Chromosome Abnormalities and Genetic Counseling , 3rd Edition

22.Parental Age Counseling and Screening for Fetal Trisomy

The maternal age association in Down syndrome (DS) was known long before its chromosomal basis. In 1909 Shuttleworth wrote that “with regard to parentage … the outstanding point is the advanced age of the mother at the birth of the child … the next point that strikes one is the large proportion of Mongol children that are lastborn, often of a long family.” He considered that either age or parity could be an etiologic factor. Subsequently, Penrose (1933, 1934) demonstrated that it was the mother's age that was the key factor. A powerful insight into the actual nature of the maternal age effect has been afforded by Battaglia et al.'s (1996) study of normal women, showing that the physical basis of the oocyte's meiotic apparatus deteriorates with age (and see Color Fig. 2-7).

Sherman et al. (1994) stated that “increasing maternal age is one of the most important factors in human reproductive failure, as well as being a leading contributor to mental retardation among live-borns.” Hassold et al. (1993) commented that “the association between increasing maternal age and trisomy is arguably the most important etiologic factor in human genetic disease. Nevertheless, we know almost nothing about its basis.” Likewise, Wolstenholme and Angell (2000) observed, “there is still no consensus of opinion as to how aneuploidy arises in man and there is a surprising lack of understanding of the basic mechanism(s) of the well-established links to maternal age.” Some suggested factors are outlined in chapter 2 (p. 31).

The maternal age effect in DS, whatever it may be, operates upon oogenesis, predisposing to nondisjunction of chromosome 21 predominantly at the first meiotic division. In more general terms, segregation of some other chromosomes is vulnerable to the maternal age effect; thus, older women who are pregnant run an increased risk for having a pregnancy with 47,+13, 47,+16, 47,+18, 47,XXX, and 47,XXY, as well as 47,+21. There is also a slight maternal age association with disorders due to uniparental disomy (Ginsburg et al., 2000), this point being discussed in more detail in Chapter 20. Advanced maternal age, or to use the preferable expression of Ginsburg et al., “mothers at advanced childbearing age,” is a common indication for prenatal diagnosis, although it is becoming less predominant because of the impact of maternal serum screening.

Paternal age generally does not usefully enter the equation, at least with respect to autosomal aneuploidies. Fathers of DS children are older than average, but simply because couples are usually of similar ages, a point determined some 70 years ago by Penrose (1934). Concerning gametic studies in older men, numerous sperm analyses have been done, with somewhat conflicting findings (Robbins et al., 1997b; Shi and Martin, 2000c; Eskenazi et al., 2002). Some have shown slight increases in some autosomal disomies, and some have shown increases in sex chromosome disomies, with XY disomy being more consistently noted. Other studies report no significant differences in at least autosomal abnormalities comparing older and younger men (one group even used testicular sperm from men in their eighties [Guttenbach et al., 2000]). By no means is XY disomy always raised. For example, in a study of men under 30 and over 60 years of age drawn from general populations, Luetjens et al. (2002) found no differences in XY disomy with respect to age. It should also be noted that XY recombination (its absence being a risk factor for nondisjunction) occurs no less frequently in older than in younger men (Shi et al., 2002). In one study of men who had actually fathered a child with XXY Klinefelter syndrome, the frequencies of 24,XY sperm did increase with increasing age of the father, although it may be an uncommon individual predisposition, rather than a primarily age-related factor, that was substantially responsible for this (Lowe et al., 2001; Eskenazi et al., 2002).

AGE COUNSELING FOR FETAL TRISOMY

Risk Figures according to Maternal Age

How old is “older,” and what is “advanced” maternal age at childbearing? Conventionally, the mid–late thirties is taken as the boundary. The risk curve for DS, the major condition of concern, begins to steepen at this period. Risk figures for individual ages with respect to this and other aneuploidies have been collected in various jurisdictions and estimates refined according to certain statistical assumptions, and the information from these studies has long been used as the basis of preconceptional and prenatal genetic counseling. These data are also useful in screening programs for fetal trisomy (see below), the woman's age-related risk being an important datum to be included, along with the various laboratory test results, to derive her overall risk estimate.

