By Dr.M.D.Mazumdar, MD
The part of the fetus that occupies the lower part of the uterus and presents itself at the cervical opening is called the presentation of the fetus.
In 96.5% of cases, it is the head (called 'cephalic presentation'), in 3 % it is the buttocks (called 'podalic or breech presentation').
In the other 0.5 %, it may be the arm, shoulder or back.
When more than one part of the fetus is at the lower part of the uterus, it is called a compound presentation - for example, arm and shoulder, hand and head etc.
The term 'lie' refers to the relationship of the fetal spine to that of the maternal spine. The longitudinal lie, where the fetal spine lies parallel to the maternal spine, is the commonest. The lie is termed a transverse lie when the fetal spine lies at a right angle to the maternal spine. It is an oblique lie when it is at an oblique angle to the maternal spine.
Very often, especially in women who have had many children and whose uterine muscles are somewhat lax, the lie may be unstable. This means that the lie of the fetus changes throughout pregnancy and early labor. It becomes stable and fixed only later on in labor when the uterine contractions become more intense. At this time, the lie may become longitudinal, transverse or oblique.
The relation of the different parts of the fetus to one another is termed the attitude of the fetus. Usually, the fetus lies in an attitude of flexion - the arms are flexed and folded in front of the chest, the head is flexed towards the chest, the legs are flexed in front of the abdomen. Sometimes though, the head may become extended and bend backwards or the legs may become extended at the knee and lie straight in front of the fetal body with the feet touching the face.
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There is usually some alteration in the shape of the fetal head as well as a reduction in its size to some extent due to the resistance it encounters during its passage in the birth canal. In normal labor, a reduction of skull diameter of upto 4 mm may occur. Moulding is physiological and harmless, and disappears within a few hours after delivery of the baby.
The fetal head is said to be engaged when the largest diameter of the head has passed the smallest diameter of the maternal pelvis. It can be diagnosed both by abdominal as well as by vaginal examination.
In the late second stage of labor, the head shows at the vaginal opening at every uterine contraction, but retracts back into the vagina when the uterus relaxes. Crowning is said to have occured when the head shows at the vaginal opening continously, without retracting into the vagina even when the uterus relaxes. It indicates that delivery is going to occur within a few minutes.
Once crowning has occured, an assessment needs to be made whether an episiotomy is required. An episiotomy is essentially a small incision made on the edge of the vaginal opening to make it wide enough to deliver the head without injury. If the opening is too small for the head and an episiotomy is not done, there may be severe lacerations and tears along the vaginal walls as well as on the edge when the head is delivered with force.
Most women in their first pregnancies need to have an episiotomy since the skin and muscles near the vaginal opening tend to be quite taut. But in women who have delivered earlier, an episiotomy may not be necessary.
Many women experience pains at later stages of pregnancy that may be mistaken for labor pains. The distinguishing features are that these pains are non-progressing and non-rhythmical, there is no dilatation of the cervix, there is no show (or expulsion of the mucus plug) and the pain may be relieved by medicines.
These are painless, intermittent, irregular uterine contractions that occur throughout pregnancy. They help to make the uterus more firm and well-defined. In early pregnancy, the pregnant woman is usually unaware of them, but in later pregnancy, when they become more prominent, they may be mistaken for the onset of labour.
During pregnancy, the cervical opening is filled up with thick mucus secreted by the cervix. This mucus serves as a barrier against infections travelling up from the vagina. This is called the mucus plug. When labor starts, the mucus plug is expelled with profuse mucus secretion. At the same time, when the cervix begins to dilate, small capillaries at the surface of the cervix ruptures and bleeds.
Some amount of bleeding also occurs from the lower part of the uterus when the amniotic membrane begins to separate from the uterine walls. The expulsion of the cervical mucus plug together with blood is termed 'show'.
Effacement, also called 'taking up', is the progressive merging of the cervix with the lower part of the uterus.
Normally, in a non-pregnant uterus, the cervical canal is about 1 inch long. As the cervix is pulled upwards by the contracting uterine muscles in labor, this length begins to decrease.
When the length of the cervix has become half that of its normal length, it is called 50% effaced. When it is fully effaced, it is called 100% effaced. In women in their first pregnancy, effacement can occur before dilation of the cervix begins.
But in women who have had children previously, effacement and dilation can occur simultaneously.
Dilation, like effacement, mainly occurs due to pulling of the cervix by the contracting and retracting uterine muscles.
Dilation is very useful for clinical diagnosis of the start of labor as well as to measure the progress of labor. Less than 4 cm dilation of the cervix means that the patient is in the latent phase of labor.
When the cervix has dilated more than 4 cm, the patient is deemed to have entered the active phase of labor. At full dilation, the cervical opening measures 10 cm - large enough for the widest diameter of a normally positioned fetal head to pass through.
It is an area of localized edema or collection of fluid on the fetal scalp that develops during labor. It occurs due to the pressure from the cervix - the cervical rim preventing the venous blood and lymphatic fluid from flowing normally. It usually disappears within twenty-four hours after delivery.
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