Myhealth Medical Centre

First Trimester Screening and NIPT

Pregnancy is an exciting time for women and their partners.  However, it can also be a period of uncertainty when we hope for a healthy baby to develop, and therefore, we want to screen for certain developmental abnormalities, including chromosomal abnormalities.

First trimester screening

First trimester screening is well established in Australia and is used to screen for common chromosomal abnormalities, such as Trisomies 21 (Down’s Syndrome), 18 and 13, between 11-14 wks gestation.  It involves a nuchal translucency ultrasound measurement of the small pocket of fluid behind baby’s neck and the foetal nasal bone combined with the mother’s results of her placental chemicals in her bloodstream.  This is combined with the maternal age to give an individualized risk for having a baby with one of the three most common trisomies.   

First trimester screening is able to detect approximately 85-90% of Trisomies 21, 18 and 13.  It is unable to detect for other chromosomal abnormalities, including sex chromosomal abnormalities. 

The result is reported as a risk ratio.  A risk of less than 1 in 1000 is considered reassuringly low and no further testing would be recommended.  A risk of greater than 1 in 1000 up to 1 in 300 is still low risk, but further testing via non-invasive prenatal testing (see below) may offer more reassurance.  Risks greater than 1 in 300 but less than 1 in 50 may be followed up with either non-invasive prenatal testing or needle diagnosis with chorionic villus sampling (CVS) or amniocentesis.  Risks of 1 in 50 or above are likely to be associated with other chromosomal abnormalities, which are not covered by the non-invasive blood test.  Hence, diagnosis with CVS or amniocentesis is recommended.

Non-invasive Prenatal Testing (NIPT) 

In recent years, non-invasive prenatal testing has become available in Australia.  This test detects the small amount of the baby’s DNA in mother’s bloodstream.  It can be done from 10 weeks onwards in singleton and twin pregnancies.  A blood test is collected from the mother and costs from $450-900 depending on the provider, with a turnaround time of less than two weeks.  It can detect more than 98% of the common trisomies and sex chromosomal abnormalities, as well as gender.  Ask your GP or obstetrician for more information.

NIPT remains a screening test and cannot diagnose chromosomal abnormalities.  It does not replace first trimester screening, as the nuchal translucency ultrasound can check on foetal viability as well as approximately 60% of early structural foetal abnormalities.   However, NIPT is particularly useful when offered in addition to the traditional first trimester screening. For example, a woman may have a 1 in 500 risk for Downs on first trimester screening but want the reassurance of a low risk NIPT result.  Alternatively, some women choose to have the NIPT at 10 weeks and the first trimester screening at 11-14 wks and receive all these results together from their obstetrician or GP.  

In summary, new advances in prenatal testing give women and their families more non-invasive testing options, at times reducing the need to progress to CVS or amniocentesis, both of which carry a risk for miscarriage.