For trisomies 13, 18, and 21, spontaneous abortion is more likely than for a normal conceptus. Thus, the prevalence of chromosome abnormality is greater at the time of prenatal diagnosis than at term, and we need access to specific figures. Looking through the three windows of observation—at chorion villus sampling (CVS) (10–11 weeks), at amniocentesis (about 15–17 weeks), and at term—the frequency of chromosomal abnormality, for a particular maternal age, progressively reduces. For trisomy 21, it is estimated that about 30% of all pregnancies existing at the time of CVS abort between then and term, and 24% abort during the period from amniocentesis to term (Spencer, 2001) (Table 22-1 and Fig. 22-1). Trisomies 13 and 18 (and monosomy X) have high rates of fetal lethality, with the majority of pregnancies aborting (Table 22-1). For XXX and XXY, in contrast, there appears to be very little selective loss in the latter part of pregnancy.

Down Syndrome

The largest body of data to be collated for the age-related risk of trisomy 21 is that of Morris et al. (2002), who examined records from a 10-year period, 1989–1998, in England and Wales. We have used their material as the basis of the age-related live-birth figures presented in Table 22-2, as probably the best available, although in fact the estimates for younger women (up to age 34 years) have been very similar in all studies, and quite similar in the 35–44 year age bracket (Morris et al., 2003). However contrary to previous interpretations, according to the Morris et al. data the risk of having a baby with trisomy 21 may actually fall (or at least not increase) in the late forties, the peak risk occurring at 43–47 years. This might reflect a greater tendency to miscarry an abnormal fetus in women in their late forties and early fifties, or hypothetically a “meiotic robustness” in some women of this age who are able to achieve pregnancy. Morris et al. do caution, however, that with so few data from this age group, the statistics are insecure (and rather susceptible to misreporting of age). Estimates for the risks of detection of trisomy 21 at prenatal diagnosis, at the times at which the procedures would be done, are given in Table 22-3, up to age 44. It would be prudent to presume that the figures for those above 44 years would be no less than those given for this age, and indeed Hook's (1992) data suggest a continuing increase with age from 44 to 49.

The only systematic calculations from non-Caucasian ethnic groups come from China, Japan, and South America. In a study encompassing nine South American countries, Carothers et al. (2001) demonstrated incidence data and maternal age correlations very similar to those recorded from other jurisdictions. In Japan, Yaegashi et al. (1998) collected data from four clinics, comprising, in all, 5484 pregnancies of women 35 years and older. The risks for trisomy 21 (and for aneuploidies) overall were, on the face of it, somewhat less than in a European population. The raw figures did, however, fluctuate somewhat, with rather small numbers of affected fetuses at each age category. A question might be raised whether some cases could have been excluded from ascertainment by earlier screening and not otherwise recorded. It may be premature to suppose that aneuploidy rates could differ to any important degree between Japanese and other races, a view that is supported by the observation of otherwise no significant differences in a Hong Kong population (Lau et al., 1998).

Other Aneuploidy

The figures for DS are of most interest, as this condition (1) produces a major mental handicap, (2) implies a major burden for parents in that survival well into adult life is now the norm, and (3) is the most common single chromosome defect in newborns. But the data for other aneuploidies are important. Women seeking advice on their age-related risk and considering prenatal diagnosis should also know that some other rather uncommon trisomies of severe effect (13 and 18) might be detected. Also, there are some age-related sex chromosome aneuploidies (XXX, XXY) that have much milder, but not trivial, effects. Tables 22-4 and 22-5 set out age-related risk estimates for these other categories of aneuploidy. There is also the possibility, irrespective of maternal age, that some other type of chromosome defect might exist. Table 22-6 sets out the risk for any chromosomal defect, whether maternal-age associated or not, to be detected at prenatal diagnosis. To put these figures into some perspective, we remind the reader that the prevalence of unbalanced chromosomal abnormality in the whole newborn population is approximately 0.4%, or 1 in 250 (Table 1-3). Another window of observation afforded in recent years is at preimplantation diagnosis, and increasing rates of aneuploidy seen in biopsied embryos, according to the mother's age, are demonstrated in Table 22-7.

