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Norsk Epidemiologi 2007; 17 (2): 103-105 103
The Medical Birth Registry of Norway –
An international perspective
Allen J. Wilcox
Epidemiology Branch, National Institute of Environmental Health Sciences, PO Box 12233, Durham, NC 27709, USA
Telephone: +1-919-541-4660 Telefax: +1-919-541-2511 E-mail: wilcox@niehs.nih.gov
ABSTRACT
Some of the most practical questions of perinatal medicine are regarding couples who have had pregnancy
problems in the past, and their risk of having such problems in future pregnancies. For example, if a couple
has a child with a birth defect, what are their chances that their next child will have a defect? The key to ans-
wering such questions is the availability of linked data such as those provided by the Medical Birth Registry
of Norway. Such linked data provide a unique resource for addressing a broad range of questions in perinatal
epidemiology. The Medical Birth Registry of Norway has been a pioneer in answering such questions.
If a mother has a stillborn baby, what are her chances
of having a healthy baby at her next pregnancy? If a
father has a birth defect, is his own child likely to have
the same birth defect? Is his child at risk for other
kinds of birth defects?
These are concrete and practical concerns of pa-
rents. Even though most pregnancies produce healthy
babies, at least a third of all couples have one or more
pregnancies with problems. These problems can range
from miscarriage to preterm delivery to an offspring
with a malformation. It is natural for couples to have
some worries when they become pregnant. Couples
who have had difficulty in a past pregnancy are likely
to be even more concerned.
What assurances can researchers provide? The
questions are simple, but the answers have been surpri-
singly difficult to generate. Data from whole popula-
tions are the gold standard for answering such ques-
tions. It is only relatively recently that epidemiologists
have had population-based data on the recurrence of
pregnancy problems. But population-based data by
themselves are still not enough.
THE IMPORTANCE OF LINKED DATA
The key to addressing this problem is the availability
of linked data. By “linked”, epidemiologists mean a
birth record that can be connected to other births from
the same woman, or to other records for the same
baby. To appreciate the importance of linked data, we
should first consider unlinked birth data. Much of what
we know about infant mortality has come from birth
certificates collected as part of vital statistics. Most
countries have laws that require collection of vital
statistics, including legal records of births and deaths.
These birth certificates typically exist in isolation,
without being linkable to other deliveries by the same
woman, or to later health problems occurring to that
baby.
Without linkage, it is difficult or impossible to ans-
wer the kinds of questions raised above. For example,
to estimate the chances of a future healthy pregnancy
among mothers who have had a stillbirth, we would
need records from a large number of mothers who
have had a stillbirth, and from a comparison group of
mothers who have had a healthy pregnancy. Then for
each mother, past records would have to be linked with
subsequent pregnancies, so that the risk at later pregn-
ancy could be calculated for the two groups of women.
Such linkage is rare. There may be legal barriers
for reasons of confidentiality. If linkage is permissible,
it may not be feasible. Birth certificates routinely
contain parents’ names but no personal identification
numbers. Many records can of course be linked by
mother’s name alone, but there are many more that
cannot: common names are shared by different mot-
hers, or women change their names, or women move
from one registration district to another. The Scandina-
vian countries are among the few places in the world
in which all births from a particular mother and father
can be systematically linked. Norway was the first to
do so.
The Medical Birth Registry of Norway has been a
pioneer among the linked registries, providing linkages
through the unique personal identification number
assigned to every person at birth. This resource has al-
lowed Norway to become a world leader in providing
information to parents about the risk of problems in
future pregnancies.
THE USES OF LINKED DATA
This capacity to link birth records for a whole nation
helped bring the Medical Birth Registry of Norway to
international attention. Within ten years of the start of
the Registry in 1967, enough women had delivered
two or more pregnancies for Norwegian researchers to
begin to analyze these linked pregnancies. The first
104 A.J. WILCOX
scientific report based on linkage was published in
1977 by Leiv Bakketeig, showing that mothers who
had one baby born preterm or low birth weight were at
increased risk of having another.
1
This was the beginning of a stream of scientific pa-
pers that have described risk in future pregnancies for
couples who have had a poor outcome. As pregnancies
accumulated in the Birth Registry, researchers were
able to describe risks with more specific outcomes. In
1984, Lorenz Irgens and his colleagues published data
showing that the relative risk of recurrence for sudden
infant death syndrome (or cot death) was 3.7, much
lower than the ten-fold increase estimated previously
by studies based on more limited samples.
