Screening for genetic abnormality

First-trimester biochemical screening for fetal chromosome abnormalities and neural tube defects

Alpha-fetoprotein (AFP), unconjugated oestriol (UE3), intact human chorionic gonadotrophin (intHCG), and the free beta subunit of chorionic gonadotrophin (F beta HCG) were investigated in a series of 21 chromosomally abnormal and 14 open neural tube defect pregnancies ascertained from a series of 14,000 prospectively collected maternal serum samples at 6-14 weeks' gestation. In 16 cases of Down's syndrome, significant reductions were found for AFP (0.65 multiples of the normal median) and UE3 (0.67 MOM). IntHCG levels were unaltered (0.97 MOM) but a significant increase was found for F beta HCG (1.96 MOM). Significant correlations were found for AFP and UE3 in the controls and for intHCG and F beta HCG in both the control and the Down's syndrome pregnancies. In a group of five trisomy 18 pregnancies, median MOMs were for AFP 0.71, for UE3 0.34, for intHCG 0.27, and for F beta HCG 0.15. None of 13 pregnancies with open neural tube defects at 8-13 weeks gestation had elevated maternal serum AFP levels, whereas matched second-trimester samples from the same pregnancies at 16-18 weeks gestation all had significantly elevated AFP levels. Thus, biochemical screening for chromosome abnormalities may be practicable in the first trimester using free beta human chorionic gonadotrophin in combination with AFP and maternal age. However, a separate screening protocol using AFP at 15-18 weeks gestation would still be required for effective detection of neural tube defects.

Do familial neural tube defects breed true?

The tendency for sibs affected by non-syndromal neural tube defect (NTD) to have the same type of lesion was assessed retrospectively in a series of 66 affected sibships from the west of Scotland. Different schemes were used to classify the lesions: in the simplest classification into either anencephaly (including anencephaly-spina bifida) or spina bifida there was a tendency for spina bifida to breed true. More detailed description of the NTD in 48 sibships permitted classification according to location on the neuraxis; in this scheme sibs had dissimilar lesions. In 48 sets of affected sibs the lesions were separable into high NTD, which had involvement above vertebral level T12, and low NTD, which did not extend above T12. Low lesions comprised a minority of the total and each one occurred in a sibship with a high lesion. These results do not support the idea that NTDs occurring above and below vertebral level T12 have a different genetic basis.

The changing incidence of neural tube defects in Scotland

The impact of neural tube defects on neonatal surgery has been declining in Scotland over the past two decades. The Scottish statistics for neural tube defects were studied from 1971 to 1988. The incidence of neural tube defects in Scotland has declined from 5.50 to 1.10 per 1,000 births over this period (3.00 to 0.58 per 1,000 births for spina bifida and 2.50 to 0.52 per 1,000 births for anencephaly). Antenatal maternal alpha-fetoprotein (AFP) screening was introduced to Scotland on a wide scale in 1976. The number of terminations for anencephaly peaked in 1980 (85), and for spina bifida in 1981 (70), and both have since declined. The Scottish birth rate has been about 67,000 per year over this period. The declining incidence of neural tube defects is not explained by the effect of antenatal screening and terminations alone. A downward trend was apparent before 1976, and although antenatal screening has had a considerable impact on anencephaly births (peak terminations 89% in 1983), it has had only a modest impact on spina bifida births (peak terminations 53% in 1984). We conclude that the natural decline in incidence of neural tube defects is the major factor in the observed decline in neonatal surgical admissions for these defects.

Patient care before and after termination of pregnancy for neural tube defects

A retrospective study was designed to examine the perception of care in women who had experienced a second-trimester termination of pregnancy (TOP) for a neural tube defect. Women were identified over a 3-year period, 1983-1985. After appropriate consent, 166 women were visited at home between 4 weeks and 7 months post-TOP and interviewed by one experienced interviewer using a structured questionnaire with open and closed questions. The majority (137, 82 per cent) felt satisfied with the care received during screening, prenatal diagnosis, and during the TOP (126, 76 per cent). Patients were less satisfied (63, 38 per cent) with post-TOP care in hospital. On leaving hospital, the post-termination sequelae were mentioned to only 25 (15 per cent) patients, which left 135 (81 per cent) confused and bewildered by the post-partum reactions of their bodies, and by their strong emotions. After-care was perceived as unsatisfactory by 113 (68 per cent). One-quarter (42, 25 per cent) did not have, and were not invited for, a post-termination appointment and thus did not have an opportunity to ask questions or to discuss the fetus. Eighty-six (51.8 per cent) had no visit from any member of the primary health-care team, yet most would have appreciated such a visit. Suggestions for improved management are presented.

Genetic aspects of prenatal diagnosis

With improved control of environmental agents, genetic conditions are now a major cause of residual handicap and mortality in all age groups. Primary prevention of this diverse group of over 5000 distinct disorders is not yet possible and effective therapy is, as yet, available for very few. Hence, the present emphasis on prevention is directed towards the identification and testing of pregnancies at risk in order to allow the option of early selective termination of pregnancy. Although over 400 distinct conditions have already been successfully prenatally diagnosed, the identification of at-risk pregnancies on the basis of a positive family history alone can identify only a minority of affected pregnancies. In contrast, genetic screening during (or before) pregnancy offers a real prospect for detection of a majority of affected pregnancies and hence for reducing the birth frequency of serious genetic conditions. This is exemplified by the impact of the maternal serum alpha-fetoprotein screening programme for neural tube defects and is under active development for autosomal aneuploidies and certain other major congenital malformations. These screening programmes will result in a reduced frequency of mental and physical handicap in the community but their implementation will require a comprehensive team approach with supraregional (or national) funding and co-ordination.