No Parental Age Effect in Some Defects

There is no discernible increasing risk with increasing maternal age for the following chromosomal abnormalities: de novo rearrangement, XYY, triploidy, and unbalanced karyotype due to transmission of parental translocation.¹ For monosomy X the risk actually lessens with increasing maternal age. With no firmly proven paternal age associations, advanced paternal age is not of itself a particular indication for chromosomal prenatal diagnosis, although a case might possibly be made for a much older (sixties and older) father.

Secular Changes in Maternal Age Distribution and Down Syndrome Prevalence

Changing maternal age profiles in a population will influence the birth prevalence of DS. In the England of Shakespeare's time, few women lived long enough to bear children in older age, and along with the effects of poor survival in DS, perhaps no more than 100 individuals with trisomy 21 then existed in that country, in a total population of 4 million (Berg and Korossy, 2001) (nevertheless, Levitas and Reid, 2003, were able to record a number of probable and possible depictions in art from centuries past). In New Zealand in the 1920s, maternal mortality was much less of an issue but family planning was rudimentary, and about 45% of all mothers were aged 30 and over. The great majority (about 90%) of all DS babies from that period, at least those surviving to the 1960s to have a chromosome study, were born to mothers in this age group. Over the next four decades, contraceptive practices became gradually more widespread. By the late 1960s, most women were completing their families while still in their twenties, and older mothers made much less contribution to the overall birth rate. Only 20% of all mothers were 30 and over; and the proportion of all DS babies born to this age group had fallen to 53% (Gardner et al., 1973a). We suppose, therefore, that the birth prevalence of DS in New Zealand progressively fell over the period 1920–1970.

Hook (1992) has reviewed the prevalences of DS in various parts of the world during the early 1980s, in relation to the proportions of mothers aged 35 and older. The former Czechoslovakia had the lowest proportion, 3.6%, of older mothers, and Northern Ireland, at 11.1%, the highest. As expected, the observed rates of DS births showed a relationship to older maternal age, with 1.06‰ in Czechoslovakia, and 1.60‰ in Northern Ireland. In the 1980s and 1990s, there has been a reversal of the maternal age trend in several parts of the world, with older mothers now closing the gap on their younger counterparts. In South Australia, for example, after falling to a low around 1975–78, the fraction of mothers over age 35 years has progressively risen, and the birth prevalence of DS was anticipated to rise from a low point of about 0.9‰ in the late 1970s to greater than 1.5‰ in 1990–94 (Staples et al., 1991). In Israel, maternal age dipped in 1978 to a low of 8% of Jewish mothers being 35 or older, and rose to 17% by 1992 (Shohat et al., 1995). These trends have continued in most affluent countries since that time.

The birth prevalence is considerably influenced by the use of prenatal diagnosis and selective pregnancy termination. In Denmark, Hook observes that with 7.1% of mothers being 35 and over, a rate of 1.35‰ was to be expected; but in fact the DS prevalence is 1.02‰. He concludes that prenatal diagnosis has led to a 25% reduction in prevalence. Likewise, in England and Wales, 14% of potential DS births were avoided by selective abortion over the period 1974–1987, reducing the birth prevalence from 1.26 to 1.08‰ (Cuckle et al., 1991). The South Australian figures noted above are estimates of the birth prevalences had termination not been used; in fact, the actual prevalences are correspondingly less (Cheffins et al., 2000). Caruso et al. (1999) analyzed the impact of prenatal diagnosis in Boston in reducing the proportion of trisomic 21 fetuses that are liveborn at term, comparing the two time frames of 1972–1974 and 1979–1994. In this jurisdiction, 86% of couples having a prenatal diagnosis chose to terminate the pregnancy. In Paris, of all cases of trisomy 21 identified over the period 1992–1997, pre-and postnatal, 71% were due to terminations (de Vigan et al., 1999), attesting to the widespread application of prenatal diagnosis in that city. The overall detection rate has risen with the institution of pregnancy screening. In Victoria, Australia, for example, the combined pre-and postnatal prevalence rose from 1.03‰ in 1989 to 1.56‰ in 2000, with the major rise observed in the mid 1990s, during which time screening was being instituted. However, the postnatal component over the 1989–2000 period fell from 0.75‰ to 0.6‰ (Riley and Halliday, 2002).

