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Antenatal screening

Dr Abhilash Sandhyala and Dr Alexandra Stanislavsky et al.

Antenatal screening and diagnosis is currently available for a select few genetic conditions, including trisomy 21 (Down syndrome), trisomy 18 (Edward syndrome), trisomy 13 (Patau syndrome) and neural tube defects.

For an overview of the conditions and their manifestations, please refer to the specific articles :

trisomy 21 (general article)
- antenatal features
trisomy 18
trisomy 13
neural tube defects

Screening

The avenues of testing include :

Maternal serum screening

Maternal blood test may be performed during either

1^st trimester : as part of the combined serum screening ^2,3
- performed in conjuction with the nuchal translucency ultrasound (see below)
- performed at 9w - 13w6d (ideally 9 - 12w)
- measures free βhCG and PAPP-A

2^nd trimester : maternal serum screening (or “quadruple test”)
- performed at 14 - 20w (ideally 15 - 17w)
- measures alpha fetoprotein (AFP), free βhCG (or total hCG), unconjugated oestriol (uE3) +/- inhibin

First trimester ultrasound

First trimester ultrasound screening involves the measurement of crown rump length (CRL) and nuchal translucency (NT). Results are combined with the serum screening to generate a risk.

For screening validity, the test must be performed at 11w3d - 13w6d, or when CRL measures 45 - 84mm (if there is discrepancy, CRL takes precendence).

Nuchal translucency :

< 3 mm is considered normal, however this must be matched with maternal age and gestational age.
please refer to the article on nuchal translucency for a discussion of correct measurement
for combined serum screening, a risk of 1 in 300 or less is considered as increased risk

Second trimester ultrasound

A second trimester fetal morphology ultrasound scan is generally performed at 18 – 20 weeks. It is not recommended as the primary screening tool for trisomy 21 or trisomy 18, although it can be used as primary screening for neural tube defects³.

Nonetheless, there are a number of structural anomalies that can be diagnosed in the second trimester.

Furthermore, numerous sonographic soft markers may be identified on antenatal ultrasound: features that do not constitute an abnormality in themselves, but are associated with an increased risk of congenital anomaly. These are discussed in detail the separate article.

Measurement

The measured value is converted to a "multiple of the median". The median level for each day equals a MoM of 1.0. A result of 1.5 MoM means the patient has one and a half times the median level

Diagnosis

Diagnostic tests are offered to high risk patients:

maternal age 37 years or older
increased risk on maternal screening
increased risk on fetal morphology scan

Invasive tests usually offered are

chorionic villus sampling : performed at 11 - 12 weeks
amniocentesis : performed at 15 - 18 weeks

Historically, the risk of miscarriage was said to be 1% for CVS and 0.5% for amniocentesis, although this is somewhat operator dependent.

NOTE : This article is currently in accordance with the Australian / NZ and UK antenatal screening guidelines. We recognise that some regional variation in the availability and timing of antenatal screening exists.