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Prenatal screening guide · 2026
Expanded panels sell on completeness. The honest question is not what they add to the list, but what a positive result would actually put you through.
Microdeletions are small missing segments of a chromosome, too small to see on a standard karyotype. The most clinically significant is 22q11.2 deletion syndrome (DiGeorge), which affects roughly 1 in 2,000 to 1 in 4,000 births and can involve heart defects, immune deficiency, palatal problems and learning difficulties. Some NIPT panels screen for it and for rarer microdeletions. Professional bodies genuinely disagree: ACOG advises against routine microdeletion screening on evidence grounds, while ACMG supports screening specifically for 22q11.2. Because these conditions are rare, positive predictive values are low and most positives are false.
| Most significant | 22q11.2 deletion syndrome, roughly 1 in 2,000–4,000 births |
|---|---|
| ACOG position | Advises against routine microdeletion screening |
| ACMG position | Supports NIPS for 22q11.2 specifically |
| Positive predictive value | Low — most positive results are false |
| A positive means | Diagnostic testing by CVS or amniocentesis with microarray |
| Counselling | Doctor consultation or specialist teleconsultation before and after testing |
A microdeletion is a small missing piece of chromosome, typically too small to see under a microscope but large enough to remove several genes. The consequences depend on which genes are lost.
22q11.2 deletion syndrome is the most common and the most clinically consequential. Features vary widely but can include congenital heart disease, immune deficiency from thymic underdevelopment, palatal abnormalities, feeding difficulties, low calcium, and developmental and learning differences. Importantly, it usually occurs de novo — it is not inherited and there is no family history to raise suspicion, which is the main argument in favour of screening.
Other microdeletions sometimes included on panels — 1p36, Cri-du-chat, Angelman and Prader-Willi — are considerably rarer, which makes the statistical problem worse.
The case against routine screening (ACOG). Evidence on performance in general obstetric populations is limited. Positive predictive values are low, so most positives are false. Each false positive can lead to an invasive diagnostic procedure with its own small risk, plus weeks of anxiety and, in some cases, decisions made on incorrect information.
The case for screening 22q11.2 (ACMG). It is common enough to matter, it occurs without family history, and the outcome is genuinely serious — but it is also actionable. Knowing before birth allows delivery in a centre with cardiac and immunology support, avoids giving live vaccines to an immunodeficient infant, and enables prompt calcium monitoring. Those are real, immediate benefits, not just information.
Both positions rest on the same data and weigh it differently. Anyone telling you the question is settled is oversimplifying. A specialist teleconsultation before testing is the appropriate way to work through it.
Decide about expanded panels before the draw, with the numbers in front of you.
Suppose a microdeletion affects 1 in 4,000 pregnancies. Even a very low false positive rate produces more false positives than true ones, simply because there are so many more unaffected pregnancies to generate them.
The practical translation: if you receive a positive microdeletion result, the most likely explanation is that it is wrong. That does not mean ignoring it — it means understanding in advance that the next step is a diagnostic procedure, and that the probable outcome of that procedure is a normal result.
The question to ask yourself before opting in is therefore not “would I like to know?” but “am I prepared to go through CVS or amniocentesis on a result that is more likely false than true?” If the answer is yes, screening may suit you. If not, declining is entirely reasonable. The general principle is explained in the NIPT accuracy guide.
Ultrasound detects many of the structural features associated with 22q11.2, particularly conotruncal heart defects. A detailed anomaly scan is part of routine care and covers ground NIPT cannot — see MedEx imaging services.
Diagnostic testing with chromosomal microarray detects microdeletions definitively, and is the appropriate route where a structural abnormality has already been seen or where there is a family history.
Questions worth asking before opting into an expanded panel:
Ask these of the provider before booking through MedEx NIPT services or genetic testing services.
A microdeletion affecting roughly 1 in 2,000 to 4,000 births, which can involve congenital heart disease, immune deficiency, palatal abnormalities, low calcium and developmental differences. It usually occurs without any family history.
Professional bodies disagree. ACOG advises against routine screening on evidence grounds, while ACMG supports screening for 22q11.2 specifically because it is actionable at birth. The decision depends on whether you would accept a diagnostic procedure on a result that is more likely false than true.
Because the conditions are rare. Even a very low false positive rate generates more false positives than true ones when the underlying condition affects only a few pregnancies in several thousand.
Diagnostic testing by chorionic villus sampling or amniocentesis with chromosomal microarray is needed to establish whether the finding is real. The most likely outcome is a normal result.
Ultrasound detects many associated structural features, particularly conotruncal heart defects, though not the deletion itself. A detailed anomaly scan is part of routine care regardless.
Where the diagnosis is confirmed, yes. It allows delivery in a centre with cardiac and immunology support, avoids live vaccines in an immunodeficient infant and enables prompt calcium monitoring.
Ultrasound covers much of the same ground and carries no false positive burden.
Medical disclaimer: This article is general health information and does not replace medical advice, diagnosis or treatment. Laboratory reference ranges differ between laboratories and results must be interpreted alongside your symptoms, medicines and medical history. Speak with a qualified clinician before starting, stopping or changing any treatment. Service details, inclusions and prices can change — confirm them with MedEx before booking.
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