Since 1967, all births in Norway have been registered in the Medical Birth Registry of Norway (MBRN). The registry includes data about perinatal mortality, i.e. stillbirths (before and during delivery) and deaths during the first week of life. The statistical calculations are based on births (alive or dead) with gestational age at least 22 completed weeks and birth weight at least 500 grams.
Country comparisons show that perinatal mortality decreases with improving living conditions. Perinatal and infant mortality are indirect public health indicators and give a picture of the national level concerning women's health, maternity care and obstetrics.
Perinatal mortality in Norway since 1967
From 1967 to 1981, perinatal mortality was almost halved in Norway, from 23 to 12 babies per 1000 births. Mortality fell both for preterm and full-term births.
Figure 1: Perinatal mortality per 1000 births since 1967, shown here for newborns of 500 grams or more. Diagram: Norhealth. Source of data: Medical Birth Registry of Norway.
The decline in perinatal mortality has continued. Perinatal mortality in Norway is now so low that we cannot expect further large reductions.
Causes of perinatal death
The largest group of perinatal deaths is stillbirths, i.e. babies who die before or during childbirth. Almost two-thirds of perinatal deaths are in this group. The proportion has increased from over half in 1967 to over three-quarters in 2013. The remaining perinatal deaths occur during the first week after birth.
Many of the babies who die have congenital malformations or other conditions that are incompatible with life.
Other contributing factors include various forms of placental dysfunction, placental abruption and preterm delivery. Factors such as maternal disease and obesity, smoking habits, maternal age, complications during delivery and multiple births are also significant.
Gestational length and perinatal death
A normal pregnancy lasts for 40 weeks when calculated from the first day of the last menstrual period. Even though the majority of the smallest preterm children survive, many do not. Children born at 22 weeks have little chance of survival, mainly because their lungs are immature. At week 22, a foetus will weigh about 500 grams.
Figure 2a Risk of perinatal death by pregnancy week for babies born in the same pregnancy week
Among babies born at week 29, 1 in 10 will die perinatally (2013 figures). Only 9 out of 1000 babies born at week 36 will die (8.7 per 1000), and among the full-term babies, born at weeks 39, 40 and 41, only about 1 in 1000 will die perinatally.
These mortality rates are illustrated in Figure 2a, where we see the risk of perinatal death by week of pregnancy per 1000 babies born in that week.
However, for a more accurate illustration of the risk of perinatal death during pregnancy, this risk should not be calculated only among those children born in a specific week of pregnancy. Figure 2b shows the number of deaths among the entire "risk population" of ongoing pregnancies, i.e. among those children born in a particular week of pregnancy, as well as all ongoing pregnancies (the “fetus-at risk” approach).
Figure 2b Deaths per 1000 babies born in the same week plus ongoing pregnancies (total risk population)
While we see that the first way to calculate risk gives a very high risk during early pregnancy and the lowest risk among children born at full-term, we see that the second calculation method gives the lowest risk of perinatal death in early pregnancy with an increased risk nearer the due date.
The highest risk is found when a pregnancy has lasted more than 42 weeks. However, the highest risk found when using the “fetus at risk” approach is still only 4.8 per 1000, from week 43 (figure 2b, based on based on data from 1999 to 2013). The highest risk when calculating mortality only among children born in a given week is almost 100 per cent (96.2 per cent in week 22, see figure 2a). Note that the scale on the vertical axis is different in the two figures (figure 2a: from 0 to 1000, figure 2b: from 0 to 5).
Pre-eclampsia is a condition often associated with placental dysfunction, resulting in intrauterine growth restriction.
Total perinatal mortality is higher when the mother has pre-eclampsia than when she does not. Excess mortality dropped dramatically during the first 20 years after the MBRN was established, from 1967 to the end of the 1980s, see figure 3.
Figure 3. Excess mortality at birth with pre-eclampsia, by birth year (5 year groups) 1973 - 2003. Excess mortality has declined, particularly since 1988. Data source: Medical Birth Registry of Norway.
The excess perinatal mortality stabilised at approximately 40 per cent. Over the last five years there has been a slight increase in mortality, which remains when we adjust for maternal age and parity.
