Post-term Pregnancy
(Postmaturity)Definition:
It means the prolongation of pregnancy beyond its normal duration. Term, by definition, is 37-42week`gestation. A very small proportion (˂5%) of pregnancies would proceed beyond 42weeks.Post-term pregnancy is a problem because at this gestation, the baby is at its maximum size, and the placenta is becoming more calcified, less efficient and more prone to failure.
Diagnosis:
In cases in which the date of the last menstrual period is absolutely certain, and in which the previous menstrual cycles were of normal length, and when this has been confirmed by an early ultrasound scan, the diagnosis of maturity can reasonably be made.If the menstrual history is uncertain an attempt can be made to assess maturity by checking the antenatal records of the uterine size between the 8th and the 14th weeks.
U/S measurements of the crown-rump length of the fetus up to about the 14th week and of the biparietal diameter of the fetal head up to about the 28th week, give a reliable indication of the duration of gestation.
Other tests for the assessment of maturity, including the date of first felt fetal movements.
There are no known tests that can predict fetal outcome post-term [an ultrasound ( if the amniotic fluid and fetal growth are normal) and CTG performed at and after 42weeks].
Clinical significance:
The delayed onset of labour might have disadvantages:After term, as the uterine blood flow diminishes and degenerative changes progress, the placenta and placental bed function may become inadequate for a large fetus that is still growing.
There may be mechanical difficulties during labour because of the increased size of the fetus.
A uterus which is slow to begin labour may also prove to be inefficient during labour.
The risk of fetal distress and fetal death during labour is greater in postmature than in normal cases (due to more difficult labour because of the larger size of the fetus and incoordinate uterine action, and due to the more frequent occurrence of oligohydramnios).
The skull of the postmature fetus is more ossified, so that moulding is less easy.
A particular hazard for the new born baby is meconium aspiration.
The risk of hypoxia in postmature cases is increased if there is also hypertension, and perhaps in the case of a relatively older mother.
Management:
Immediate induction of labour or delivery postdates should take place if:
There is reduced amniotic fluid on scan.
Fetal growth is reduced.
There are reduced fetal movements.
The CTG is not perfect.
The mother is hypertensive or suffers a significant medical condition.
So, when counseling the parents regarding waiting for labour to start naturally after 42weeks, it is important that the woman is aware that no test can guarantee the safety of her baby, and that perinatal mortality is increased (2-fold) beyond 42weeks.
The cervix should always be checked for ripeness (length, softness, position and dilatation) before a decision is made to induce labour.
Labour may be induced by insertion of vaginal prostaglandin pessaries or gel.
If labour does not quickly follow, amniotomy is performed and intravenous syntocin given.
When the fetal head is well down in the pelvic cavity and the cervix is soft and taken up, labour follows induction without delay.
Very careful monitoring of the progress of the labour and of the condition of the fetus is required.
Particular attention is paid to whether or not amniotic fluid is draining, and to thick meconium.
If there is cardiotocographic evidence of fetal hypoxia during the first stage of labour, preferably confirmed by examination of a sample of fetal blood obtained by scalp puncture, Caesarean section may be necessary.
In the second stage clinical signs of fetal distress call for forceps delivery without delay.
Note: A labour induced post-term is more likely to require Caesarean section; this may partly be due to the reluctance of the uterus to contract properly, and the possible compromise of the fetus (abnormal CTG).
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