Psychological/social aspects of prolonged pregnancy

over 4000 or 4500 g, that is, macrosomic babies, rises with prolonged pregnancy (7).

Large babies tend to have longer labours, and because the pla­cental reserve tends to fall at later gestational ages they tend to be less able to withstand the stress of labour.

Factors associated with increased risks from prolonged pregnancy

If the mother has other complications of pregnancy such as diabetes, hypertension, pre-eclampsia, intrahepatic cholestasis of pregnancy, or fetal growth restriction then the risks associated with prolonged pregnancy are increased. Induction of labour prior to 40 weeks' ges­tation is commonplace for conditions associated with an increased risk of late antepartum stillbirth.

Fetal growth restriction

Babies weighing less than the tenth centile are at increased risk from prolonged pregnancy than those on or above the tenth centile. In a large, relatively old cohort study of over 400,000 babies comparing term with post-term babies prior to the introduction of a policy of routine induction of labour for prolonged pregnancy, the authors found that of the post-term babies, those less than the tenth centile had an adjusted relative risk of perinatal mortality of 5.68 compared to babies on or above the tenth centile (8).

Diabetes

The association between pregestational diabetes and stillbirth is widely known (9). The perinatal mortality rate for women with pre­existing diabetes (type 1 or 2) is 32 per 1000, compared with 9 per 1000 in the general population. In the United Kingdom, the National Institute for Health and Care Excellence (NICE) recommends that pregnant women with diabetes with a normally grown baby should be offered elective delivery, either by induction of labour or cae­sarean section if appropriate, from 38 weeks (10).

Past obstetric history

Women who have experienced a stillbirth in their first pregnancy have an increased risk of stillbirth in their second pregnancy (still­birth rate 16 per 1000 total births) (11).

Parental preferences

Parental and family views about prolonged pregnancy vary widely. Many people have a strong preference to avoid intervention and are content to allow pregnancy to continue for a long period after the due date. Others place relatively little value on avoiding intervention and become fed up waiting for the birth even in the weeks leading up to the due date. Such opinions are often influenced by the previous experiences of other family members or close friends.

In a prospective questionnaire study of 400 women with uncom­plicated pregnancies performed in 1991, completed at 37 weeks and 41 weeks, the authors found that 45% of women at 37 weeks had a preference for conservative management (with serial antenatal monitoring) versus induction of labour if undelivered at 41 com­pleted weeks, falling to 31% of women expressing a preference for conservative management at 41 weeks (13). In a more recent study of 508 women performed in Sweden at 41 weeks, 74% of women preferred to be induced than have serial antenatal monitoring (14). It seems reasonable to conclude therefore that a significant minority of women have a preference for conservative management of pro­longed pregnancy.