By Yolanda Smith, BPharm
All pregnant women in most countries worldwide are currently offered prenatal screening for spina bifida and other birth defects. The test results are not 100% accurate and there are some false positive and false negative results, making subsequent tests a requirement to confirm the findings.
Since the introduction of pregnancy screening, the incidence of spina bifida has reduced significantly due to the option of expectant parents to opt for the termination of a pregnancy with spina bifida.
Maternal Serum Alpha-Fetoprotein Test
The maternal serum alpha-fetoprotein (MSAFP) test is commonly used initially to screen for spina bifida.
A blood sample of the expectant mother is taken and sent to a laboratory to be analysed for the presence of alpha-fetoprotein. This protein is usually produced by the baby and a small amount crosses the placenta into the mother’s bloodstream. An abnormally high concentration of the protein in the blood sample is suggestive of a neural tube defect, such as spina bifida or anencephaly.
The test does not have a high level of specificity or selectivity, as various other factors can manipulate the level of alpha-fetoprotein. For this reason, subsequent blood tests and other screening methods may be required.
Other blood tests may detect human chorionic gonadotropin (HCG), inhibin A or estriol hormones, to suggest the presence of other complications such as trisomy 21, also known as Down syndrome.
Ultrasonography is a useful screening technique for spina bifida and is commonly used in practice if results from the MSAFP blood test are higher than normal.
Ultrasound imaging uses high-frequency sound waves to form an image of bodily tissues, according to the way the bounce off the tissues. The images can help to determine the reason for abnormally high levels of alpha-fetoprotein in the mother’s blood sample, such as multiple babies and gestational age.
Advanced ultrasound technology can also detect evidence of spina bifida including the characteristic opening of the spine or abnormalities in the brain of the foetus.
Amniocentesis is commonly used when an abnormal result in the MSAFP screening test is found, but there is no evidence of spina bifida or other abnormalities in the ultrasound.
This technique involves taking a sample of the amniotic sac fluid with a needle, which is analysed to determine the level of alpha-fetoprotein. Similarly to the maternal blood test, this sample usually contains a low level of the protein but is higher when there is an open neural tube defect. This is due to the opening in the baby’s spine that leads to leakage of the protein into the amniotic sac.
As this screening test is associated with a small risk of unplanned pregnancy termination, it is reserved to be the last line screening test and the benefits and risks of the tests should be considered before proceeding.
Termination of Pregnancy
The expectant couple has the choice to continue or terminate the pregnancy with the knowledge of the pregnancy screening tests for spina bifida. Since the introduction of screening, the incidence of spina bifida has reduced significantly due to the choice of many couples not to continue the pregnancy to full-term. This has also been associated with improved quality life of patients.
Last Updated: Sep 28, 2015