ANTENATAL TESTS

Why test?

If you’re over 35 and therefore have an increased risk of having a child with a disability, or if there is a history of inheritable disability in your family, or if you are just a plain worrier, then you may want to have a test to check your baby. Before having any tests however you should consider how you would respond if the baby has a significant abnormality and what you would gain from having the tests. There are five common antenatal tests, each with advantages and disadvantages to be considered.

Anatomy Scan

The ultrasound anatomy scan is routinely offered. The purpose of the scan is to look for structural abnormalities in the brain, face, spine, heart, stomach, bowel, kidneys and limbs. The scan will also confirm due dates, multiple pregnancies and whether the baby is developing at the expected rate. The scan is non-invasive, it allows you to see your baby, and many problems may be picked up. If necessary you are referred to a specialist for further testing. Due to the position of the baby or the limitations of the technology, many things may not be identified during the scan and a clear scan does not guarantee a baby in good health.

Maternal Serum Testing

A simple blood test may be conducted around 15 weeks to analyze the amount of maternal serum alpha-fetoprotein (AFP) and specific hormone levels in the mother’s blood. If the levels are higher than expected for your age, height, weight and other factors, then you may be considered to have a higher risk of having a child with an abnormality. The tests give indicators of chromosomal abnormalities such as Down syndrome and neural tube defects such as spinal bifida. They involve no risk to the mother or fetus. Maternal serum test results are not conclusive. They only give you an idea of the risks of abnormality. If your level of risk is higher than for most women, your LMC will probably suggest a more invasive test, such as amniocentesis, to clarify the results.

Nichol translucency thickness test or Nichol fold

An ultrasound scan takes accurate fetal measurements including the space at the back of the fetus’ neck known as the Nichol fold. This measurement along with your age and the baby’s gestational age are entered into a formula and your baby’s chances of having an abnormality are calculated. In general, the thicker the fold at a given gestational age, the higher the chance of a problem. Babies with abnormalities such as Down syndrome tend to have more fluid in this tissue at the back of the neck in the first trimester, meaning that the measurement will be larger. The Nichol fold test is non-invasive, painless, and is performed early in the pregnancy, usually between 11 and 13 weeks. The results are available immediately. The test has limitations. It only shows the chance of having a baby with Down syndrome or similar disability i.e. results can show either a 1:300 chance or more than 1:300 chance of a baby with Down syndrome. With results of more than 1:300 an amniocentesis is generally recommended.

Amniocentesis

The first step in an amniocentesis is an ultrasound scan to check the position of the fetus and placenta. A needle is inserted through the abdomen and into the uterus to draw out some of the amniotic fluid which surrounds your baby. The procedure takes about one minute. Amniocentesis is 99 per cent accurate in picking up a wide range of genetic abnormalities, such as Down syndrome, and can also pick up problems such as spinal bifida. The test must be taken in the second trimester, usually between 15 and 18 weeks, and the results take two to take two to three weeks. If a problem is found you are faced with the difficult decision of whether or not to have a late abortion. There is also a 1: 200 risk of miscarriage after amniocentesis.

Chorionic Villous Sampling (CVS)

Guided by an ultrasound scan, some of the chorionic villi, the tiny projections on the placenta, are removed either through cervix or through the wall of the abdomen and tested for abnormalities. CVS is a technique requiring a high level of skill and is only available in some centers. Results are usually available within seven to 10 days. The test is slightly less accurate than an amniocentesis in detecting general genetic disorders and chromosomal abnormalities and so is used only for detecting specific disorders such as cystic fibrosis, if this has been a problem in your family. The advantage of CVS is that it can be done early, it is usually performed between weeks 10 and 12, and that it is very accurate for some specific conditions. However, CVS cannot diagnose some abnormalities such as spinal bifida or structural problems, and that there is an increased risk of miscarriage compared with amniocentesis.