The Ultrasound Scan at 18-23 Weeks
The guidelines recommend management of common obstetric conditions at primary and referral level, based on local experience and evidence-based practice. They are not intended as strict protocols nor as standard operating procedures, but rather as a guide to safe effective management of common obstetric problems. The guidelines should be most useful to interns, medical officers and registrars.
Irrespective of the outcome of the 11-14 week scan, another scan at 18-23 weeks is recommended to identify structural abnormalities. This is particularly important in fetuses with increased nuchal translucency with normal karyotype at 11-14 weeks, because of the higher risk of cardiac abnormalities and genetic syndromes. In addition, the assessment of the feto-maternal circulation & the cervical length continue to provide important information.
Down’s syndrome detection & screening from 15 to 23 weeks
Biochemistry screening
Maternal serum levels of intact hCG, unconjugated estriol, & alphafeto-protein (triple test) may be measured to compute a Down’s syndrome risk at 16-18 weeks. Women with ‘positive’ tests are counselled for possible amniocentesis. Biochemistry screening currently not available in public hospitals.
Ultrasound detection of soft markers
A soft marker is structural anomaly visible on ultrasound scan rarely of postnatal significance but associated with a chromosomal abnormality. The presence of one or more of the following may suggest the possibility of Down’s syndrome, & therefore counseling for amniocentesis:
- Cardiac abnormalities
- Renal pyelectasis
- Short femur and/or short humerus, Sandal gap
- Hypoplasia of the middle phalanx of the fifth finger & clinodactyly,
- Nuchal fold thickening
- Absent or hypoplastic nasal bone
Structural abnormalities (hard markers)
Hard markers are serious structural abnormalities frequently associated with Down’s syndrome & other chromosomal abnormalities, indicating a need for fetal karyotyping by amniocentesis or cordocentesis. These include:
- Duodenal atresia
- Omphalocele
- Cystic hygroma
- Hydrothorax
- Atrial septal defect &/or ventricular septal defect of the heart
- Cerebral ventriculomegaly
The Non Invasive Prenatal Test (NIPT)
DNA isolated from maternal blood not only contains maternal DNA, but also a small amount of fetal DNA: the fetal fraction of cell-free DNA in maternal blood from 10 weeks onwards is about 10-15% of all cell-free DNA.
It is possible to analyze this cell-free DNA to detect fetal trisomies such as Down syndrome.
The NIPT test is a very accurate screening test for Trisomy 21, 18, 13 & sex chromosomal abnormalities with an accuracy of >99%. Patients who screen positive for a chromosomal abnormality with the NIPT still need to have an invasive test for confirmation of the chromosomal abnormality (a diagnostic test). In addition to the above chromosomal abnormalities, a number of common microdeleltions such as Di George, Prader Willi, Angelman Synrome, Cri du Chat, can be tested as well. NIPT is only available in the private sector.