Introduction

Clinical use of amniocentesis (AC) and fetal chromosome analysis was introduced in Denmark in the early seventies. Up to 1978, the number of prenatal diagnoses (PND) carried out was limited and AC only offered to selected high-risk groups of pregnant women.

In 1978, the Danish legislation made resources available in order to increase the capacity of the genetic departments offering PND. During the following years, AC and PND became accepted preventive procedures in the Danish population and today more than 10% of our pregnancies are monitored by PND.

After the first investigation on chorionic samples by Hahnemann and Mohr [1] in 1968, chorionic villus sampling (CVS) was introduced for clinical use in Denmark in 1983 and has now become a popular alternative to AC among Danish women. Approximately half of all prenatal analyses are now carried out by CVS.

Over the years, many techniques and procedures associated with PND have been introduced in Denmark as in the other European countries. The use of DNA techniques in the diagnosis of monogenic diseases and fluorescent in situ hybridisation (FISH) as a supplement to cytogenetic analysis has been important. Improved ultrasound equipment is used for fetal heart scanning and for guiding cordocentesis and filter AC [2].

This paper provides an overview of the present state of PND in Denmark with special emphasis on prenatal cytogenetic analysis.

National Organisation

Prenatal chromosome analysis in Denmark is carried out by 5 genetic departments: (1) Department of Clinical Genetics, Rigshospitalet, Copenhagen; (2) The John F. Kennedy Institute, Glostrup; (3) Department of Clinical Genetics, Vejle County Hospital; (4) Department of Clinical Genetics, University Hospital of Aarhus, and (5) Department of Clinical Biochemistry and Genetics, Odense. Prenatal metabolic or DNA analyses are performed at 4 laboratories, analysis of biochemical markers in maternal serum at local laboratories or the State Serum Institute, and genetic counselling at 6 genetic departments.

All 5 genetic institutes are public. Each institute is servicing a certain part of the 16 Danish counties corresponding to certain geographical regions. All prenatal cytogenetic diagnoses are recorded in the Danish Central Cytogenetic Register (DCCR). AC is performed at 17 and CVS at 12 obstetrical/ultrasound departments throughout the country. Very few samples were taken in private clinics (2-4 per year), and they were all analysed at the public genetic institutes. Screening by the triple test or ultrasound scan is offered in one county. The expenses related to PND are covered by the National Health Service. The number of live births per year in Denmark is given in table 1.

Table 1 Live births in Denmark 1990–1993
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Sources of Information

The sources of information on PND in Denmark are: Danish Central Cytogenetic Register, which registers sampling technique (AC or CVS), prenatal chromosome diagnoses, maternal age, indication, terminations, county, hospital, and genetic laboratory. Danish Register of Congenital Malformations worked until 1995; the information is since then included in the Danish Birth Register. During 2 years (1994–1996), a working group has investigated the possibilities for a Danish Register of Genetic Diseases, but finally concluded that it is at present not possible. Local registers and research registers exist at e.g. Department of Medical Genetics, Copenhagen University (muscular dystrophy), Department of Ophthalmology, Copenhagen (hereditary eye diseases), and Register on hereditary tumour syndromes. A complete survey was attempted 1995–1996 without success.

The EUCROMIC register at the John F. Kennedy Institute contains detailed information on ambiguous prenatal diagnoses (mosaicism and discordance) after diagnostic CVS, AC, chordocentesis, coelocentesis, and/or fetal cells in maternal blood. By 1996, it included information from all Danish cytogenetic centres.

A yearly report from the Danish Cytogenetic Central Register makes it easy to follow the activity of PND from year to year, and it is possible to obtain information about individual cases to be used for genetic counselling. Data for 1994 and 1995 are not yet available. The register has also proved useful for a large number of research projects [e.g. 3, 4]. There has been little interest among the genetic institutes to record DNA and metabolic analyses for monogenic diseases in the register. Consequently, it has not been possible to include this activity.

The data presented below are all obtained from the register and illustrate the kind of information that is recorded.

Figure 1 gives an overview of the development of PND based on AC and CVS from 1980 to 1993. The fraction of analyses based on CVS is rapidly increasing and has probably reached 50% by now.

Fig. 1.
figure 1

Development of prenatal cytogenetics in Denmark 1980–1993 (DCCR).

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Figure 2 shows that the fraction of pregnancies that are monitored by PND has settled around 13%. Note the increase of deliveries since 1983.

