Down syndrome and maternal age in South Glamorgan

Cytogenetic study in children with down syndrome among kosova Albanian population between 2000 and 2010

Conflict of interest: none declared.

Comparison of models of maternal age-specific risk for Down syndrome live births

OBJECTIVES

To display and compare the different published formulae that specify the association between maternal age and the risk of a Down syndrome live birth.

METHODS

Papers published since 1987 on the prevalence of Down syndrome live births in relation to maternal age were located using MEDLINE and the references given in other papers. The data series and the models fitted to them were plotted to obtain a visual idea of their similarities and differences.

RESULTS

The observed and modelled age-specific rates for Down syndrome births were remarkably similar in all published series of data for women up to the age of 35, were reasonably similar for women aged 35 to 45, but differed for women older than 45.

CONCLUSION

In practice, the overall small differences in age-related risk between the different studies did not materially affect the performance of antenatal screening for Down syndrome. If a choice is to be made, the analysis based on the National Down Syndrome Cytogenetic Register (NDSCR) has marginal advantages since it is based on the largest data set and the corresponding model fits the data well. More data is needed to clarify the pattern of risk with maternal age among women over 45 years of age.

International variation in reported livebirth prevalence rates of Down syndrome, adjusted for maternal age

Reported livebirth prevalence of Down syndrome (DS) may be affected by the maternal age distribution of the population, completeness of ascertainment, accuracy of diagnosis, extent of selective prenatal termination of affected pregnancies, and as yet unidentified genetic and environmental factors. To search for evidence of the latter, we reviewed all published reports in which it was possible to adjust both for effects of maternal age and for selective termination (where relevant). We constructed indices that allowed direct comparisons of prevalence rates after standardising for maternal age. Reference rates were derived from studies previously identified as having near complete ascertainment. An index value significantly different from 1 may result from random fluctuations, as well as from variations in the factors listed above. We found 49 population groups for which an index could be calculated. Methodological descriptions suggested that low values could often be attributed to under-ascertainment. A possible exception concerned African-American groups, though even among these most acceptable studies were compatible with an index value of 1. As we have reported elsewhere, there was also a suggestive increase in rates among US residents of Mexican or Central American origin. Nevertheless, our results suggest that "real" variation between population groups reported to date probably amounts to no more than +/-25%. However, reliable data in many human populations are lacking including, surprisingly, some jurisdictions with relatively advanced health care systems. We suggest that future reports of DS livebirth prevalence should routinely present data that allow calculation of an index standardised for maternal age and adjusted for elective prenatal terminations.

Maternal age specific risk rate estimates for Down syndrome among live births in whites and other races from Ohio and metropolitan Atlanta, 1970-1989

Our primary objective was to estimate, by one year and five year intervals, maternal age specific risk rates for Down syndrome among whites and among other races from two different populations, metropolitan Atlanta and south west Ohio, using live birth and prenatally diagnosed cases ascertained during 1970-1989. The five year estimates were also calculated separately for each of the five four year periods during these 20 years. Additionally, we compared two different methods of estimating these risk rates by using a third population of whites, and compared two different statistical methods of smoothing the risk rates. The results indicate good agreement between the metropolitan Atlanta and south west Ohio estimates within races, but show a statistically significant difference between the two race categories. Because 86% of live births in the "other races" category in the combined population are to blacks, these data may be seen as the first estimates of maternal age specific risk rates for Down syndrome among blacks calculated by one year intervals. We found excellent agreement in the risk rate estimates among the five four year time periods, between the estimates obtained by using the two different methods of estimation, and between the estimates obtained using the two different methods of statistical smoothing. Our estimated risk rates for white women in their 20s strongly reinforce those from previous studies currently being used for genetic counselling purposes. While we did find somewhat higher rates for women under 20, and increasingly higher rates for those over 30 years of age, these differences are not substantial. Thus, this study in general supports the risk rates estimated from data collected mostly during the 1960s and 1970s.

