The 12 week Ultrasound


A few of my colleagues have remarked on the fact I am yet to discuss any obstetric topics so I thought I would get them to suggest one. They suggested combined first trimester screening and non invasive prenatal testing (NIPT).

The initial reason for the ultrasound between 11 weeks and 13+6 weeks of pregnancy was to screen for some chromosomal abnormalities. The ultrasound checks the thickness of the fluid behind the baby’s neck. Along with 2 blood tests (hCG and PAPP-A) and the mother’s age (cFTS), it is combined to assess how likely your baby is of having down’s syndrome (3 chromosomes at number 21). It also checks for two other chromosomal abnormalities including Edwards syndrome ( 3 chromosomes at number 18) and Patau syndrome (3 chromosomes at number 13).

This test is a screening test is classified as low or high risk and it is a reasonably accurate test.

If your test is a low risk result (<1:300 chance that your baby is affected) you will not necessarily be offered further testing. If your result is high risk (>1:300 chance of the baby being affected) you will be offered further testing.

NIPT is a blood test that detects cell free DNA in the mother’s blood. It can be done from 10 weeks and screens for the 3 chromosomal abnormalities and as well as sex chromosome ones and several others. It is extremely accurate and has over a 99% accuracy. But it is still important to know that it is only a screening test. While it is really accurate, there are some babies that will be born with chromosomal abnormalities.

It is also still worth getting an US at 12 weeks even if you have had the NIPT blood test. This is because, while US is not that accurate at detecting any structural concerns in a baby so early, it can detect issues with spina bifida and anencephaly- where the bottom of the brain and skull is affected.

It is important to know that while NIPT is new and very accurate, it costs approx $400 and not covered by medicare.

Of course it is up to you as to what test you would like to have. If you have a low risk result for first trimester screening with cFTS, you will not be offered further testing with NIPT.

You will be offered it if you have a high risk result. Your other option is a chorionic villus sampling (CVS) where a piece of placenta is tested or amniocentesis where a sample of fluid is taken from around the baby from 15 weeks. These both carry risks of miscarriage and is a topic for another time.

As I always say, every person’s situation is different and you should always check with your GP or obstetrician if you have any specific question.