideal foetal position for a smooth passage through the birth canal?
The most common and ideal foetal position for childbirth is the cephalic presentation, specifically the occiput anterior (OA) position. This means the baby is positioned head-down, facing the mother's back, with its chin tucked to its chest. This orientation presents the smallest diameter of the baby's head to the birth canal, facilitating an easier and safer delivery.
6. How does a foetus navigate and rotate through the curved path of the birth canal?
A foetus navigates the birth canal through a series of passive movements known as the mechanisms of labour. Because the pelvic inlet and outlet have different shapes, the foetus must rotate to pass through. Typically, the head enters the pelvis in a transverse (sideways) position, rotates to an anterior position to pass the mid-pelvis, and then extends its neck as it emerges from under the pubic bone. These rotations align the widest part of the baby with the widest part of the maternal pelvis at each stage.
7. Why is the bony structure of the maternal pelvis so critical for the function of the birth canal?
The maternal pelvis forms the rigid, unyielding boundary of the birth canal. Its specific size and shape are critical because they determine if there is enough space for the foetal head to pass through. A condition known as cephalopelvic disproportion (CPD), where the baby's head is too large for the mother's pelvis, can obstruct labour and necessitate a Caesarean section, highlighting the pelvis's crucial role as the framework for the birth canal.
8. What are some potential complications that can arise for the baby during its passage through the birth canal?
While natural birth is a normal physiological process, several complications can occur within the birth canal, including:
- Shoulder Dystocia: An obstetric emergency where the baby's head is delivered, but one of the shoulders becomes stuck behind the mother's pubic bone.
- Umbilical Cord Prolapse: The umbilical cord drops into the cervix ahead of the baby, leading to cord compression that can cut off the baby's oxygen supply.
- Foetal Distress: The baby may experience stress, indicated by an abnormal heart rate, due to prolonged labour or pressure within the canal.
9. Can a Caesarean section be performed if the baby has already entered the birth canal?
Yes, a Caesarean section can be performed even after the baby has descended into the birth canal, although it is a more complex procedure. This is known as a 'second-stage C-section'. It is typically performed as an emergency intervention due to 'failure to progress' in labour, sudden foetal distress, or an unexpected obstruction that makes a vaginal delivery unsafe for the mother or baby.
