A screening report is classified as Screen Positive when a pregnancy was previously affected with Down's syndrome or neural tube defects because screening for Down's syndrome and neural tube defects is not simply based upon risk estimation (see reference). It is also based upon having previously had an affected pregnancy (see reference). Although the absolute risk may still be low, the fact that a woman has had an affected pregnancy is itself, in effect the result of a "positive screening test". The test or question is "Have you had a Down's syndrome or neural tube defect pregnancy?" Yes is positive, and No is negative in this screening enquiry.
The practical reason for treating a previously affected pregnancy in this way is to alert medical staff to the need for specific counselling. Otherwise, the fact could be overlooked and the woman may not receive appropriate counselling, may not have an amniocentesis when she would have wished to have one and there would be the possible view that medical care offered was not what it should have been.
In practice, a woman's risk of having an affected pregnancy can be presented to her in the context of her having had a previously affected pregnancy. She can then make a decision as to whether she wants an amniocentesis. With appropriate explanation the policy should be clear and understood.
A similar approach is used for pregnancies previously affected with pre-eclampsia.