Every pregnant woman hopes for a healthy baby. Indeed, the vast majority of babies are healthy at birth. However all women, whatever their age, have a small risk of having a baby with impaired physical or developmental function. In some cases, this is caused by a fetal chromosomal abnormality.
In recent years, powerful new screening techniques have been developed to help identify pregnant women who may be at an increased risk of having a baby with a chromosomal problem. One such screening test is called First Trimester Screening. This screen uses a series of markers to determine a woman’s unique risk of having a baby affected with the most common genetic disorders that affect pregnancies. These include Down’s Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13).
First Trimester Screening must be completed between 10 and 14 weeks of pregnancy. It is a two part screening test.
The first part involves obtaining a maternal blood sample. There are two markers in the maternal blood (PAPP-A and free BhCG) whose values can sometimes be abnormal in the presence of a fetal chromosomal problem.
The second part of the screen involves performing an ultrasound exam on the baby to look for two additional markers.
The first of these markers is called the nuchal translucency. This is a small pocket of fluid that is present at the back of the baby’s neck. A significantly enlarged pocket of fluid at this gestational age can sometimes be predicative of a genetic problem. The final marker is the presence or absence of the baby’s nasal bone. An absent nasal bone at this point in the pregnancy can also indicate a higher risk.
Once both parts of this test have been completed, the results will be combined, together with other factors such as the woman’s age, to generate a risk assessment for her current pregnancy.
It is important to remember that, as with all genetic screening tests, this test is not designed to diagnose a chromosomal problem. Rather, it will provide an assessment of risk that will be much more accurate than determining risk based on age alone. If the screen result is positive (ie: the risk is determined to be higher than average), then the next step would be to perform a diagnostic test in the form of genetic amniocentesis. This would definitively determine the presence or absence of a chromosomal disorder.
To ensure quality assurance, the First Trimester Screen must be performed by individuals certified in this specialized ultrasound procedure.
A unique feature of First Trimester Screening is its ability to provide a genetic screen result prior to14 weeks of pregnancy (typically within a day or two of completing the screen). The other available screening options typically provide a final result closer to 18 weeks of pregnancy.
At present, First Trimester Screening is not funded by the Medical Services Plan. It is offered through the Early Pregnancy Assessment Unit in Victoria.