Aetiology of prolonged pregnancy

It is likely that the majority of prolonged pregnancies represent the upper range of a normal distribution. The effects of anencephaly and of placental sulphatase deficiency on the duration are interesting examples of extreme post-term pregnancy but are unhelpful in elucidating the aetiology of the majority of cases of prolonged pregnancy.

There may be a genetic factor regulating the onset of labour. Prolonged pregnancy tends to repeat itself. Analysis of data from the Norwegian Birth Registry showed that a woman delivering post-term in her first pregnancy had a relative risk of a second post-term pregnancy of 2.2 and a woman with two post-term pregnancies had a 3.2-fold relative risk of a third post-term pregnancy [11]. Swedish

Medical Birth Registry data confirm these results and also indicate a tendency for daughters of mothers who deliver post-term to have prolonged pregnancies [12] but overall these factors account for only a small proportion of the overall population attributable risk for post-term pregnancy. Male fetuses may be associated with a higher risk of prolonged pregnancy.

Low vaginal levels of fetal fibronectin at 39 weeks are predictive of an increased likelihood of post-term pregnancy [13]. Ramanathan et al. [14] showed how transvaginal measurement of cervical length at 37 weeks predicts both prolonged pregnancy and failed induction. These observations suggest that a defect or delay in the remodelling of the cervix that takes place prior to successful initiation of labour, may cause prolonged pregnancy and may also be associated with some of the apparent increase in dystocia associated with prolonged pregnancy.

Prolonged pregnancy could result from variations in the corticotrophin releasing hormone (CRH) system during pregnancy, such as an alteration in the number or expression of myometrial receptor subtypes, altered signal-transduction mechanisms or increase in the capacity of CRH binding hormoneprotein to bind and inactivate CRH. Prospective, longitudinal studies have shown that women destined to deliver preterm tend to have a more rapid exponential rise in CRH in mid-pregnancy while women who go on to deliver post-term babies have a slower rate of rise [15]. Efforts currently directed towards researching the initiation of labour preterm may lead to greater understanding of the aetiology of post-term pregnancy.

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Pregnancy And Childbirth

Pregnancy And Childbirth

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