Early pregnancy screening for fetal abnormalities

Screening tests determine if your pregnancy is at a low or high risk of some common genetic abnormalities.

Screening Tests

There are now two choices available to screen for genetic problems in the fetus between 10 and 14 weeks gestation. These tests give you an indication if you are at high risk for genetic conditions such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or Patau syndrome (trisomy 13).

Down syndrome (trisomy 21); mild to moderate intellectual impairment, characteristic facial features, growth delay and possibly other health issues such as cardiac abnormalities.

Edwards syndrome (trisomy 18); severe intellectual and development problems, physical abnormalities, less than 10% survive to 1 year old.

Patau syndrome (trisomy 13); severe intellectual and development problems, usually multiple physical abnormalities, less than 10% live past the first year of life.

Choices for Genetic Screening

1. Combined First Trimester Screening This has been available since 1996

It involves an ultrasound to check the anatomy of the fetus and measure a small pouch of fluid at the back of the baby’s neck “the nuchal translucency” which gets thicker with some genetic problems (this can only be performed between 10+0 and 13+6 weeks of pregnancy).

It also involves a blood test, looking for 2 small pregnancy proteins in the mothers blood “PAPP-A and free BhCG” (which can be taken between 9 and 13+6 weeks gestation).

The results are combined with the mother’s age and size of the fetus to give a risk assessment for Down syndrome, Edwards syndrome and Patau syndrome.

If your blood test has been performed prior to the ultrasound appointment, the risk can be calculated and discussed with you immediately after the ultrasound has been performed.

It picks up Down syndrome about 90% of the time. However 5% of women with a normal baby have a high risk result, which can lead to a stressful few weeks for these families.

Twin Pregnancies and Higher Order Multiples

Accuracy is lower for twins with a pick-up rate for Down syndrome of 80 to 85%.

A nuchal translucency scan can be performed on each fetus however the blood test result is a combination of all fetuses.

2. Non Invasive Prenatal Test (NIPT) This has been available since 2012.

There is an out of pocket cost of $450. It can be performed from 10 weeks gestation.

This newly available test takes a sample of the mother’s blood (by a standard blood test) and looks for a very small amount of cell free fetal DNA in the sample to determine the baby’s genetic make-up.

After blood collection it takes up to 2 weeks for the results to be available.

It picks up Down syndrome about 99.9% of the time and only 0.1% of women with a normal baby have a falsely high risk result, so the test is extremely accurate.

There is also the option of testing for fetal sex and fetal sex chromosome abnormalities.

About 2% of patients have a failed result, the test can be repeated free of charge, however it may fail a second time (because the amount of fetal DNA in the mothers blood may be insufficient to detect a result). This figure is higher for women who are overweight.

If the result is abnormal (showing a high risk for a genetic condition), it must be confirmed by an amniocentesis or chorionic villus sampling (CVS) to be certain it is accurate before proceeding to any further management.

Twin and other Multiple Pregnancies

NIPT is available for twin pregnancies however the failure rate may be higher and the accuracy may be lower. Testing for the fetal sex or sex chromosome abnormalities is not available.

NIPT is not available for triplets or higher order multiples.

For either of these screening tests (combined screening or NIPT) with a high risk result there is an option to have an invasive test (CVS or amniocentesis) which will provide a definite answer to whether your fetus is affected by Down syndrome, Edwards syndrome or Patau syndrome, as well as some other genetic problems.

Diagnostic Tests

1. Chorionic Villus Sampling (CVS) from 11 weeks gestation. 

2. Amniocentesis from 15 weeks gestation

Chorionic Villus Sampling (CVS)

Is a procedure performed in pregnancy from 11 weeks gestation which samples a small number of cells from the placenta to test whether the fetus has any genetic abnormalities.

An ultrasound probe is used to guide a very thin needle into the placental tissue to sample a few cells. The cells are cultured (grown up) and usually represent the genetic material from the fetus.

They can accurately determine whether the fetus has Down syndrome, Edwards syndrome or Patau syndrome as well as a large number of other genetic abnormalities. The fetal sex can also be determined accurately.

There is a small risk of having a miscarriage as a result of the procedure (up to 1%).

There are also small risks (1%) that there will be “mosaicism” in the placenta, meaning the cells do not represent the genetic material of the fetus or the cells do not culture as expected. If this occurs an amniocentesis needs to be performed from 15 weeks gestation to provide a diagnosis.

Amniocentesis

Is a procedure performed in pregnancy from 15 weeks gestation which samples fluid from around the baby to test whether the fetus has any genetic abnormalities.

An ultrasound probe is used to guide a very thin needle into the fluid around the baby. Within this fluid are cells shed off from the fetus, which are cultured (grown up) and represent the genetic material from the fetus.

They can accurately determine whether the fetus has Down syndrome, Edwards syndrome or Patau syndrome as well as a large number of other genetic abnormalities. The fetal sex can also be determined accurately.

There is a small risk of having a miscarriage as a result of the procedure (up to 0.5%).

There is a small risk (<0.1%) that the fetal cells do not culture as expected, and no result is obtained. In this case the test needs to be repeated to obtain a diagnosis.

Genetic Testing

Even though a wide spectrum of genetic abnormalities are tested for with both a CVS and an amniocentesis, there are also a lot of genetic abnormalities that are not tested for. It is not possible to exclude all genetic abnormalities in any pregnancy.

Twins, triplets and more

In a twin pregnancy it is likely that a sample of genetic material will need to be taken from each twin. This means that a CVS or amniocentesis is performed for each fetus (ie more than one needle pass is required).

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