American Journal of Medical Genetics 37961-264 (1990)
Maternal Serum hCG and SP1 in Pregnancies With Fetal Aneuploidy Iris Bartels, Marion Thiele, and Mark H. Bogart Institut fur Humangenetik, Uniuersitat Gottingen, West Germany (I.B., M.T.); Department of Medicine, Division of Medical Genetics, University of California, San Diego (M.B.)
In a retrospective study, maternal serum levels of chorionic gonadotropin (hCG) and pregnancy specific pl-glycoprotein (SP1) from 63 pregnancies with aneuploid fetuses were compared to the levels observed in pregnancies with a chromosomally normal fetus. Thirty-eight percent of the abnormal pregnancies had elevated levels (>2.0 multiples of the normal median [MOM])of hCG and 14% had depressed levels (c0.25 MOM). With a false-positive rate of 5%,44%of the 42 fetuses with trisomy 21 would have been detected by elevated hCG levels. With the same false-positive rate, only 21% had elevated SP1 levels. hCG was significantly depressed in 12 pregnancies affected by fetal trisomy 18.
KEY WORDS: Down syndrome, chromosome abnormalities, prenatal diagnosis, chorionic gonadotropin, pregnancy specific pl-glycoprotein INTRODUCTION Various steroids and proteins produced by the fetalplacental unit have been demonstrated to be potentially important for identifying pregnancies a t increased risk of a fetus with a chromosome disorder. The mean maternal serum alpha-fetoprotein level (AFP) [Merkatz et al., 19841 and unconjugated estriol level [Canick et al., 19881have been shown to be significantly lower in pregnancies affected by fetal Down syndrome (DS) than in normal control pregnancies. In the second trimester, maternal serum levels of both chorionic gonadotropin (hCG) [Bogart e t al., 1987; Wald et al., 1988; Bogart et al., 19891and pregnancy specific pl-glycoprotein (SP1) [Bartels and Lindemann, 19881 are increased in pregnancies with fetal DS. The results of the hCG studies Received for publication October 30, 1989; revision received March 9, 1990. Address reprint requests to Mark Bogart, Ph.D., University of California, San Diego Division of Medical Genetics M-039, La Jolla, CA 92093.
0 1990 Wiley-Liss, Inc.
suggest that approximately 60% of affected pregnancies could be detected with a false-positive rate of approximately 5%. The initial study on SP1 showed that serum levels of 24 affected pregnancies were on average twice the median of the normal range. A subsequent study [Wald et al., 19891 found a weaker association between elevated levels of SP1 and DS. The aim of this study was to assess further the diagnostic value ofhCG and SP1 for antenatal screening ‘for fetal chromosome abnormalities.
MATERIALS AND METHODS Maternal serum samples from forty-three pregnancies with fetal trisomy 21, twelve pregnancies with fetal trisomy 18, two pregnancies with trisomy 13, two pregnancies with monosomy X, one with triploidy, one with triple X, and one pregnancy with both trisomy 21 and a n additional sex chromosome (48,XXY, + 21) were collected between the 14th and 24th week of gestation. Gestational age was based on sonograph assessment. Sera from 173 uncomplicated chromosomally and sonographically normal singleton pregnancies of the same stage of gestation were analyzed as controls. Fifty-one samples from normal pregnancies and 42 samples from mothers with a chromosomally abnormal fetus were collected a t the University of California, San Diego. hCG results from 27 of the abnormal pregnancies were reported previously [Bogart et al., 1987, 19891. Serum samples from 122 normal pregnancies and 22 samples from abnormal pregnancies were collected a t the University of Gottingen. An additional 84 serum samples from normal pregnancies were available for SP1 evaluation only. Aliquots of sera were shipped on dry ice from Germany to California and vice versa. All hCG evaluations were performed a t the University of California using the “Tandem-E-hCG Immunoenzymetric Assay” (Hybritech Inc., San Diego, CA) calibrated to the First International Preparation. SP1 was measured a t the University of Gottingen using the “Enzygnost-SP1” immunoenzymetric assay (Behringwerke AG, Marburg, FRG) according to the manufacturers’ instructions. The SP1 median values for unaffected pregnancies were calculated separately for weeks 14115, 16, 17, 18, and 19/20. Because there were too few sera available for estimating reliable median values a t advanced gestational ages, linear regression values to week 25 were
262
Bartels et al.
