Maternal Serum Screening

Maternal serum screening estimates the chance of selected fetal conditions; it does not establish whether a fetus has a chromosomal anomaly or neural-tube defect. A result such as 1:100 means that, among pregnancies with a similar calculated profile, approximately one would be expected to have the condition and 99 would not. It is a higher-risk result than 1:1,000, even though both are written with the same format. In Canada, prenatal screening for the common chromosomal anomalies—trisomy 21 and trisomy 18—and for neural-tube defects should be routinely offered to every pregnant person, regardless of age. The decision to accept screening remains the patient’s. Screening should be paired with an explanation of what a positive or negative result can and cannot answer. A screen-positive result is not an emergency and is not a diagnosis. It means that further counselling and diagnostic options should be offered. A screen-negative result lowers the estimated probability but does not exclude every chromosomal or structural abnormality. The second-trimester anatomy scan remains part of routine prenatal assessment because serum screening cannot evaluate fetal anatomy comprehensively. First-trimester ultrasound is...