First Trimester Genetic Screening

During your first trimester of pregnancy, you may be offered a genetic screening test.  A screening test is used to show if a patient’s risk is high or low for a specific disease or condition.  If the risk is high, then further diagnostic testing can be done. The most common screening tests for pregnant women estimate the risk of the baby having either Down syndrome or spina bifida.

A blood test and an ultrasound are used for these screening tests.  Human choriaonic gonadotrophin (hCG) and pregnancy associated plasma protein PAPP-A are measured using a blood test.  Women are more likely to have a baby with Down syndrome if hCG is high and PAPP-A is low.

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An ultrasound is also used to indicate the risk of Down syndrome.  The nuchal translucency, or the amount of space between the spine and the baby’s neck, is measured.  During my screening, the tech indicated that they like to see a space of less than 3mm at my gestational age (12w 5d).

Both the blood test and ultrasound results are used in conjunction with other risk factors like the mother’s age to determine the risk.  The combined accuracy rate for the screen to detect the chromosomal abnormalities mentioned above is approximately 85% with a false positive rate of 5%.  If the risk is higher than normal, many providers will arrange for you to meet with a genetic counselor to talk about what these results mean and decide if further diagnostic testing is right for you and your family.

It is important to remember that Ultrasound technology, while useful for medical screening when necessary, should not be over used.  According to the American Pregnancy Association, “The ultrasound is a noninvasive procedure that, when used properly, has not demonstrated fetal harm. The long term effects of repeated ultrasound exposures on the fetus are not fully known.”

Genetic screenings are not required, and you can of course decide that it isn’t right for you.  It is important to make an informed decision before consenting to any testing or procedure during pregnancy.  For more information about genetic screening tests visit the American Pregnancy Association page.

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