calculated. The normal median hCG values were calculated for weeks 15, 16, 17, and 18-25 weeks. All hCG and SP1 values are expressed as multiples of the median (MOM)value for normal pregnancies of the corresponding gestational ages. Statistical analysis was performed using the rank sum test, Wilcoxon, Spearman rank correlation test, and chi square test.
RESULTS Figure 1 shows the individual maternal serum hCG and SP1 level for each pregnancy with a n abnormal fetal karyotype expressed in MOMof unaffected pregnancies. Maternal hCG and SP1 values were significantly higher in DS pregnancies than in unaffected pregnancies (Wilcoxon test with normal approximation, hCG: N(0,l) = 6.0, P < SP1: N(0,l) = 2.39, P = 0.017). The median values in the affected pregnancies were 2.18 (hCG) and 1.54 (SP1) MOMfor normal pregnancies. Table I shows the numbers and proportion of affected and unaffected pregnancies with maternal serum levels of hCG and SP1 greater than or equal to specified values for all chromosome abnormalities and trisomy 21 and trisomy 18 separately. The total detection rate for both elevated and depressed hCG (>2.0 MOMor 3.0 MOM), while SP1 values were normal in all but 2 cases. Only few data were available on sex chromosome aneuploidy, trisomy 13, and triploidy. In those sera, MOM values from hCG and SP1 were: trisomy 13: 1.0710.36 and 1.8710.94; 48,XXY, + 21: 2.910.77; 47,XXX: 2.8310.9; 45,X 2.9711.75 and 3.5911.08; triploidy: 0.0710.56. To compare the specificity of hCG and SP1, the 95th centile of the normal range was used as a cutoff (corresponding to 1.83 MOM for hCG and 2.0 MOM for SP1)
(Fig. 1).Using this limit, 38% of the 63 chromosomally abnormal pregnancies and 44% of the DS pregnancies showed elevated hCG levels, while only 18% and 21% respectively would have been detected by SP1 testing. Four cases of DS (10%) with normal hCG values had elevated SP1 levels. Thus, 54% of DS cases had either elevated hCG or SP1. hCG and SP1 levels were positively correlated in unaffected pregnancies (Spearman rank correlation test, r = 0.35, P and DS pregnancies (r = 0.205, P = 0.186). Analysis of the data on control samples from the 17th week of gestation collected a t the two centers showed significantly higher hCG values in the 31 sera from the West German when compared to 67 sera from the United States (P < 0.01).
DISCUSSION Our findings for hCG are consistent with previous reports indicating t h a t maternal serum hCG is elevated in pregnancies with fetal aneuploidy. Wald et al. [1988] report 55% detection for DS at a 2.0 MOM cutoff, and Petrocik et al. [1989] report a 63% detection rate in DS using 2.0 MOM as the cutoff level. The percentages of women carrying a n unaffected fetus and having a serum concentration >2.0 MOM (false-positive rate) were different in these studies. Wald et al. [19881 had a 7.2% false-positive rate, Petrocik et al. [19891 a 14.8% falsepositive rate and we observed a 6.6% false-positive rate in the present study. The detection rate (related to a 5% cutoff level) in each study was 35, 37, and 44%, respectively. The specificity of hCG testing in this study is somewhat less than has been observed in previous studies [Bogart et al., 1987, 19891 perhaps due to the apparent difference in the median values for the normal control women that were combined for this study. Despite the methodological differences in all these studies, i t is apparent that hCG evaluation will detect approx-
hCG ( M O M )
Fig. 1. Maternal serum levels of hCG and SP1 in pregnancies with a chrornosomally abnormal fetus (0 = trisomy 21; A = trisomy 18;0 = others).90th centiles for both hCG and SP1 are shown by the dotted lines.
