Choosing Your Prenatal Test: Understanding NIPT and OSCAR Screening

When considering first-trimester prenatal screening, first-trimester screening NIPT analyses cell-free fetal DNA in maternal blood to estimate the risk of selected chromosomal conditions, while OSCAR combines maternal blood markers with ultrasound findings to provide a first-trimester risk assessment. Both are screening tests rather than diagnostic procedures. Any high-risk result may require confirmation through diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis.

The most appropriate screening approach depends on factors such as gestational age, clinical circumstances, personal preferences, and discussions with the obstetrician.

What OSCAR Screening Measures

Performed between weeks 11 and 14 of pregnancy, the OSCAR (One-Stop Clinic for Assessment of Risk) test combines a maternal blood test with an ultrasound measurement of nuchal translucency, the fluid-filled space at the back of the baby’s neck. These findings are assessed alongside maternal factors such as age and weight to estimate the risk of chromosomal conditions, including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).

Beyond Chromosomal Screening

The ultrasound component of OSCAR provides information that is not available through blood-based screening alone. During the scan, the sonographer assesses early foetal anatomy, confirms gestational age, identifies multiple pregnancies, and evaluates early markers of fetal development.

An increased nuchal translucency measurement may be associated with a higher risk of certain structural abnormalities, including congenital heart defects. Depending on the findings, your doctor may recommend additional assessment, such as a fetal echocardiogram or specialist review.

How NIPT Analyses Foetal DNA

Available from week 10 of pregnancy, Non-Invasive Prenatal Testing (NIPT) analyses cell-free placental DNA fragments circulating in maternal blood to estimate the risk of selected chromosomal conditions, including Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13).

Expanded Screening Options

In addition to the common trisomies, some NIPT providers offer expanded screening panels that assess the risk of selected sex chromosome conditions and certain microdeletion syndromes, where small segments of genetic material are missing.

NIPT may also provide information about fetal sex earlier in pregnancy than is typically possible through routine ultrasound assessment, although this is not the primary purpose of the test.

Comparing Screening Performance

Both NIPT and OSCAR are screening tests designed to estimate the likelihood of specific chromosomal conditions. Clinical studies have shown that NIPT generally has higher detection rates and lower false-positive rates than OSCAR for the common trisomies.

However, neither test is diagnostic. Screening results should always be interpreted within the context of clinical findings, ultrasound assessment, and discussions with your obstetrician.

Understanding What Screening Results Mean

Detection rate refers to how often a screening test correctly identifies pregnancies affected by a particular condition, while false-positive rate refers to how often a test indicates increased risk in an unaffected pregnancy.

The purpose of prenatal screening is to identify pregnancies that may benefit from further assessment. A high-risk screening result does not confirm a diagnosis, and diagnostic procedures such as CVS or amniocentesis may be required for confirmation.

Timing and Practical Considerations

NIPT can generally be performed from week 10 of pregnancy onwards, offering greater flexibility in scheduling. Results are typically available within one to two weeks, although turnaround times may vary depending on the laboratory and provider.

In contrast, OSCAR must be performed within a specific window between weeks 11 and 14 of pregnancy. Results are often available within a few days, depending on the clinic’s processes and reporting arrangements.

When Results Are Inconclusive

In some cases, NIPT may produce an inconclusive result due to an insufficient amount of cell-free placental DNA in the maternal blood sample. This is more likely to occur when testing is performed earlier in pregnancy or in certain maternal circumstances, including higher body weight.

Depending on the clinical situation and gestational age, your doctor may recommend repeating the test after a period of time or considering alternative screening approaches, including OSCAR, if the pregnancy remains within the appropriate screening window.

Situations Where OSCAR May Be Considered

OSCAR may be considered when parents wish to combine chromosomal risk assessment with a first-trimester ultrasound evaluation. The ultrasound component provides information about fetal development, confirms gestational age, identifies multiple pregnancies, and assesses certain early structural markers.

It may also be considered by patients seeking a first-trimester screening option that incorporates both blood-test and ultrasound findings.

Situations Where NIPT May Be Considered

NIPT may be considered by patients who wish to undergo screening for common chromosomal conditions from week 10 of pregnancy. Clinical studies have shown that NIPT generally has higher detection rates and lower false-positive rates than traditional first-trimester screening methods for the common trisomies.

The doctor may discuss NIPT as part of the screening process based on factors such as maternal age, family history, previous pregnancy history, and individual preferences.

