The 11-14 weeks scan - KH Nicolaides, NJ Sebire, RJM Snijders, AP Souka

Chapter 1

DIAGNOSIS OF CHROMOSOMAL ABNORMALITIES

Non-invasive diagnosis

During the last 30 years, extensive research has aimed at developing a non-invasive method for prenatal diagnosis based on the isolation and examination of fetal cells found in the maternal circulation. About 1 in 103–107 nucleated cells in maternal blood are fetal. The proportion of fetal cells can be enriched to about 1 in 10–100 by techniques such as magnetic cell sorting (MACS) or fluorescence activated cell sorting (FACS) after attachment of magnetically labelled or fluorescent antibodies on to specific fetal cell surface markers. The resulting sample is unsuitable for traditional cytogenetic analysis because it is still highly contaminated with maternal cells. However, with the use of chromosome-specific DNA probes and fluorescent in situ hybridization (FISH), it is possible to suspect fetal trisomy by the presence of three-signal nuclei in some of the cells of the maternal blood enriched for fetal cells.

On the basis of currently available technology, examination of fetal cells from maternal peripheral blood is more likely to find an application as a method for assessment of risk, rather than the non-invasive prenatal diagnosis of chromosomal defects. The sensitivity of this method is comparable to serum screening. However, unlike serum biochemistry testing, which is relatively easy to apply for mass population screening, analysis of fetal cells from maternal blood is both labor intensive and requires highly skilled operators. The extent to which the techniques for enrichment of fetal cells could be improved, to achieve a higher yield of the necessary cells, as well as become automated, to allow simultaneous analysis of a large number of samples, remains to be seen.

Recent interest has focused on the presence of cell-free fetal DNA in maternal plasma and the ability to quantify the concentration of male fetal DNA in pregnancies with male fetuses using real-time quantitative PCR. There is contradictory evidence concerning the concentration of cell-free fetal DNA in trisomy 21 pregnancies with some studies reporting that the levels are increased and in others there was no significant difference from chromosomally normal pregnancies. The extent to which cell-free fetal DNA will become another maternal serum marker in screening for trisomy 21 remains to be seen.

Non-invasive diagnosis

• Examination of fetal cells from maternal peripheral blood is more likely to find an application as a method for assessment of risk, rather than the non-invasive prenatal diagnosis of chromosomal defects.
• There is contradictory evidence concerning the concentration of cell-free fetal DNA in trisomy 21 pregnancies.

Invasive diagnosis

Amniocentesis

There is only one randomized trial which compared the risks of amniocentesis to controls. In this study, 4,606 low-risk, healthy women, 25–34 years old, at 14–20 weeks of gestation, were randomly allocated to amniocentesis or ultrasound examination alone (Tabor et al 1986). The total fetal loss rate in the patients having amniocentesis was 1% higher than in the controls. The study also reported that amniocentesis was associated with an increased risk of respiratory distress syndrome and pneumonia. Amniocentesis is feasible from the first-trimester. However, randomized studies have demonstrated that after amniocentesis at 10-13 weeks the rate of fetal loss is about 2% higher and the incidence of talipes equinovarus is 1.6% higher than after first-trimester chorionic villus sampling or second-trimester amniocentesis.

Chorionic villus sampling

Randomized studies have demonstrated that the rate of fetal loss following first-trimester transabdominal chorionic villus sampling is the same as with second-trimester amniocentesis. There is controversy as to whether the rate of fetal loss after transcervical chorionic villus sampling is higher or not. It is likely that in centres with experience in ultrasound guided invasive procedures the risks of amniocentesis and chorionic villous sampling, irrespective of route, are the same. There is an association between chorionic villus sampling before 10 weeks and fetal transverse limb abnormalities, micrognathia and microglossia. It is therefore imperative that chorionic villus sampling is performed only after 11 weeks by appropriately trained operators.

Invasive testing

• Diagnosis of fetal chromosomal abnormalities requires invasive testing.
• The risk of miscarriage from chorionic villus sampling in the first trimester is the same as for amniocentesis in the second trimester
• Amniocentesis should not be performed before 15 weeks
• Chorionic villous sampling should not be performed before 11 weeks
• Invasive tests should be carried out by appropriately trained and experienced operators.