Periodic health examination, 1996 update: 1. Prenatal screening for and diagnosis of Down syndrome. Canadian Task Force on the Periodic Health Examination

OBJECTIVE

To make recommendations to physicians providing prenatal care on (1) whether prenatal screening for and diagnosis of Down syndrome (DS) is advisable and (2) alternative screening and diagnosis manoeuvres.

OPTIONS

"Triple-marker" screening of maternal serum levels of alpha-fetoprotein, human chorionic gonadotropin and unconjugated estriol; fetal ultrasonographic examination; amniocentesis; and chorionic villus sampling (CVS).

OUTCOMES

Accuracy of detection of DS in fetuses, and risks to the mother, including psychologic distress, and to the fetus from the screening and diagnostic interventions.

EVIDENCE

A MEDLINE search for relevant articles published from Jan. 1, 1966, to Mar. 31, 1994, with the use of MeSH terms "Down syndrome," "prenatal diagnosis," "screening," "prevention," "amniocentesis," "chorionic villus sampling," "ultrasonography," "anxiety," "depression" and "psychological stress" and a manual search of bibliographies, recent issues of key journals and Current Contents.

VALUES

The evidence-based methods and values of the Canadian Task Force on the Periodic Health Examination were used. A high value was placed on providing pregnant women with the opportunity to determine whether they are carrying a fetus with DS and to make choices concerning the termination of the pregnancy. The economic issues involved are complex and were not considered.

BENEFITS, HARMS AND COSTS

Triple-marker screening identifies an estimated 58% of fetuses with DS, but it has an estimated rate of true-positive results of 0.1% and of false-positive results of 3.7% (given a risk cut-off of one chance in 190 of DS). These rates vary with maternal age and the risk cut-off chosen. Women with a known risk of having a fetus with DS (e.g., those who have had a previous child with DS) may benefit from a reduction in anxiety after confirmation that their fetus does not have DS. Screening allows women at low risk of having a child with DS to detect fetuses with the syndrome, but may cause psychologic distress if there is a false-positive screening test result. Up to 20% of women with positive results of screening tests may decline to undergo a subsequent amniocentesis. Amniocentesis and CVS are very accurate in diagnosing DS in fetuses and have a very low rate of serious complications for the mother. Amniocentesis is associated with a 1.7% rate of fetal loss when it is performed after 16 weeks' gestation, whereas the rate among controls is 0.7% (for a difference of 1%, 95% confidence interval 0.3% to 1.5%). CVS entails a greater risk of fetal loss than amniocentesis (odds ratio 1.32, 95% confidence interval 1.11 to 1.57). There is little evidence from controlled trials of significant associations between amniocentesis or CVS and neonatal morbidity or malformations; however, samples have been too small to show differences in rare outcomes. Results from some case-control studies suggest that CVS increases the risk of transverse limb deficiency. Costs were not considered because they are beyond the scope of this review.

RECOMMENDATIONS

There is fair evidence to offer triple-marker screening through a comprehensive program to pregnant women under 35 years of age (grade B recommendation). Women given detailed information about serum-marker screening show more satisfaction with the screening than those not given this information. There is fair evidence to offer amniocentesis or CVS to pregnant women 35 years of age and older and to women with a history of a fetus with DS or of a chromosome 21 anomaly (grade B recommendation). Information on the limitations and advantages of each procedure should be offered. Triple-marker screening may be offered as an alternative to CVS or amniocentesis to pregnant women over 35.

VALIDATION

Recommendations concerning prenatal diagnosis are similar to those of the US Preventive Services Task Force, the Society of Obstetricians and Gynaecologists of Canada, the Canadian College of Medical Geneticists and the Cochrane Pregnancy and Childbirth Group. No previous specific recommendations concerning triple-maker screening exist.

SPONSORS

These guidelines were developed and endorsed by the Canadian Task Force on the Periodic Health Examination, which is funded by Health Canada and the National Health Research and Development Program.

