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Pregnancy Bliss | Reproductive Health Hub

Causes of prolonged pregnancy
What causes the pregnancy to be prolonged?
This is unknown. In a very small proportion of affected mothers, some factors have been identified which could explain the phenomenon. These may include some forms of fetal abnormalities (easily ruled out by an ultrasound scan), certain placental enzyme deficiencies and low thyroid hormone levels.

These, however, constitute an exceedingly small proportion of those women with prolonged pregnancy. Of more significance is a group of women where dates may have been wrong from the start, where no verification of the gestation was made in the first half of the pregnancy.

It is by no means rare to find a pregnancy over­-estimated by up to four weeks, usually the error discovered at the routine 18-20 weeks ultrasound scan. If such a pregnancy slips through the net at that stage of the pregnancy, it is practically impossible to correct the error in late pregnancy. At a supposedly 40 weeks visit, when she will actually be 36 weeks, she will be expected to deliver within two weeks. If this does not happen,, she will be categorized as post-term. Lesser errors of one to two weeks are even more common.

Surely an ultrasound scan performed in late pregnancy should be able to correct the error?
Unfortunately not. At about 28 weeks and beyond, the boat has been well and truly missed. The ultrasound scan is too inaccurate at dating pregnancy at this late stage.
This is not due to the weakness of the technology itself. It is rather the nature of pregnancy. Babies will grow to quite different sizes because of a variety of factors. These differences are usually quite marked in the third trimester. Because ultrasound dating is based on measuring various parts of the baby, any attempt to date a pregnancy for the first time at this stage is futile.

What factors will move a pregnancy from the category of low-risk?
There are various factors that may make the pregnancy regarded as no longer being low-risk. These may include:
Unsatisfactory growth rate
Reduced amniotic fluid around the baby
Vaginal bleeding in pregnancy (antepartum haemorrhage)
Hypertensive disorders, including pre-eclampsia .
This is why proper antenatal care is important, in order to detect factors that may increase risk to a pregnancy and to have them dealt with in good time.

Apart from recognised risk factors, what could prompt intervention before 42 completed weeks?
The expectant mother's wishes. The obstetrician will try to allay unfounded worries as much as possible. This is not always possible and if a woman is beside herself with worry, however baseless her fears might be, there is little justification to deny her induction of labour, just to prove a point.

It is, however, important that she is made aware of a small but definite increase in the risk of caesarean section associated with induction of labour, especially if associated with an unfavourable cervix.

Fetal Well-being: Next Page
Expected date Term After 42 weeks Predisposing factors Fetal wellbeing Labour in postmaturity