hCG and SP1 in Aneuploid Pregnancies
263
TABLE I. Number (Percentage) of Chromosomally Abnormal and Unaffected Pregnancies According to MOM Values of Maternal Serum hCG and SP1 Levels Maternal serum hCG levels ~
Maternal serum hCG value (MOM) 50.25 21.0 21.5 22.0 22.5 23.0
No (%) of unaffected pregnancies (N = 183)
No (%) of trisomy 21 pregnancies (N = 43)
No (’3%) of trisomy 18 pregnancies (N = ~~12).. ~
1 ( 0.5) 88 (48) 42 (23) 12 ( 6.6) 7 ( 3.8) 1 ( 0.5)
1 ( 2) 37 (86)* 27 (63)* 19 (44)* 13 (30)* 8 .(19)* .
7 (58)* 2 (16) 1( 8) 1 1 1
~
~~~~~~
.
No (%) of all abnormal pregnancies (N = .63) ~ ~
~
~
~
9 (14)* 45 (71)** 33 (52)* 24 (38)* 18 (29)* 10 (16)* ~~~
~
Maternal serum SP1 levels Maternal serum SP1 value (MOM)
No (%I of unaffected pregnancies ( N = 262)
50.25 2 ( 0.8) 21.0 133 (50.8) 46 (17.2) 21.5 22.0 13 ( 4.9) 4 ( 1.5) 22.5 0 23.0 * P < 0.001,using f test. **P < 0.05 using x -test.
No (9%)of trisomy 21 pregnancies (N = 43)
No (%) of trisomy 18 pregnancies (N = 12)
0 29 (67)* 16 (37)* 9 (21)* 7 (16)* 4 ( 9)*
0 7 (58) 2 (17) 2 0 0
imately 50% of pregnancies with fetal aneuploidy. Increased detection rates have been claimed using additional parameters such a s the free gonadotropin alphasubunit [Bogart e t al., 1987,19891, unconjugated estriol [Wald et al., 1988; Osathanondh et al., 19891, and AFP [Arab et al., 1988; White et al., 1989; Petrocik et al., 19891. Significantly depressed serum hCG levels seem to be useful for identification of women a t risk for having a fetus with trisomy 18, though occasionally a DS pregnancy may also have depressed hCG levels. In our study, a t a 0.5% false-positive rate 58%of trisomy 18 cases and 14% of all aneuploid cases would have been detected by very low (less than 0.25 MOM)levels (Table I). Predictiveness of low hCG levels should be considered in combined multiparameter analyses because it may improve sensitivity and specificity of screening. Previous reports on SP1 levels in maternal serum indicate that SPl levels are elevated in pregnancies with fetal DS [Bartels and Lindemann, 1988; Wald et al., 19891. Our findings confirm this observation. However, the proportion of values above the 90th centile was suggested to be approximately 75% in the initial study [Bartels and Lindemann, 19881,but recently Wald et al. [19891 and Knight e t al. [1989] observed only 31% and 42%,respectively, of DS pregnancies with values above the 90th centile. In this study, we observed that 30% (Fig. 1)of DS pregnancies had SP1 values above the 90th centile. At this time it does not appear that SP1 evaluation substantially improves detection over evaluation of hCG alone. However, the wide variation of percentages of cases detected with SP1 in different studies points to the inadequacy of retrospective studies.