NIPT in Twin Pregnancies

NIPT can be performed in twin pregnancies, although interpretation may be more complex than in singleton pregnancies. Factors such as zygosity and fetal fraction can influence result interpretation, particularly in dizygotic (non-identical) twins.

The obstetrician can advise whether NIPT is suitable for your pregnancy and explain any limitations relevant to twin gestations.

Combining Both Tests

Some parents choose to undergo both OSCAR and NIPT as part of their prenatal screening journey. OSCAR provides first-trimester ultrasound findings alongside chromosomal risk assessment, while NIPT provides additional screening information for selected chromosomal conditions.

The suitability of this approach depends on individual clinical circumstances, personal preferences, and discussions with the obstetrician. Undergoing both tests may increase overall costs and extend the screening process.

What Happens After Screening

Following screening, your results will help guide discussions about any further assessment that may be appropriate.

• Low-Risk Results: Indicates a lower likelihood of the screened chromosomal conditions. However, no screening test can exclude all genetic or structural conditions, and screening does not provide a definitive diagnosis.
• Intermediate-Risk Results: May require further discussion with your obstetrician. Depending on your circumstances, additional screening, closer monitoring, or diagnostic testing may be considered.
• High-Risk Results: Does not confirm that a condition is present. The doctor may recommend diagnostic testing, such as CVS or amniocentesis, to obtain additional information and confirm the diagnosis where appropriate.

Preparing for Your Screening Decision

Choosing a prenatal screening approach involves considering both clinical factors and personal preferences.

• Review Your Medical and Family History: Inform the doctor about any family history of genetic conditions, chromosomal disorders, or previous pregnancy complications that may influence screening discussions.
• Consider What Information Is Important to you: Different screening options provide different types of information. Some parents prioritise first-trimester ultrasound findings, while others may wish to explore additional chromosomal screening options.
• Understand Possible Outcomes: Before screening, it may be helpful to consider how you would approach low-risk, high-risk, or inconclusive results and whether you would consider diagnostic testing if recommended.
• Discuss Options with Your Obstetrician: The obstetrician can explain the benefits, limitations, timing, and suitability of each screening option based on your individual circumstances.

When to Speak With Your Obstetrician

• Vaginal bleeding during pregnancy
• Severe abdominal pain or cramping
• Questions about which screening option may be suitable for your situation
• A family history of genetic or chromosomal conditions
• Previous pregnancy complications or pregnancy losses
• Concerns or anxiety related to prenatal screening and testing

Commonly Asked Questions

Can I skip prenatal screening entirely?

Prenatal screening is optional. Some parents choose to undergo screening, while others prefer not to. Our obstetrician can explain the purpose, benefits, and limitations of screening, as well as how declining screening may affect prenatal care planning.

Does a low-risk NIPT result mean my baby definitely doesn’t have Down syndrome?

No. A low-risk NIPT result indicates a lower likelihood of the screened condition, but no screening test can provide absolute certainty. Diagnostic procedures such as chorionic villus sampling (CVS) or amniocentesis are required to confirm or exclude chromosomal conditions definitively.

Why is NIPT generally more expensive than OSCAR?

NIPT involves laboratory analysis of cell-free placental DNA using specialised sequencing technology, whereas OSCAR combines ultrasound assessment with maternal blood testing. Differences in laboratory processes and technology contribute to the variation in cost.

What happens if my NIPT result is inconclusive?

An inconclusive result may occur when there is insufficient cell-free placental DNA in the blood sample for analysis. Depending on your gestational age and clinical circumstances, our doctor may discuss options such as repeating the test or considering alternative screening approaches.

Does NIPT detect all genetic conditions?

No. NIPT screens for selected chromosomal conditions included in the testing panel. It does not detect all genetic disorders, structural abnormalities, or conditions outside the panel being tested. A low-risk result does not exclude all possible fetal conditions.

Next Steps

NIPT and OSCAR are both prenatal screening tests with different strengths, timing requirements, and limitations. NIPT generally has higher detection rates and lower false-positive rates for common trisomies, while OSCAR combines first-trimester ultrasound assessment with chromosomal risk estimation. A high-risk result from either test should be confirmed with diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis, before clinical decisions are made.

If you are considering prenatal screening or have questions about your screening results, consult our obstetrician to discuss the most appropriate option based on your pregnancy and individual circumstances.