How is first trimester screening done, and how do I understand my risk result?

[Anonymous]

How is first trimester screening done, and how do I understand my risk result?

[Genetic Counselor]

First trimester screening combines three pieces of information. This is why it is often called the “combined test”. In India, the blood test part is commonly called the “double marker” test.

• The nuchal translucency scans an ultrasound scan measures a small pocket of fluid at the back of the baby’s neck, called the nuchal translucency. All babies have some fluid here. Babies with chromosome conditions tend to have more. The scan must be done when the baby measures between 45 and 84 millimetres from head to bottom, which is roughly 11 weeks to 13 weeks and 6 days. It needs a trained and certified sonologist because a difference of a fraction of a millimetre changes the result. The scan also checks the baby’s heartbeat, confirms the due date, counts the babies, and looks at early body structure.
• The blood test A sample of the mother’s blood is checked for two pregnancy hormones: free beta human chorionic gonadotropin and pregnancy-associated plasma protein A. In Down syndrome, the first is often higher than usual and the second is often lower.
• The mother’s age and details Age, weight, number of babies, and some other factors are added in.

A computer program combines all three to give a single number for each condition, such as “1-in-2,500” or “1-in-150”. This means that out of 2,500 (or 150) pregnancies with the same result, 1 baby would be expected to have the condition. Each laboratory uses a cut-off, often around 1-in-250 or 1-in-300. A number with a smaller chance than the cut-off is reported as “low risk”. A number with a higher chance is reported as “high risk” or “increased chance”.

Combined screening finds about 82 to 87 of every 100 babies with Down syndrome, while flagging about 5 in every 100 pregnancies overall as high risk. A low-risk result is reassuring, but it does not rule out a chromosome condition completely.

[Author]

Posts