No (%) of all abnormal pregnancies (N = 63) 0 39 (62)** 19 (30)** 11 (18)* 7 (11)* 4 ( 6)*
The underlying cause for the elevated levels of hCG and SP1 is unknown. Wald et al. [19881 suggested that the high concentration of hCG may be due to placental immaturity. This suggests that trisomic trophoblasts produce the greater amounts of hCG characteristic of a n unaffected pregnancy a few weeks earlier in gestation. The same reasoning suggests that a delay in development of the placenta should cause low SP1 levels in affected pregnancies. However, SP1 levels are shown to be high in contrast to the expected lower values for less mature placentas. Further, there is no evidence of immaturity of placental hCG production during the first trimester LCuckle et al., 1988; Bogart et al., 19891 but rather a lack of appropriate decline in hCG levels from the normal first trimester range. Therefore, it is worth considering that alterations of the placental mass or secretory capabilities of the cytotrophoblast may cause the high maternal serum levels of hCG and SP1 in women carrying a DS fetus. In the present study, the distribution of normal control hCG values was discordant between the sera collected in our 2 centers despite the same criteria being used for patient selection. Variations of biochemical values among studies are well known for maternal serum AFP screening [Hook, 19881. Methodologic differences between the assay systems can be excluded as a n explanation for the discrepancies in our study because hCG was measured in all samples by a single investigator. Hook [ 19881 suggested that geographic variation might be a possible explanation €or different results obtained in different countries. Such “geographic” variation can be caused by environmental factors such as smoking and alcohol consumption, frequency of diabetes
264
Bartels et al.
and obesity, and ethnic differences. Detailed data for these possible factors was not available for our 2 study populations. Further studies are currently underway to more carefully evaluate the possibility of ethnic or other causes of variation in normal hCG values. The recent expansion of biochemical screening procedures in various countries should soon provide more data for establishing the diagnostic value of prenatal screening test. At this time, the most practical biochemical screeningprotocol for fetal aneuploidy appears to be a combination of age, hCG, and AFP evaluation [Petrocik et al., 19891.However, exact evaluation of the predictive value of screening test for the recognition of pregnancies with a DS fetus or other chromosome abnormality requires prospective studies on unselected pregnancies.
ACKNOWLEDGMENTS We thank Dr. Dati of Behringwerke AG for kindly providing SP1 assay kits and M. Krawczak for statistical help. This work was supported in part by the Bundesministerium fur Forschung und Technologie (grant No. 0706534). REFERENCES Arab H, Siegel-Bartelt J , Wong PY, Doran T (1988): Maternal serum beta human chorionic gonadotropin (MSHCG) combined with maternal serum alpha-fetoprotein (MSAFP) appears superior for prenatal screening for Down syndrome (DS)than either test alone. Am J Hum Genet 43:A225. Bartels I, Lindemann A (1988): Maternal levels of pregnancy-specific pl-glycoprotein (SP-1) are elevated in pregnancies affected by Down’s syndrome. Hum Genet 80:46-48.
Bogart MH, Golbus MS, Sorg ND, Jones OW (1989):Human chorionic gonadotropin levels in pregnancies with aneuploid fetuses. Prenat Diagn 9:379-384. Bogart MH, Pandian MR, Jones OW (19871:Abnormal maternal serum chorionic gonadotropin levels in pregnancies with fetal chromosome abnormalities. Prenat Diagn 7:623-630. Canick JA, Knight GJ, Palomaki GE, Haddow JE. Cuckle HS, Wald NJ (1988):Low second trimester maternal serum unconjugated oestriol in pregnancies with Down’s syndrome. Br J Obstet Gynaecol 95:330-333. Cuckle HS, Wald NJ, Barkai G, Fuhrmann W, Altland K, Brambati B, Knight G, Palomaki G, Haddow J E , Canick J (1988):First-trimester