Some centers offer a slightly different picture, with the shift to the right in the maternal age curve counteracting the influence of prenatal testing. In Switzerland, the mean maternal age rose from 26 years in 1980 to 30 years in 1996, and the incidence of DS remained practically unchanged (Mutter et al., 2002). Prevalence is also influenced by the greater survival of children with DS in recent decades. The survival figure to age 1 year for Western Australia has risen from 83% in those born during 1966–1975 to 94% for the period 1991–1996. Survival to age 10 years is now 85% (Leonard et al., 2000).

SCREENING FOR FETAL TRISOMY

Down syndrome is a preventable condition, in that prenatal diagnosis (amniocentesis or CVS) is possible, and a known abnormal pregnancy can be terminated. Definitive prenatal diagnostic procedures could not realistically be offered to the whole population of pregnant women, but screening allows a subgroup at increased risk to be identified, who then can be offered definitive testing. The screening tools are the taking of a blood sample, and the performing of an ultrasonogram. Certain biochemical markers in the mother's serum may have altered concentrations, whether increased or decreased, if she is carrying a trisomic pregnancy; presumably, these differences reflect perturbation in the trisomic fetoplacental unit. Subtle differences in fetal morphology are also detectable on ultrasonography. The screening approach differs according to the timing in pregnancy, whether in the first or second trimester (or possibly both: “integrated screening”). If the calculated risk is greater than that of a certain threshold risk figure (usually taken as 1 in 250), the pregnancy is regarded as being at increased risk, and definitive testing is then offered. Since other aneuploidies can also influence the measured indices, the test procedure in practice becomes broader than just a trisomy 21 screen.

Second-Trimester Biochemical Screening

The analytes currently measured in second-trimester screening include α-fetoprotein (AFP), estriol, the β component of human chorionic gonadotrophin (hCG), and inhibin-A. An assessment is made of the degree to which each level differs from expectation, and these data are factored into an algorithm that takes into account the prior risk due to maternal age. Sophisticated computer packages are employed to calculate an overall risk figure. The proportions of trisomy 21 and other aneuploidies detected by this approach are approximately equal (Sheridan et al., 1997). About a quarter of women aged 37 and older will get an increased risk result, and the remainder a low risk result; as noted above, some in this latter group might then choose to forego amniocentesis.

Second-Trimester Ultrasonography

Several “soft signs” on second-trimester ultra-sonographic fetal assessment point to an increased likelihood for DS. An advantage is that this procedure is often done routinely, as part of normal obstetric management, and thus a DS screen can be added on essentially at no additional cost. However, the observations do not lend themselves to a ready analysis in terms of adjusting the level of risk; further, the frequency of these “soft signs” in normal fetuses leads to a high false-positive rate. In contrast, the recently described association of absence of the nasal bone with DS appears to demonstrate a rather powerful positive and negative predictive value (Bromley et al., 2002; Cicero et al., 2002).

