Can you tell me what it mean: a neural tube defect or pre-eclampsia, because screening does not detect all affected preg?

Neural Tube Defects : are malformations of the fetal spinal cord, brain & vertebrae that occur the 3rd-8th week of pregnancy. The most common NTD is Spina Bifida, a split of the vertebral arches; 90% of cases are diagnosed on fetal ultrasound before 18 weeks' gestation. Pre-eclampsia, with maternal high blood pressure, edema, protein in the urine, & more, usually starts after 20 weeks & can harm mother & fetus.