biochemical screening for Down syndrome. Lancet 2:851-852. Hook EB (1988): Variability in predicted rates of Down syndrome associated with elevated maternal serum alpha-fetoprotein levels in older women. Am J Hum Genet 43:160-164. Knight GJ, Palomaki GE, Haddow J E , Johnson AM, Osathanondh R, Canick JA (19891: Maternal levels of the placental products hCG, hPL, SP1, and progesterone are all elevated in cases of fetal Down Syndrome. Am J Hum Genet 45:A257. Merkatz IR, Nitowsky HM, Macri J N , Johnson WE (1984):An association between low maternal serum alpha-fetoprotein and fetal chromosomal abnormalities. Am J Obstet Gynecol 148:886-894. Osathanondh R, Canick JA, Abell KB, Stevens LD, Palomaki GE, Knight GJ, Haddow J E (19891: Second trimester screening for trisomy 21. Lancet 2:52. Petrocik E, Wassman RE, Kelly J C (1989):Prenatal screening for Down syndrome using maternal serum chorionic gonadotropin (hCG).Am J Obstet Gynecol 161 #5:1168-1173. Wald N, Cuckle H, Densem J (1989): Maternal serum specific beta,glycoprotein in pregnancies associated with Down’s syndrome. Lancet 2:450. Wald NJ, Cuckle HS, Densem JW, Nanchahal K. Royston P, Chard T, Haddow J E , Knight GJ, Palomaki GE, Canick J A (1988):Maternal serum screening for Down’s syndrome in early pregnancy. Br Med J 297:883-887. White I, Papiha SS, Magnay D (1989):Improving methods of screening for Down’s syndrome. N Engl J Med 320:401-402.
Maternal Serum hCG and SP1 in Pregnancies With Fetal Aneuploidy Iris Bartels, Marion Thiele, and Mark H. Bogart Institut fur Humangenetik, Uniuersitat Gottingen, West Germany (I.B., M.T.); Department of Medicine, Division of Medical Genetics, University of California, San Diego (M.B.)
In a retrospective study, maternal serum levels of chorionic gonadotropin (hCG) and pregnancy specific pl-glycoprotein (SP1) from 63 pregnancies with aneuploid fetuses were compared to the levels observed in pregnancies with a chromosomally normal fetus. Thirty-eight percent of the abnormal pregnancies had elevated levels (>2.0 multiples of the normal median [MOM])of hCG and 14% had depressed levels (c0.25 MOM). With a false-positive rate of 5%,44%of the 42 fetuses with trisomy 21 would have been detected by elevated hCG levels. With the same false-positive rate, only 21% had elevated SP1 levels. hCG was significantly depressed in 12 pregnancies affected by fetal trisomy 18.
KEY WORDS: Down syndrome, chromosome abnormalities, prenatal diagnosis, chorionic gonadotropin, pregnancy specific pl-glycoprotein INTRODUCTION Various steroids and proteins produced by the fetalplacental unit have been demonstrated to be potentially important for identifying pregnancies a t increased risk of a fetus with a chromosome disorder. The mean maternal serum alpha-fetoprotein level (AFP) [Merkatz et al., 19841 and unconjugated estriol level [Canick et al., 19881have been shown to be significantly lower in pregnancies affected by fetal Down syndrome (DS) than in normal control pregnancies. In the second trimester, maternal serum levels of both chorionic gonadotropin (hCG) [Bogart e t al., 1987; Wald et al., 1988; Bogart et al., 19891and pregnancy specific pl-glycoprotein (SP1) [Bartels and Lindemann, 19881 are increased in pregnancies with fetal DS. The results of the hCG studies Received for publication October 30, 1989; revision received March 9, 1990. Address reprint requests to Mark Bogart, Ph.D., University of California, San Diego Division of Medical Genetics M-039, La Jolla, CA 92093.
0 1990 Wiley-Liss, Inc.
suggest that approximately 60% of affected pregnancies could be detected with a false-positive rate of approximately 5%. The initial study on SP1 showed that serum levels of 24 affected pregnancies were on average twice the median of the normal range. A subsequent study [Wald et al., 19891 found a weaker association between elevated levels of SP1 and DS. The aim of this study was to assess further the diagnostic value ofhCG and SP1 for antenatal screening ‘for fetal chromosome abnormalities.