First-Trimester Ultrasonographic Screening

Ultrasonographic scanning is more precise, in terms of enabling a measurable risk estimate for fetal DS, in the first trimester, during the window of 11–14 weeks² inclusive. One specific parameter is assessed: the degree to which the skin at the neck is separated from the underlying tissue by fluid. Since this fluid does not reflect the sound wave on the scan, it is referred to as “nuchal translucency.” An increased nuchal translucency is associated with DS. Absence of the fetal nasal bone has promise as another useful marker for DS and other aneuploidy, according to preliminary data (Zoppi et al., 2003).

First-Trimester Combined Ultrasonography and Biochemical Screening

A better detection may be achieved through a combination of first-trimester nuchal translucency assessment and the measurement of certain analytes in the maternal serum. At present, the two first-trimester analytes most commonly used are β-hCG and pregnancy-associated plasma protein-A (PAPP-A), the former typically high and the latter low in a DS pregnancy. If the blood test is done first, these results can be held pending the ultrasound, and the combined figure can be available shortly after the scan is done. Detection rates may be as high as 85%–90%, for a false-positive rate of 5% or less (Cuckle and van Lith, 1999; Spencer et al., 1999).

Integrated Screening

In theory, the best detection rate could be achieved from a combination of first- and second-trimester screening, as high as 94%, for a 5% false-positive rate, the result offered as a single report following the second-trimester testing (Wald et al., 1999). In practice, from a 3½-year study in Hong Kong, Lam et al. (2002) reported an 86% detection rate, which, while less than 94%, was a better figure than that achieved with either first- or second-trimester screening alone. Adding in the factor of presence/absence of the nasal bone should improve the figures yet more: potentially retaining 90% detection but reducing the false-positive rate to 0.5%, or achieving a 97% detection for a 5% false-positive rate (Cicero et al., 2003). Integrated screening needs to be approached carefully: there are subtle complexities and potential for confusion in advising those women who have already been given the result of a first-trimester test and who then go on to have second-trimester screening (Hackshaw and Wald, 2001a,b).

Couples achieving a pregnancy by in vitro fertilization (IVF) may find screening particularly attractive, since it involves no invasive procedure that might put at risk a pregnancy in which there has been so much investment (Meschede et al., 1998b). In assessing the degree of risk for aneuploidy, note that it is the age of the woman (whether of the pregnant woman herself or a donor) at the time the ovum was collected that counts, not her current chronological age.

Interpretation of Screening Results

What do these various figures mean? A little epidemiology is in order. Imagine a group of 10,000 pregnant women, of all ages. Assuming a birth prevalence for DS of 1.2‰, we can take it that 12 would otherwise give birth to a baby with DS. If the particular screening approach has a detection rate of, say, 85%, 10/12 of these DS pregnancies would recognized as being at increased risk, and could be identified at prenatal diagnosis. The remaining 15% who are carrying a DS fetus (2/12) would fail to be recognized. If the false-positive rate is, say, 4%, 400 women would have an increased-risk report following screening, but followed by a normal result from the amniocentesis or CVS. Putting these figures in the conventional format, we have the following:

The sensitivity (detection rate) of the test is 10/12 (~85%). Thus, 15% of women with a trisomic 21 fetus will be missed by the test. The positive predictive value of the test is only 10/410 (2.4%).³ Thus, 97.6% of women returning an increased-risk result will not have a DS baby. The negative predictive value is 9588/9590 (99.98%); in other words, a low-risk result means a 99.98% chance for an unaffected baby. Each service needs to determine its own detection and false-positive rates, and should continuously review these.