Fig. 2.
figure 2

Pregnancies monitored by prenatal cytogenetics 1980–1993 (DCCR)

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Figure 3 shows that the fraction of women > 35 years who accepted PND has declined from 76% in 1983 to 65% in 1993. From the yearly reports of the register it can be seen that the decrease has taken place mainly in the larger urban areas, which in the past showed great interest in PND; the fraction of women having PND is now more or less the same all over the country.

Fig. 3.
figure 3

Pregnant women ≥ 35 years monitored by prenatal cytogenetics 1980–1993.

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The number of deliveries by women of 35 years of age or more has doubled from 1980 to 1993. This increase of deliveries among older women is significantly larger as compared to the smaller increase for all women as seen in figure 2. Whether this increase has something to do with the possibility of having PND or not is unknown. Another explanation could be that an increasing fraction of women are employed outside home and for this reason decide to get children later in life.

Impact of Prenatal Diagnosis

Tables 2 and 3 show indications, the number of analyses, and number of induced abortions because of abnormal findings at AC and CVS, respectively, for the period 1980–1993. Advanced maternal age is clearly the largest group for PND by CVS or AC. Also the group with chromosomal disease or mental retardation in the close family and the group having the test for psychological reasons are large. The fact that the frequency of induced abortions because of abnormal findings is the same in these last two groups is probably the reason that numerical chromosome aberration in the close family is no longer regarded as a risk group requiring invasive PND. The guideline for this group is now only to offer the triple test (table 1). 1.4 and 2.4% of all AC and CVS, respectively, result in induced abortions because of abnormal findings.

Table 2 AC in Denmark 1980–1993 (DCCR)
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Table 3 CVS in Denmark 1983–1993 (DCCR)
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Table 4 shows the number of different chromosome abnormalities detected prenatally after AC and CVS from 1980 to 1993 and the corresponding number of induced abortions. Autosomal trisomies constitute about 55% and unexpected structural aberrations about 8% of prenatally detected chromosome aberrations [5].

Table 4 Induced abortions related to chromosome abnormalities detected after prenatal examination in Denmark 1980–1993 (DCCR)
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Almost all cases of autosomal trisomies, polyploidies and unbalanced translocations result in induced abortion. Most of those who do not have an induced abortion in these groups experienced a spontaneous abortion before the result of the PND was available. There have been no cases of prenatally detected trisomy 21, where the women/couples decided to continue the pregnancy.

The picture is quite different for the sex chromosome aberrations, where 60–70% choose interruption of pregnancy.

Figure 4 illustrates the impact of PND on the number of children born with trisomy 21 [6]. It is assumed that all liveborn children with Down syndrome (DS) are karyotyped within the first year after birth. It is noticed that over the years about 40% of the trisomy 21 cases are detected prenatally. Increasing use of CVS raised expectations towards an increased fraction of prenatally detected trisomy 21 cases (tables 2 and 3). On the other hand, the declining fraction of women ≥ 35 years having PND will influence the figure in the opposite direction (fig. 3). The triple test is not widely used in Denmark and may only slightly influence the results.

Fig. 4.
figure 4

Fraction of trisomy 21 fetuses detected prenatally 1980–1993 (DCCR).

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The changes in severe malformations in liveborns or stillbirths are listed in table 5.

Table 5 Severe malformations in livebirths or stillbirths in Denmark
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Diagnostic and Screening Procedures Available

The recommended indications for PND and choice of method have been updated in 1994 by the National Health Service (table 6). They should only be regarded as guidelines, and it is stressed in the comments that genetic counselling associated with PND should allow the women/couples to make their own choice of procedure according to their own wishes and to the choices they are given.

Table 6 Indications and choice of procedure (according to Danish Board of Health)
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Sampling Procedures

CVS was previously performed in the 7th to 12th week [7–10], but is now usually carried out in the 10th to 12th week of gestation [11]. Second-trimester CVS is used to achieve a quicker result as compared to AC or because of oligohydramnios [12]. As for AC, both standard and high-resolution banding techniques were developed [13, 14]. Lately, 2nd-trimester CVS or amniocentesis has been used for obtaining a result within 24 h by using interphase FISH [15].

Amniocentesis is usually performed at the 15th to 16th week of gestation, however, a new technique, filter AC, has been introduced where amniotic fluid is aspirated and reinjected after passage through a filter which keeps the amniotic cells [2]. This technique is performed at the 12th week of gestation and the result of karyotyping is available within 10–12 days. Second-trimester CVS or AC in week 13 are not routine procedures in Denmark.