MATERIALS AND METHODS Maternal serum samples from forty-three pregnancies with fetal trisomy 21, twelve pregnancies with fetal trisomy 18, two pregnancies with trisomy 13, two pregnancies with monosomy X, one with triploidy, one with triple X, and one pregnancy with both trisomy 21 and a n additional sex chromosome (48,XXY, + 21) were collected between the 14th and 24th week of gestation. Gestational age was based on sonograph assessment. Sera from 173 uncomplicated chromosomally and sonographically normal singleton pregnancies of the same stage of gestation were analyzed as controls. Fifty-one samples from normal pregnancies and 42 samples from mothers with a chromosomally abnormal fetus were collected a t the University of California, San Diego. hCG results from 27 of the abnormal pregnancies were reported previously [Bogart et al., 1987, 19891. Serum samples from 122 normal pregnancies and 22 samples from abnormal pregnancies were collected a t the University of Gottingen. An additional 84 serum samples from normal pregnancies were available for SP1 evaluation only. Aliquots of sera were shipped on dry ice from Germany to California and vice versa. All hCG evaluations were performed a t the University of California using the “Tandem-E-hCG Immunoenzymetric Assay” (Hybritech Inc., San Diego, CA) calibrated to the First International Preparation. SP1 was measured a t the University of Gottingen using the “Enzygnost-SP1” immunoenzymetric assay (Behringwerke AG, Marburg, FRG) according to the manufacturers’ instructions. The SP1 median values for unaffected pregnancies were calculated separately for weeks 14115, 16, 17, 18, and 19/20. Because there were too few sera available for estimating reliable median values a t advanced gestational ages, linear regression values to week 25 were
262
Bartels et al.
calculated. The normal median hCG values were calculated for weeks 15, 16, 17, and 18-25 weeks. All hCG and SP1 values are expressed as multiples of the median (MOM)value for normal pregnancies of the corresponding gestational ages. Statistical analysis was performed using the rank sum test, Wilcoxon, Spearman rank correlation test, and chi square test.
RESULTS Figure 1 shows the individual maternal serum hCG and SP1 level for each pregnancy with a n abnormal fetal karyotype expressed in MOMof unaffected pregnancies. Maternal hCG and SP1 values were significantly higher in DS pregnancies than in unaffected pregnancies (Wilcoxon test with normal approximation, hCG: N(0,l) = 6.0, P < SP1: N(0,l) = 2.39, P = 0.017). The median values in the affected pregnancies were 2.18 (hCG) and 1.54 (SP1) MOMfor normal pregnancies. Table I shows the numbers and proportion of affected and unaffected pregnancies with maternal serum levels of hCG and SP1 greater than or equal to specified values for all chromosome abnormalities and trisomy 21 and trisomy 18 separately. The total detection rate for both elevated and depressed hCG (>2.0 MOMor 3.0 MOM), while SP1 values were normal in all but 2 cases. Only few data were available on sex chromosome aneuploidy, trisomy 13, and triploidy. In those sera, MOM values from hCG and SP1 were: trisomy 13: 1.0710.36 and 1.8710.94; 48,XXY, + 21: 2.910.77; 47,XXX: 2.8310.9; 45,X 2.9711.75 and 3.5911.08; triploidy: 0.0710.56. To compare the specificity of hCG and SP1, the 95th centile of the normal range was used as a cutoff (corresponding to 1.83 MOM for hCG and 2.0 MOM for SP1)
(Fig. 1).Using this limit, 38% of the 63 chromosomally abnormal pregnancies and 44% of the DS pregnancies showed elevated hCG levels, while only 18% and 21% respectively would have been detected by SP1 testing. Four cases of DS (10%) with normal hCG values had elevated SP1 levels. Thus, 54% of DS cases had either elevated hCG or SP1. hCG and SP1 levels were positively correlated in unaffected pregnancies (Spearman rank correlation test, r = 0.35, P and DS pregnancies (r = 0.205, P = 0.186). Analysis of the data on control samples from the 17th week of gestation collected a t the two centers showed significantly higher hCG values in the 31 sera from the West German when compared to 67 sera from the United States (P < 0.01).
