Prenatal Screening, Diagnosis and Intervention

What are the possibilities for prenatal diagnosis of NTDs?

Prenatal diagnosis of NTDs and other severe congenital anomalies is possible using available pregnancy screening tests. Maternal serum alphafeto protein (MSAFP) screening detects approximately 85% to 90% of NTDs. When it is combined with second trimester fetal ultrasound screening, the detection rates for anencephaly and spina bifida are virtually 100% and 95% respectively. The most difficult NTDs to detect are small sacral meningoceles and lipomeningoceles, with the reliability of ultrasound and MSAFP screening decreasing significantly when the lesions are skin covered.

Although prenatal screening for congenital anomalies is available in many jurisdictions, most provinces/ territories do not have organized prenatal screening programs. Screening for congenital anomalies should be available to all women wanting these services, irrespective of location or income. The advantages of identifying NTDs prenatally include the ability to prepare emotionally and logistically for the delivery of an affected infant, the option to terminate an affected pregnancy and, in the future, the increasing potential for in utero treatment (see below).

The choice of whether or not a couple will continue with an NTD-affected pregnancy is a very difficult one. Clearly, many factors influence this decision. It is imperative that all pregnancies identified as having fetal abnormalities be evaluated by specialists in perinatology and genetics, and that couples receive appropriate counselling before making their decision about the pregnancy.

What about management of a pregnancy and delivery of a fetus diagnosed prenatally with an NTD or other congenital anomaly?

Referral is recommended to a high-risk fetal diagnosis centre with combined perinatology, medical genetics and other specialties. It is imperative that the nature of the structural anomalies be delineated by sophisticated ultrasound and other prenatal testing to confirm preliminary ultrasound findings.

Most pregnancies with an NTD-affected fetus are evaluated and delivered by an obstetrician or perinatalogist at a medical centre that can provide neonatology and surgical consultation and treatment of the newborn. One of the delivery concerns is ventriculomegaly due to underlying Arnold-Chiari malformation, which may pose practical difficulties. It remains controversial whether a Cesarian birth is indicated in order to reduce the likelihood of rupturing a meningocele or encephalocele sac, causing further neurological damage.

In utero treatment may be available for some types of congenital anomalies as part of a clinical investigative trial. Currently, in utero surgery is being done on fetuses with spina bifida and fetuses with diaphragmatic hernias on an experimental basis at several centres in the United States. It is not yet clear whether there will be any long term benefits to in utero treatment, both with respect to maternal complications and acceptable newborn outcomes.

What is being done to monitor outcomes of prenatal screening and diagnosis programs?

All prenatal diagnosis programs in Canada have built-in quality control activities. Embryofetal pathology assessment is done on all aborted fetuses with parental consent. Morbidity and mortality reviews assess the performance of prenatal screening programs. The limitation of these programs is that they rely on the delivering physicians and specialists to report back to the prenatal programs regarding the outcome of term infants. No mechanism is currently in place for verifying the outcome of pregnancies with normal prenatal screening tests. Collaboration among prenatal genetics, prenatal obstetrics and other related services would allow for better monitoring of the effects of prenatal screening and diagnosis programs.

At the national level, the Canadian Congenital Anomalies Surveillance System (CCASS) monitors the birth prevalence of congenital anomalies in live births and stillbirths, using hospitalization data and data from the Alberta Congenital Anomalies Surveillance System (ACASS). ¹⁹ CCASS cannot ascertain fetal anomalies from pregnancies less than 20 weeks' gestation. At the provincial/ territorial level, only Alberta routinely or regularly reports on these anomalies. ⁵¹ As a result, special efforts are required to determine the national impact of secondary prevention of NTDs through the detection by MSAFP screening and fetal ultrasound resulting in selective termination of pregnancies.

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