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First Trimester Pregnancy Screenings
Choosing or Refusing First Trimester Screenings
By Katherine Bontrager
One of the newer tests is referred to as a first trimester screening and is comprised of a biochemical and an ultrasound component.
"The ultrasound is performed between 11 and 14 weeks and measures a region of the fetus referred to as the nuchal translucency," says Weinblatt. "This is located at the base of the fetal head and neck. The measurement is designed to measure the space between the fetal skin and muscle. This area is filled with fluid, but if the fluid accumulates in excess, it is associated with an increased risk for Down syndrome and trisomy 18. Trisomy 18 is caused by an extra chromosome 18 (Down syndrome is an extra chromosome 21) and is associated with severe mental retardation and multiple fetal problems including heart defects, growth delay and extremely shortened life span."
The ultrasound is only the first step, though, and is paired with a biochemical test that measures the amount of free Beta HCG (human chorionic gonadotropin) and PAPP-A (pregnancy-associated plasma protein type A).
"Elevated levels of free Beta HCG and low levels of PAPP-A are associated with Down syndrome, while low levels of both proteins are associated with trisomy 18," says Weinblatt, who also works at Pennsylvania's Arcadia University as an adjunct professor for a graduate-level class that explores prenatal counseling and testing in high-risk patients.
What follows is a mathematical equation that consists of the parameters derived from the testing combined with the mother's maternal age. The end result is a fraction, or in other words, the risk to the pregnancy.
"A person whose age-related risk for Down syndrome might be one in 1000, could have first trimester screening done, and the new predicted risk could be one in 50, or conversely, one in 10,000," Weinblatt says. "The test will detect (depending on the lab) 80 percent to 90 percent of pregnancies with Down syndrome as well as trisomy 18, but will also leave about 5 percent of patients with false positive results, meaning 5 percent or so of patients with normal pregnancies will have screens which indicate an increased risk, even though their pregnancies are normal."
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