Skip Ribbon Commands
Skip to main content

Methods perinatality

The parameters "residence" and "nationality" refer to the mother.
Natality rate:
The ratio of the number of live births in an area to the population of that area; expressed per 1000 population per year.
Age specific fertility rate:
The ratio of the number of live births during a year to women in a particular age group, usually per 1000 women in an area.
The denominator used to calculate the rates (infant mortality, preterm delivery,...) consists of the number of children born in Belgium in the corresponding years. For stillbirth and perinatal mortality rate the denominator is the sum of all children born alive and stillbirths. For the early neonatal mortality rate, the rates of death after the 7th day (late neonatal and postneonatal) and the infant mortality rate only live births are included in the denominator. The birthweight was not a criterion for including or excluding a child from the denominator. The potential effect of excluding children with a very low birthweight from the denominator was small given the low number of cases.
Confidence Interval (CI):
The 95% CI were calculated on the basis of the observed values. Several methods were applied according to the type of indicator (proportion, mortality rate) and the objectives of the analysis.
The classical formula is based on a Binominal distribution (p = proportion in a geographical area, n = number of births in this area). This method was especially used for the CI of the proportions of preterm delivery and low birthweight, particularly from the comparison between provinces and between regions.
For the analysis by district, and more specifically for the geographical presentation of the data, a method, proposed by Black and MacFarlane (Reference: Black N, MacFarlane A. Methodological kit: monitoring perinatal mortality statistics in a health district. Community Medicine 1982;4:25-33), was applied (P = proportion observed on national level, n = number of births in the district).
 Based on the CIs, the districts are classified in 4 groups to create a map:
    • < CI: proportion in district is statistically significantly smaller than national proportion (p < CI);
    • CI (<= P): proportion in district is smaller, but not statistically different from national proportion (CI <=p <=P);
    • CI (> P): proportion in district is higher, but not statistically different from national proportion (P < p <= CI);
    • > CI: proportion in district is statistically significantly higher than national proportion (CI < p).

CIs for the different mortality rates (per 1000 births) were calculated according to the square root transformation using a Poisson distribution (O = number of cases in an area, number of births in this area).


Box-and-Whisker plot:A Box-and-Whisker plot represents the distribution of the values observed in each district. This figure presents the dispersion between the 25th and the 75th percentile (Interquartile Range (IQR)) by a rectangle. The median (50th percentile) is the horizontal line in the rectangle. Certain values are considered as outside the dispersion, with a distinction between extreme values and outliers. Extreme values and outliers are minimum 3 and 1,5 times the IQR respectively. The vertical lines that start from these limits of the rectangle go to the minimum or maximum value, or in case of outliers, to the observation that is not an outlier.

Grouping of causes of death and morbidity:
The causes of death and the health status of the child at birth are coded according to the ICD9-classification. This classification is available in Links. To simplify the analysis, the causes were classified in 8 groups. This classification happened from an operational point of view: bringing together the causes related to the same problem 

Groups ICD9
Congenital abnormalities 740 - 759
Pregnancy related-perinatal condition 651, 760 - 763, 766, 767, 771 - 778
Prematurity 764, 765
Anoxia - Respiratory Distress Syndrome 768 - 770
SIDS 798
Others 1 - 650, 652 - 739, 800 - 999
Ill defined conditions 779 - 797, 799
Not specified -


The morbidity of the mother during the pregnancy and/or the delivery classified

Groups ICD9t>
Infections 1 - 139, 647, 760.1
General conditions not related to pregnancy 140 - 629, 680 - 759, 760.2 - 760.8
Complications of pregnancy 630 - 641, 643, 645 - 646, 761
Toxemia 642 + 760.0
Premature labor 644
General conditions related to pregnancy 648
Multiple pregnancy 651
Obstetric or post-partum complications 652 - 676, 762 - 778
Ill defined conditions 760.9, 779 - 999
Not specified -