Episode 8 Aneuploidy Screening

This page contains information transcribed directly from the MOGCAST podcast.  You can listen to the episode on Spotify.

"Hi, everyone. I am Dr. Aekta Neel. I am a consultant obstetrician and gynaecologist who works primarily at Western Health in the Joan Kirner Women's and Children's and also at the Royal Women's Hospital. And today, I am here to talk to you briefly about aneuploidy screening.

So, firstly, what is aneuploidy screening? So, in Australia, we offer all pregnant women screening tests for the common chromosomal abnormalities that are detected in the pregnancy. The main one is trisomy 21 or Down syndrome, which comprises almost half of the chromosomal abnormalities that are detected antenatally. The other more common ones are trisomy 18 and trisomy 13. Together these three chromosomal abnormalities make up about 66 percent of the major aneuploidies that are detected by prenatal screening.

So why might people choose to do aneuploidy screening? Well, there are a few different reasons for this. One of the main reasons is that new or expecting parents just want more information about any potential complications or issues that they might have in their pregnancy. Some women choose to do aneuploidy screening because if there was an abnormality detected they would choose to terminate or abort the pregnancy and end the pregnancy. And others choose to do aneuploidy screening just because they want more information but at the time of screening, they actually aren't sure what they would do if the result came back positive. There are also a group of people who choose not to have any screening, and that's because they've decided that regardless of what abnormalities the pregnancy may have it wouldn't change their outcome and they would continue the pregnancy regardless.

And so, it's really important before we even do the screening to ask the patient in front of you why she's wanting the screening and what she's hoping to get out of that.

There are three main screening tests that we do in Australia. The first one is the first trimester combined screen, often abbreviated as FTCS, or sometimes CFTS, combined first trimester screen.

That one is performed between 11 plus zero to 13 plus six weeks. And exactly as the title suggests, it's a combined screen, which means it uses an ultrasound specifically looking at the nuchal translucency on the fetus at the back of the neck. And also combined with a serum or blood test that looks for the Pregnancy Associated Plasma Protein, or PAPP-A, and also a beta hCG level.

The advantage of doing the first trimester combined screen is that there is some rebate from Medicare. You also do an ultrasound, which confirms further viability of the pregnancy at that point and can also detect if there is any significant abnormality. The combined first trimester screen is 85 percent sensitive and 95 percent specific.

The second type of aneuploidy screening is the Maternal Serum Screening Test, or MSST. And this one is done in the second trimester between 14 weeks and 20 plus 6 weeks. This test can detect pregnancies that are at an increased risk of trisomy 21, trisomy 18, or neural tube defects. It is a blood test that looks for the alpha fetoprotein, unconjugated estriol, free beta hCG, and inhibin A. The Maternal Serum Screening Test has a sensitivity of only 70 to 75 percent and a specificity of 93%.

The third and final type of aneuploidy screening is the cell free DNA based screening or the NIPT or noninvasive prenatal testing. Some of the common trade names that you may have heard is the Harmony or the Percept test. This is a maternal blood test that can be done from 10 weeks in the pregnancy. It looks at the cell free DNA in the maternal plasma that is released by the developing placental tissue. The NIPT test is a bit more costly, it's between 400 and 500, and it is not covered by Medicare. The NIPT test also doesn't include an ultrasound as part of the testing, but it's still recommended that any woman have one to confirm the pregnancy is still viable and also to detect any potential structural abnormalities. The NIPT test is 99 percent sensitive and specific.

Those are the three aneuploidy screening tests that are available in Australia. The combined first trimester screen, the second trimester maternal serum screening test and the non-invasive prenatal testing or cell free DNA screening. It's important to note that all three of these are screening tests, and if anyone screens positive, they are offered further testing in the form of diagnostic testing.

A diagnostic test will definitively confirm the chromosomal abnormality and can then further guide decision making. The diagnostic tests available are usually a CVS or chorionic villus sampling from 9 to 14 weeks, or an amniocentesis, often done after 15 or 16 weeks.”