Prenatal Testing Sequence
Prenatal Sequential Screening: An Early Detection Method for Fetal Abnormalities
Prenatal sequential screening is a two-stage process that offers expectant mothers an early and non-invasive way to assess the risk of their unborn child having chromosomal abnormalities or neural tube defects. The screening takes place during the first and second trimesters of pregnancy.
In the first trimester (9 to 13 weeks), a combined serum screening is conducted, which involves measuring specific maternal blood markers and performing a nuchal translucency ultrasound to evaluate fetal risk. This stage is crucial in detecting common trisomies such as Down syndrome with moderate sensitivity.
The first trimester combined screening consists of testing two important proteins: PAPP-A and hCG. Low levels of PAPP-A increase the likelihood of Trisomy 18 or Down syndrome. On the other hand, high levels of hCG may suggest Down's syndrome, while lower amounts may indicate Trisomy 18.
Approximately one per cent of all women show a positive result after stage one testing. If results are inconclusive or indicate a higher risk, a second trimester blood test, known as the "quadruple test," is performed, measuring additional markers, to refine risk assessment.
In the second trimester (15-17 weeks), the quadruple test involves checking the levels of four proteins: Alpha-foetoprotein (AFP), unconjugated estriol, hCG, and Dimeric inhibin A. If AFP levels in the mother's blood are lower than normal, Down's syndrome and Trisomy 18 may be implicated. Conversely, high levels of hCG may suggest Down's syndrome, while lower amounts may indicate Trisomy 18.
The prediction accuracy for open neural tube defects in the second stage of sequential screening is around 80%. However, the accuracy significantly improves for Trisomy 18 and Down syndrome, with an accuracy of around 90%.
It is essential to note that sequential screening tests are not enforced, as many people might be uncomfortable or unwilling to know such details. Pregnant women, especially those with complicated pregnancies or a family history of birth problems, are recommended to undergo these tests.
A positive result in sequential screening might indicate a foetal defect when there actually isn't one (false-positive). In such cases, a genetic counsellor may be recommended to help understand the risks and provide advice. The final sequential screening results come in one of two forms: positive or negative.
Down syndrome and spina bifida occur once in every 1000 babies, while Trisomy 18 shows up at one in every 7000 babies. Early detection allows better preparation, potential interventions, or counseling. The sequential approach, combining biochemical and ultrasound data in the first trimester and serum tests in the second, maximizes detection rates while limiting false positives, which is crucial in prenatal care for reducing pregnancy loss due to unnecessary invasive procedures and improving outcomes.
These tests form a stepwise strategy in prenatal care to efficiently and safely screen for fetal chromosomal abnormalities.
Read also:
- Impact of Alcohol Consumption During Pregnancy: Consequences and Further Details
- The cause behind increased urination after alcohol consumption is explained here.
- Toe joint arthritis: Signs, triggers, and further details
- West Nile Virus found in Kentucky for the first time; residents advised to take protective measures