Some of the decline in excess perinatal mortality associated with pre-eclampsia may be due to improved monitoring of women with this pregnancy complication and from inducing births in women with more severe pre-eclampsia. This is shown by the increase in induced births from 35 per cent (1967-73) to 64 per cent (2009-13) in women with pre-eclampsia, while the increase was from 8 per cent to 15 per cent for women without pre-eclampsia over the same time period.
Maternal age and perinatal death
Low and high maternal age are associated with an increased risk of perinatal death. The perinatal mortality rate is lowest when the mother is between 20 and 35 years. This has been the case since the Medical Birth Registry was established in 1967, and it remains today.
Figure 4 below shows the relationship between maternal age and perinatal mortality among singletons born during three periods between 1979 and 2013. Although perinatal mortality has declined in all maternal age groups over the time periods, the difference between the age groups within each time period is quite consistent.
The increasing age of women giving birth today may lead to a new increase in the overall perinatal mortality in Norway.
Maternal age must also be seen in the context of assisted reproduction. Since the age of women trying to become pregnant is rising, there is an increased demand for in vitro fertilisation or other assisted reproduction technologies.
Figure 4: Perinatal mortality related to maternal age over three periods. Mortality is greatest at low and high maternal age in all three time periods. Data source: Medical Birth Registry of Norway
Maternal lifestyle: obesity and smoking
Conditions such as daily smoking and obesity also affect the risk of losing a child perinatally. The MBRN has registered maternal smoking habits since 1999 if the mother does not object, and since 2006, together with maternal height and weight.
The figures from the registry show that women who are daily smokers at the end of pregnancy have a 50 per cent increased risk of losing the child perinatally compared with non-smokers. This increased risk is unchanged when adjusting for age, which should be done since more younger than older women smoke daily.
In Norway, daily smoking during pregnancy has decreased considerably over the years since smoking habits have been registered. Over 18 per cent of women said they smoked daily by the end of pregnancy in 1999, while this proportion had fallen to 4.5 per cent in 2013 (among women where smoking information exists).
Data about smoking habits at the end of pregnancy are lacking for about 20 per cent of women. It appears that the group of women without smoking information have an equally high risk of perinatal death as daily smokers. Among women without smoking information there is a 60 per cent increased risk, and among daily smokers there is a 50 per cent increased risk. These figures are not statistically different.
The risk of perinatal death also increases if the mother is overweight, and the risk increases with the degree of obesity. Compared with women who had normal weight before pregnancy, overweight women have a 30 per cent increased risk of perinatal death, while for women with obesity, the risk was doubled. Overweight in this context is defined as a body mass index (BMI) of 25 - 29 kg / m2. Obesity means BMI> 30.
The women who were significantly underweight also had an increased risk, see Figure 5.
Figure 5: Perinatal mortality (per 1000 singleton births) related to maternal body mass index (BMI) before pregnancy. (Kroppsmasseindeks = body mass index)
More stillbirths among immigrant women
Non-Western immigrant women have an increased risk of stillbirth and they experience a failure of antenatal care more often than western women. This is shown in a review of stillbirths in Oslo and Akershus in the period 1998-2003. Of 356 stillbirths in this period, 110 were for non-Western women. Failure of pregnancy and maternity care is often caused by linguistic and cultural barriers.
In general, some stillbirths could be prevented if all cases of reduced foetal growth in pregnancy were detected. Particularly for non-Western immigrant women, maternity care can be improved to detect all the cases where the foetus has problems, or if labour is not progressing well. (Saastad E, Vangen S, Frøen JF).
According to a WHO study, female genital mutilation in women increases the risk of birth complications and perinatal mortality. The risk that a child would die in connection with birth was 15-55 per cent higher if the woman was circumcised, with the highest risk with grade III mutilation. It is estimated that per 1000 births in African hospitals, 10-20 children will die because of female genital mutilation. The study included over 28 000 births at hospitals in six African countries. There are no figures for home births (WHO, 2006)
The World Health Organization has statistics of perinatal deaths in the Health For All database, such as the map below.
References and links
- Medical Birth Registry of Norway statistics bank
- Norhealth - find birth statistics from 1967, see left menu, births/abortions
- Saastad E et al. Suboptimal care in stillbirths - a retrospective audit study. Acta Obstetrica & Gynecologica 2007; 86:222-50.
- WHO Study Group, 2006. Female genital mutilation and obstetric outcome and Lancet 2006; 367: 1835-41.
About the article
Text: Medical Birth Registry of Norway. Figure 1 updated 19.12.2017