Along with the rapid biomedical development of prenatal screening tests, target groups' attitudes and decision-making about, and the acceptance of, screening procedures have come into focus. To understand users' decision-making, it is essential to understand users' knowledge and perceptions of a procedure. The aim of this study was to examine Finnish women's knowledge and perceptions of, and stated reasons to participate in, two prenatal screening tests: serum screening and mid-trimester ultrasound screening. Subjects (n=1035) for the serum screening survey were catered for in the maternity care centres of two Finnish towns, where serum screening is available for all pregnant women. After one reminder, 88 per cent returned the questionnaire. Subjects (n=497) for the mid-trimester ultrasound screening survey were catered for in the obstetrical and gynaecological outpatient clinic of the city hospital of another town; the response rate was 85 per cent. Women's perceptions of the studied prenatal screening tests, serum screening and mid-trimester ultrasound screening, differed significantly, even though both are used to detect fetal malformations. Serum screening was far more often perceived to be connected with finding diseases or abnormalities than ultrasound screening. Another interesting finding was that the stated reasons for screening in general and the subjective reasons for participation were different. Reassurance was the personal reason most often mentioned in both the serum screening and the ultrasound group. Almost all women had the most superficial knowledge about serum screening; they knew whether it had been offered and that it is done to screen for Down syndrome. The greatest gaps in knowledge concerned the sensitivity of serum screening, its use in screening for congenital nephrosis, and diagnostic tests and their risks. Knowledge was poorer among women without a high school education. When counselling women about prenatal screening tests, more emphasis should be given to the sensitivity of serum screening, all of its screening uses, and the possible diagnostic tests and their risks. The fact that ultrasound screening can detect conditions which may lead to the possibility of a selective abortion should also be explained more fully.