2
In 1994,
Rolv Terje Lie and his colleagues looked at risk of
birth defects among couples whose first baby had a
birth defect.
3
On average, such couples had eight times
the background risk of having the same birth defect in
their second child. However, this risk was very small
in absolute terms – only a few percent of second ba-
bies were affected with the same birth defect. Further-
more, the couples’ risk of having a baby with any
other type of defect was not much different than for
other couples. Thus, among couples who had one
affected child, the vast majority – around 95% – could
expect their next baby to be free of any recognized
malformations at birth.
LINKAGE BETWEEN REGISTRIES
Researchers soon recognized that they could link
births from the Medical Birth Registry with later
health outcomes recorded in other Registries. Thus, in
1985, Gayle Windham and her colleagues explored
whether babies with birth defects are at higher risk for
childhood cancers (they are not).
4
Researchers have
also been able to consider whether a woman’s preg-
nancies affects her own risk of later disease. Lars
Vatten and his colleagues showed that a woman with a
preeclamptic pregnancy has a lower risk of breast
cancer than other women, for reasons not yet under-
stood.
5
One of the most influential papers in this regard
was by Henrik Irgens and his colleagues.
6
These inves-
tigators assessed the later cardiovascular mortality of
women who had had a preeclamptic pregnancy. They
found that the risk of mortality from cardiovascular
disease was increased eight-fold among women who
had had a premature baby from a preeclamptic preg-
nancy. This finding has generated interest in the link
between preeclampsia and heart disease, and suggests
that preeclampsia may be an early expression of a
woman’s risk of heart disease.
TWO-GENERATION STUDIES
By the mid-1990s, yet another opportunity began to
unfold: persons born into the Registry were becoming
old enough to have children of their own. This allowed
a new type of study that assessed familial risk across
generations. In two widely-cited papers,
7,8
Norwegian
investigators described the chances that babies with a
birth defect would grow up to have offspring with
birth defects. Once again, the results were reassuring.
While the affected parents had an increased risk of the
same defect in their own baby, this risk was very small
in absolute terms. Overall, 95% of the babies born to
affected fathers or affected mothers had no birth defect
recorded in the Registry.
SURVEILLANCE
One of the original purposes of the Medical Birth
Registry was to establish a tool for surveillance, a tool
that could be used to identify newly emerging risks.
Surveillance tools are useful because they can address
questions that were not even imagined at the outset.
For example, the Chernobyl accident in 1986 exposed
Norwegians to radiation through direct airborne con-
tact and through contamination of local foods. There
was understandable concern about possible health ef-
fects including birth defects. An analysis of the rate of
birth defects before and after the accident showed no
increase in any of the types of birth defects associated
with radiation exposure.
9
Such assurance would have
been impossible if the Registry had not been establis-
hed and in full operation before the accident occurred.
THE INTERNATIONAL IMPACT OF THE
MEDICAL BIRTH REGISTRY OF NORWAY
In 1995, the US Centers for Disease Control and the
US National Institutes of Health organized an interna-
tional symposium on maternally-linked pregnancy out-
comes. Along with the excellent linked Registries of
Sweden and Denmark, the Medical Birth Registry of
Norway was one of the centerpieces of that sympo-
sium.
10
Ten years later, in 2005, a second international
symposium was held, and once again, Norwegian re-
searchers played a prominent role.
11,12
Over the past 40 years – and especially in the past
20 years – the Medical Birth Registry of Norway has
made major scientific contributions to medicine and
public health. During this time, more than 20 papers
have been published in the leading weekly clinical
journals – the most influential journals in the field of
medicine and public health. Thirty papers have been
published in the international specialty clinical
journals in pediatrics, obstetrics and other fields, and
nearly 80 papers have appeared in major epidemiology
journals. This prodigious scientific productivity is a
credit to the researchers of Norway, and most espe-
cially to the founders of the Medical Birth Registry.
The founders had faith that the seeds they planted
would contribute to the health of the Norwegian
people. What the founders could not have foreseen
was the benefits that the Registry would provide to the
world at large.
THE MEDICAL BIRTH REGISTRY OF NORWAY – AN INTERNATIONAL PERSPECTIVE 105
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