The National Board of Health recommended, 1994, that AC should be performed by professionals with a prior education of 100 samples and subsequent routine of 200 samples per year. For CVS the recommended number of samples was given as 250 (education) and 400 (routine).

Cordocentesis is used on special indications and is normally carried out at about the 20th week.

Screening Procedures

Ultrasound type I scanning is available in all 29 obstetrical departments; 16 of these offer routine ultrasound examinations in early pregnancy and in 7 of these, the fetus is routinely screened for malformations (type II scan). The most advanced type III scanning with Doppler technique, e.g. for congenital heart defects is only available at the larger university hospitals.

Serum AFP screening for neural tube defects combined with type II ultrasound scanning is offered routinely in a few hospitals.

Triple test combined with type II ultrasound scanning is currently offered routinely in one of the counties.

Current Methods in use

Fetal karyotyping using various conventional banding techniques (Q, G, R, C, AgNOR) is routinely performed in all cases of PND including monogenic diseases. Special techniques were developed for the fragile X syndrome [16–19]. Recently FISH has been introduced to solve complicated structural rearrangements and to diagnose microdeletion syndromes [20–23]. Interphase FISH for chromosomes 13, 18, 21, X, and Y is used in special cases, where a fast result is required [15]. Comparative genomic hybridisation (CGH) has also been used in attempts to clarify complicated structural rearrangements [24].

A large number of inborn errors of metabolism are currently diagnosed by enzymological metabolic detection procedures [25–27] and other monogenic diseases are diagnosed by various DNA techniques [28–30]. The current list of about 150 routine analyses is continuously updated. The Cu deficiency syndrome, Menkes’ disease, is diagnosed by analysis of the Cu content in chorionic villi or amniocytes [31–35].

All amniotic fluid samples are routinely analysed for AFP and most of them are also analysed for AChE.

Areas under Development

A large controlled study comparing interphase FISH with conventional CVS and AC, respectively, has recently been conducted in Denmark. Probes for chromosomes 13, 18, 21, X, and Y were used and the results are promising. It is not yet clear whether the FISH interphase technique will be used as an adjunct to conventional cytogenetics to achieve a quick result (1–2 days) or whether the technique will be used as an alternative, relatively cheap, and fast stand-alone technique.

Much developmental work in the prenatal genetic institutes is devoted to metaphase FISH as to diagnose an increasing number of microdeletion syndromes and to disclose complicated structural rearrangements which in the past have been difficult or even impossible to diagnose with conventional banding techniques [36]. As mentioned above, even CGH has proved to be usable both for detecting aneuploidies and for clarifying complicated structural rearrangements [24]. It is still not clear whether the method will be available for routine purposes.

At Rigshospitalet in Copenhagen, attempts to detect fetal nucleated erythroblasts in maternal blood have been going on for some years, but a definitive breakthrough has not been made yet.

As the Human Genome Project progresses, an increasing number of monogenic diseases can be diagnosed prenatally by various kinds of DNA technology. Techniques for pre-implantation diagnosis are also investigated.

Genetic screening for common monogenic diseases like cystic fibrosis is under consideration.

First-trimester biochemical screening and ultrasound screening for trisomy 21 is gradually coming up.

The diagnostic and clinical problems after chromosomal mosaicism, which is encountered both after AC [37] and CVS [38] are investigated in a European multicentre study funded by the EU Commission (EUCROMIC, BMH1-CT93-1673).

Current Legislation

Termination of pregnancy is self-determined until 12 weeks. If severe disease is detected later in pregnancy, termination is accepted after approval by a local committee. There is not yet any legislation defining severe disease.

Pre-implantation diagnosis is not allowed at present. However, a new law is under debate in the Danish parliament.

Problems to Face in the Future

In the recent instructions from the National Board of Health on PND in Denmark, the indications have been tightened. The consequence is limited access to prenatal diagnosis for women with DS in the close family and for women who want the test because they are afraid to get a DS child. One may fear that further limitations may be established.

During the years there has been intense debate between supporters of invasive PND and supporters of biochemical screening for DS (triple test). The supporters of the invasive procedure fear that the National Board of Health may change its policy and reduce the access to CVS and AC for women > 35 years and offer this group the triple test instead. Most cytogeneticists feel that this would be a step backwards.

In the past, certain groups, e.g. The Ethical Council, attempted to introduce rules for specifying which prenatal detection is allowed and consequently, for which diseases termination of pregnancy would be acceptable. We feel, that such limitations would be most unfortunate for pregnant women and would mean that they can no longer make their own decisions.