Perinatal mortality

Tables 22.1 and 22.2 summarize some of the epidemio-logical studies that attempt to compare perinatal mortality rates of babies delivered at 42 weeks or over with those delivered between 37 weeks and 41 weeks and 6 days.

The methodological problems are considerable. There may be errors or biases in recording of information relating to gestational age. Women with uncertain dates have been repeatedly shown to be at increased risk of perinatal mortality [16,17]. Their inclusion may inflate the apparent perinatal risks of prolonged pregnancy. Older studies of perinatal outcome in post-term pregnancy showed that about 25% of the excess mortality risk in post-term pregnancy relates to congenital malformations [18]. Of the studies quoted in Tables 22.1 and 22.2 only that by Smith [19] specifies that cases of lethal congenital malformation have been excluded from the analysis. Hilder et al. [20] re-analysed the data presented in their 1998 [10] study after correcting for congenital malformation, showing that the outcomes presented were not biased by fetuses with congenital malformation being preferentially represented among post-term pregnancies. Another potential bias is the interval between intrauterine death and delivery. Afetus that dies in utero at 41 weeks and is delivered at 42 weeks will be counted as a perinatal death at 42 weeks' gestation.

Yudkin et al. [21] questioned the validity of using perinatal mortality rates as a means of relating outcome to gestational age, arguing that the population at risk of intrauterine fetal death at a given gestational age is the population of fetuses in utero at that gestational week and not those delivered at that week. However, the population at risk of intrapartum and neonatal complications such as cord prolapse or meconium aspiration syndrome is clearly the population of babies delivered at that week of pregnancy [19]. These issues are clearly explained by Smith [19], who related the perinatal risks at each gesta-tional week to the appropriate denominators. Antepartum deaths were related to the number of ongoing pregnancies, intrapartum deaths to all births at that gestational age, excluding antepartum stillbirths and neonatal deaths were related to the number of live births. Yudkin et al. [21] expressed the prospective risk of stillbirth for the next fortnight of the pregnancy; Hilder et al. [10] expressed the risk as a rate over the next week; Cotzias et al. [22] generated considerable controversy [20,23], by expressing the risk of

Table 22.1 Perinatal mortality rates term versus post-term pregnancies

Authors

Source

Outcome

37-41 [86]

42 Weeks and over

Campbell et al. [25]

444,241 births Norway 1978-87

Relative risk of perinatal death

1

1.30 (1.13-1.50)

Fabre et al. [86]

547,923 births Spain 1980-92

Stillbirth rate

3.3

3.6

Early neonatal mortality rate

1.7

2.8

Perinatal mortality rate

4.9

6.4

Olesen et al. [87]

78,033 prolonged pregnancies

Adjusted odds ratio - stillbirth

1

1.24 (0.93-1.66)

Danish birth register 1978-93

Adjusted odds ratio - neonatal death

1

1.60 (1.07-2.37)

5% sample of deliveries at term

Adjusted odds ratio - perinatal death

1

1.36 (1.08-1.72)

Table 22.2 Perinatal outcomes by week of gestation, 37-43 weeks

Author

Source

Outcome

38-39

39-40

40-41

41-42

42-43

43 and more

Bakketeig &

157,577 births

Perinatal mortality

7.2

3.1

2.3

2.4

3

4

Bergsjo [11]

Sweden 1977-78

rate

Ingemarsson &

914,702 births

Stillbirth rate in

2.72

1.53

1.23

1.86

2.26

Kallen [24]

Sweden 1982-91

nulliparae

Neonatal mortality

0.62

0.54

0.54

0.9

1.03

rate nulliparae

Stillbirth rate in

2.1

1.42

1.35

1.4

1.51

multiparae

Neonatal mortality

0.55

0.45

0.53

0.5

0.86

rate multiparae

Divon et al. [88]

181,524 singleton

Odds ratio for fetal

1

1.5

1.8

2.9

pregnancies.

death

Reliable dates,

40 weeks or more

Sweden 1987-92

Hilder et al. [10]

171,527 births

Stillbirth rate

3.8

2.2

1.5

1.7

1.9

2.1

London 1989-91

Infant mortality rate

4.7

3.2

2.7

2

4.1

3.7

Stillbirth rate per 1000

0.56

0.57

0.86

1.27

1.55

2.12

OP*

Infant mortality rate

0.7

0.83

1.57

1.48

3.29

3.71

per 1000 OP

Caughey & Musci

Hospital based

fetal death rate per

0.36

0.4

0.26

0.92

3.47

[89]

California 1992-2002

1000 OP

45 673 births after

37 weeks

Smith [19]

700, 878 births in

Cumulative

0.0008

0.0013

0.0022

0.0034

0.0053

0.0115

Scotland

probability of

1985-96

antepartum

Stillbirth

Multiple births and

Estimated probability

0.0006

0.0005

0.0006

0.0006

0.0006

0.0008

congenital

of intrapartum &

anomalies excluded

neonatal death

* Ongoing pregnancies prospective stillbirth for the remainder of the pregnancy. This is a counter-intuitive concept for most obstetricians, whereas many find the concept of the prospective risk of stillbirth over the coming week an accessible concept, particularly in pregnancies of 40-42 weeks' gestation. ('If this woman remains undelivered in the next seven days, what is the chance of a fetal death occurring in uteroT).

The outcomes presented in Table 22.1 compare pregnancies fulfilling the epidemiological definition of prolonged pregnancy with those delivered at 'term'. In modern obstetric practice, women with epidemiological and obstetric risk factors are more likely to be delivered before 42 weeks. Women with twin pregnancies, pre-eclampsia, diagnosed intrauterine growth restriction, antepartum haemorrhage, previous perinatal death are likely to be over-represented in the 37-41 week population and under-represented among those delivered at 42 weeks and later.

Table 22.2 addresses the argument that the duration of pregnancy is a continuum and that perinatal risks are unlikely to alter abruptly on day 294 of a pregnancy. Outcomes are presented by week of gestational age from 37 weeks up to and including 43 weeks' gestation. Outcome statistics are presented in a variety of forms as discussed above.

Both tables show that prolonged pregnancy is associated with an increased risk of perinatal death. However, there is no consistency between studies as to the timing of that increased risk from fetal death before labour, to antepartum death to early neonatal death or even infant mortality. The studies summarized in Table 22.1 suggest that an increased risk of neonatal death is the main source of the increased perinatal risk. However, Table 22.2 shows that when pregnancies ending at 42 weeks are compared with those delivered at 41 weeks, every adverse outcome is increased with the exception of the 'estimated probability of intrapartum and neonatal death' from the Smith study [19]. When pregnancies ending at 41 weeks are compared with those ending at 40 weeks, this outcome is again unchanged, as is the neonatal mortality rate in multiparae in the Ingemarsson and Kallen [24] series and the infant mortality rate in the Hilder [10] series. All other outcomes deteriorate from 40 weeks to 41 weeks and again from 41 weeks to 42 weeks.

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