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Screening for Down syndrome during the first trimester

Screening for Down syndrome is based on an ultrasound scan and blood samples from the mother. The screening allows for detection of Down syndrome in a large proportion of foetuses with the condition as early as during the first trimester. Participation in the screening and the possible further examinations is voluntary.

The purpose of the screening is to find the expectant mothers with a higher probability of giving birth to a child with Down syndrome and to steer them to further examinations. This probability is calculated based on the mother's age, the measurement of nuchal translucency determined based on an ultrasound and the content of markers determined from the mother's blood samples. The final probability is calculated by a computer programme, which has been developed for this purpose. A positive screening test indicates that the foetus in question has a higher probability of having Down syndrome.

In five percent of pregnancies, the screening test is positive, i.e. the foetus has a higher probability of having Down syndrome. This five percent groups includes over 80% of all those pregnancies, where the foetus does have Down syndrome. It is good to note that in most pregnancies with a positive screening test, the foetus does not have Down syndrome.

The current method is better than screening based on the mother's age alone, and it can therefore better find those pregnancies with higher probability of the foetus having Down syndrome. The mothers found with the help of this screening can be steered towards further examinations, if they so wish. Further examinations include examining the foetus' cells by taking samples of the placenta or the amniotic fluid. If the foetus is diagnosed with Down syndrome, the family can either prepare for the birth of a child with special needs or decide to terminate the pregnancy, which is possible up until the 24th week of pregnancy, according to Finnish law.

Screening through clinic referral

Screening for a higher risk pregnancy is implemented through a referral from the maternity clinic at the outpatient maternity clinic of the delivery hospital. The invitation to the ultrasound scan is sent from the hospital. A blood sample will be taken during the pregnancy weeks 10-11+6, after which an ultrasound is carried out between weeks 11+0 and 13+6. This ultrasound determines the size of the foetus as well as the measurement of nuchal translucency. After this, a computer program will calculate the probability. The results are available about one week after the ultrasound. The blood sample result may be available in omaKanta before this, but the final results cannot be based on the blood sample alone.

The mother is informed of a negative screening test, i.e. low probability of Down syndrome, by a letter. This means that the probability of Down syndrome in said pregnancy is lower than the age-specific probability of a 35-year-old mother (risk score is lower than 1: 250).

About 95% of mothers receive a negative screening result. However, a negative screening result is never a guarantee that the foetus is fully healthy.

A positive screening test, i.e. a higher probability, means that the pregnancy in question has a higher than 1: 250 probability of Down syndrome and that further examinations for determining the chromosomes of the foetus are possible, if the family so wishes. The family is informed of a positive screening result by phone or by letter from the Fetomaternal Medical Center (FMC) of the Women's Hospital. In connection with this, matters related to the further examinations will also be clarified and an appointment made, if necessary.

The only sure way to diagnose Down syndrome is by examining the cells of the foetus, which can be accessed either from the placenta or through an amniotic fluid sample. Both methods carry a slight risk of miscarriage (below 1%).


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