The Understanding of Women Who Have Screening

The interpretation of a maternal serum test result to the patient is fraught with potential for confusion. The major pitfall is that an increased-risk test result may sometimes be understood by the woman and her medical advisor to mean that the pregnancy is likely to be affected. As we showed above, the great majority of women screening positive will yet go on to have a normal baby. Counselors doing this work need a clear awareness of these issues, so that they can enable their patients to understand, intuitively or explicitly, the concept and relevance of a low positive predictive value. The counselor is referred to Macintosh's (1994) essay “Perception of Risk” for a very readable and practical commentary upon these issues, and to Marteau and Dormandy (2001) for an overview of the complexity of issues. The ideal is that anyone having a screening test for DS should have a basic awareness of the condition, and of the rationale of the screening procedure. That women's understanding is less than complete may be drawn from the analysis of Marini et al. (2002), who made the seemingly incongruent observation that 52% of women of advanced childbearing age who returned an increased-risk screen nevertheless decided against amniocentesis, while 13% with a low-risk result chose in favor of the procedure. More directly, Chilaka et al. (2001) showed that only 30% of women being offered screening in the English city of Leicester had a good understanding of what DS is, with non-Caucasians and poor English speakers being the least well informed. Coming to grips with the numerical expression of risk is not necessarily straightforward, and Grimes and Snively (1999) show that rates (e.g., 2.6 per thousand) may be better understood than proportions (1 in 384).

Concerning the facts about DS itself in the context of pregnancy screening, leaflets are the simplest means of conveying information, and many clinics and jurisdictions produce their own material, the quality of which may vary considerably (Murray et al., 2001). Videotapes of pertinent information may be helpful (Hewison et al., 2001). It is a fine matter to judge what should be the level and tone of the information. Bryant et al. (2001) reviewed the leaflets produced in a number of clinics in the United Kingdom, and considered that the viewpoints expressed were, in the main, weighted unduly negatively toward DS. It is true that information ought to be couched in such terms that it will be useful (in the fullest sense of that word) to the wide range of people for whom it is intended (and see p. 15). Equally, the comment can be made that attempting to neutralize negative aspects of DS may send a mixed message, since being given the option of abortion to avoid having a DS child rather plainly implies that having such a child may not be a desirable outcome. The view that is offered should be clear, accurate, and even-handed.

Effect of Screening on Prevalence of Down Syndrome

In several centers the increasing acceptance of maternal serum screening has been associated, as expected, with a reduction in the number of DS babies being born, above that achieved by maternal age–targeted amniocentesis. Cheffins et al. (2000) assessed the situation in South Australia. The fraction of pregnant women having screening rose from 17% at the time of its introduction in 1991 to 76% by 1996. Consequently, the proportion having amniocentesis rose from 6% to 10%, with younger mothers being the main contributor to this rise. The resulting increase in the detection of fetal trisomy 21, and with termination of the affected pregnancies, caused the birth prevalence of DS to fall by more than half—by 60%, to be precise— from 1.05 to 0.42 per 1000 births. This fall took place in spite of a natural increase in prevalence due to the mothers in 1996 being older. A very similar picture is seen in Belgium (Verloes et al., 2001). During 1984–1989, 244 cases of trisomy 21 were diagnosed in the Genetic Centers of Liège and Loverval, 17% of these at prenatal diagnosis. A decade later (1993–1998), of the 294 diagnoses of trisomy 21, the fraction detected prenatally had risen to 56%, and over 90% of these pregnancies had been terminated. Theoretically (and very probably actually), this reduced the birth prevalence from 1.26 to 0.62 per 1000, a fall of just over a half. More broadly, the pick-up rate at prenatal diagnosis for all aneuploidies has increased, due to the identification of increased risk pregnancies afforded by these screening programs.

If screening is not widely offered, an inappropriate section of the population may be targeted for prenatal diagnosis, and the reduction that is achieved in DS incidence will be less. In Denmark over the period 1980–1998, there was a rather high level of CVS and amniocentesis procedures being done, but mostly in lower-risk pregnancies, and this yielded about a 30%–40% reduction in the expected DS incidence. This figure could have been higher, for a lesser number of invasive procedures, had screening allowed a preselection of women at increased risk (Larsen et al., 2001).

Notes

1. Except for a possible association between maternal age and 3:1 disjunction (p. 83).

2. To be precise, the period is from 11 weeks 3 days to 13 weeks 6 days, during which the crown–rump length goes from 45 to 84 mm.

3. The positive predictive value achievable at combined first-trimester screening may be higher, in the vicinity of 8%.