First & Second Trimester Screenings
In this article, we describe and compare the first and second trimester screenings, and non-invasive prenatal testing (NIPT). Please note that the first and second trimester screening dates on this page are taken from the Women’s and Children’s Health Network (SA), however dates may differ slightly among states/territories.
Prenatal screening is offered during pregnancy to calculate the chance that a pregnancy may be at higher or lower risk of a chromosomal or genetic condition. This is usually discussed and offered at your first antenatal appointment, the booking visit, at about 10 – 12 weeks. All prenatal screening is optional, each type of screen looks for different conditions and none are diagnostic.
Some women choose to decline screenings because an abnormality is of no concern to them. Some women wish to know if they are at higher risk so that further diagnostic testing can be done.
The first trimester screen can assess the risk for Down Syndrome (trisomy 21), Edwards Syndrome (trisomy 18) or Palau Syndrome (trisomy 13). This screen is a two- step process and includes information from a maternal blood test and an ultrasound. The blood test is done between 9 and 14 weeks and measures a protein known as PAPP-A (pregnancy-associated plasma protein) and a hormone called beta-human chorionic gonadotropin (βHCG). The ultrasound is done between 11 and 14 weeks and measures the thickness of fluid at the back of the baby’s neck, this is called a nuchal translucency screening. The information gained from these is then combined with maternal age and weight and the gestational age to determine the risk of a chromosomal abnormality.
The second trimester maternal serums screen assesses the chance of Down Syndrome, Edwards Syndrome and spina bifida. This screen uses only a maternal blood test and is taken between 14 weeks and 20 weeks, no ultrasound is needed. This blood test measures the levels of a different protein called alpha-fetoprotein (AFP) and two hormones: βHCG and unconjugated estriol (UE3). The information gained from the blood screen is then combined with maternal age and weight and the gestational age to determine the risk of a chromosomal abnormality or spina bifida.
There is a possibility that your screening results may also show an increased risk for other complications (which are not the purpose of these screenings). For example, if a protein or hormone is out of the normal expected range, there may be an increased risk of pre-eclampsia or miscarriage. If your results are abnormal, a management plan will be discussed with you.
Non-invasive prenatal testing (NIPT) is a relatively new way to screen for genetic conditions. You may have heard of it as the Harmony prenatal test or the Nest prenatal test – these are brand names for the same screening. These screenings use maternal blood taken from 10 weeks and have an accuracy rate of 75 – 99%, depending on what is being screened for.
Both the Nest and Harmony screenings work by detecting cell-free DNA from the baby. About 10% of DNA circulating in the maternal blood stream is fetal DNA, which has come from the placenta. The fetal DNA is then sequenced to determine if a pregnancy is high or low risk for a variety of conditions including: Down Syndrome, Edwards Syndrome, Palau Syndrome and sex chromosome aneuploidy (abnormalities of the X and Y chromosomes). They can also determine the gender of your baby if you wish.
Neither of these screens are covered by Medicare and will be charged to you. Their fees range from $425 to $445 (current 2020).
It is important to understand that any result you receive from any of these screenings are NOT diagnostic. They cannot tell you that your baby does have an abnormality, nor that your baby does not have an abnormality with absolute certainty.
Low risk ≠ no risk
High risk = might have an abnormality, based on your individual data
If your result comes back as high risk. You your care provider should offer you genetic counselling and diagnostic testing. Diagnostic testing involves taking fetal DNA directly from the placenta, called chorionic villus sampling, or from the amniotic fluid surrounding the baby, called amniocentesis (we will cover this next month). These tests carry a higher risk than the screenings explained on this page and are only offered to families that are at high-risk of a genetic condition.