Methods for maternal age-standardization of the incidence of congenital abnormalities

In order to compare the birth incidences of particular congenital abnormalities in different populations, it is often necessary to allow for the effects of maternal age. Three age-adjusted indices are defined, based on analogous indices from the literature on mortality studies, namely, the Standardized Mortality Ratio, the Comparative Mortality Figure and Kerridge's Inverse Method. In most practical situations the differences between them are likely to be trival. However, the first index is maximally efficient under multiplicative risk models and is easily adjusted for incomplete data. The second is the only one to provide valid comparisons under additive, as well as multiplicative, models. The third has the advantage that it does not require a knowledge of the maternal age distribution of all births in the population. The use of the three indices is illustrated with published data on Down's syndrome.

The imprecision in rates of Down syndrome by 1-year maternal age intervals: a critical analysis of rates used in biochemical screening

Current algorithms to determine eligibility for prenatal cytogenetic diagnostic services depend critically on the accuracy and precision of the underlying rates of cytogenetic abnormality used in the calculations. We examine the maternal age-specific rates of Down syndrome livebirths in eight studies of European-origin populations, pooled rates from which are widely used for baseline calculations in biochemical screening. These studies vary significantly in such factors as methods of ascertainment of cases, likelihood of complete ascertainment, and methods of correction, if any, for underascertainment. Restriction of analysis to those two studies among the eight whose methods suggest the greatest likelihood of complete ascertainment for Down syndrome generates rates significantly higher than those in widespread use. Confidence intervals about previously reported and currently derived rates indicate that even with large-scale data, there is considerable residual uncertainty in derived rates.

Estimating a woman's risk of having a pregnancy associated with Down's syndrome using her age and serum alpha-fetoprotein level

The risk of an individual woman having a pregnancy associated with Down's syndrome was estimated from her age and her serum alpha-fetoprotein (AFP) level at 14-20 weeks gestation. The estimates were made using published data on the risk of Down's syndrome in relation to maternal age from 4528 affected and over 5 million unaffected pregnancies, and on the risk in relation to serum AFP from 68 affected and 36,645 unaffected pregnancies. Separate estimates were derived for AFP levels using gestational age based on the time since the first day of the last menstrual period and on an ultrasound biparietal diameter measurement. In each case this was done with and without adjusting AFP levels to take account of maternal weight. The same sources of data were also used to construct six Down's syndrome screening policies, each combining information on maternal age and serum AFP. For example with one policy the detection rate would be 28% and would involve selecting 2.8% of unaffected pregnancies for amniocentesis; using age alone the same detection rate could only be achieved by selecting 4.3% of unaffected pregnancies for amniocentesis--an increase of 50%. In general, screening for Down's syndrome using both maternal age and serum AFP is more efficient than either alone.

The distribution of Down's syndrome

A recent increase in the incidence of Down's syndrome has been found, mainly affecting young mothers. It is concluded that this increase is due to the environment, and an association with the contraceptive pill is sought. The cause of non-disjunction is discussed and the probability of prevention suggested.

The incidence of Down's syndrome in Nigeria

Studies of Down's syndrome covering a period of 9 years revealed an incidence of 1 in 865 livebirths in a Nigerian hospital. Cytogenetic analysis in 386 patients showed 369 (95.5%) cases to be the result of regular trisomy 21, and translocation trisomy 21 was found in nine (2.5%) patients. Six (1.5%) patients were mosaics and the remaining two (0.5%) cases were classified as miscellaneous. A high incidence of cases among young mothers was recorded, but a search for environmental factors contributory to non-disjunction in this relatively young age group was unrevealing. The study has shown that Down's syndrome occurs as commonly in Negroes as in other races. Epidemiological studies like this are necessary to heighten the awareness of health planners in communities that have for a long time considered haemoglobinopathies to be the major genetic disorder, in order to prepare the ground for preventive measures.