DISCUSSION Our findings for hCG are consistent with previous reports indicating t h a t maternal serum hCG is elevated in pregnancies with fetal aneuploidy. Wald et al. [1988] report 55% detection for DS at a 2.0 MOM cutoff, and Petrocik et al. [1989] report a 63% detection rate in DS using 2.0 MOM as the cutoff level. The percentages of women carrying a n unaffected fetus and having a serum concentration >2.0 MOM (false-positive rate) were different in these studies. Wald et al. [19881 had a 7.2% false-positive rate, Petrocik et al. [19891 a 14.8% falsepositive rate and we observed a 6.6% false-positive rate in the present study. The detection rate (related to a 5% cutoff level) in each study was 35, 37, and 44%, respectively. The specificity of hCG testing in this study is somewhat less than has been observed in previous studies [Bogart et al., 1987, 19891 perhaps due to the apparent difference in the median values for the normal control women that were combined for this study. Despite the methodological differences in all these studies, i t is apparent that hCG evaluation will detect approx-
hCG ( M O M )
Fig. 1. Maternal serum levels of hCG and SP1 in pregnancies with a chrornosomally abnormal fetus (0 = trisomy 21; A = trisomy 18;0 = others).90th centiles for both hCG and SP1 are shown by the dotted lines.
hCG and SP1 in Aneuploid Pregnancies
263
TABLE I. Number (Percentage) of Chromosomally Abnormal and Unaffected Pregnancies According to MOM Values of Maternal Serum hCG and SP1 Levels Maternal serum hCG levels ~
Maternal serum hCG value (MOM) 50.25 21.0 21.5 22.0 22.5 23.0
No (%) of unaffected pregnancies (N = 183)
No (%) of trisomy 21 pregnancies (N = 43)
No (’3%) of trisomy 18 pregnancies (N = ~~12).. ~
1 ( 0.5) 88 (48) 42 (23) 12 ( 6.6) 7 ( 3.8) 1 ( 0.5)
1 ( 2) 37 (86)* 27 (63)* 19 (44)* 13 (30)* 8 .(19)* .
7 (58)* 2 (16) 1( 8) 1 1 1
~
~~~~~~
.
No (%) of all abnormal pregnancies (N = .63) ~ ~
~
~
~
9 (14)* 45 (71)** 33 (52)* 24 (38)* 18 (29)* 10 (16)* ~~~
~
Maternal serum SP1 levels Maternal serum SP1 value (MOM)
No (%I of unaffected pregnancies ( N = 262)
50.25 2 ( 0.8) 21.0 133 (50.8) 46 (17.2) 21.5 22.0 13 ( 4.9) 4 ( 1.5) 22.5 0 23.0 * P < 0.001,using f test. **P < 0.05 using x -test.
No (9%)of trisomy 21 pregnancies (N = 43)
No (%) of trisomy 18 pregnancies (N = 12)
0 29 (67)* 16 (37)* 9 (21)* 7 (16)* 4 ( 9)*
0 7 (58) 2 (17) 2 0 0
imately 50% of pregnancies with fetal aneuploidy. Increased detection rates have been claimed using additional parameters such a s the free gonadotropin alphasubunit [Bogart e t al., 1987,19891, unconjugated estriol [Wald et al., 1988; Osathanondh et al., 19891, and AFP [Arab et al., 1988; White et al., 1989; Petrocik et al., 19891. Significantly depressed serum hCG levels seem to be useful for identification of women a t risk for having a fetus with trisomy 18, though occasionally a DS pregnancy may also have depressed hCG levels. In our study, a t a 0.5% false-positive rate 58%of trisomy 18 cases and 14% of all aneuploid cases would have been detected by very low (less than 0.25 MOM)levels (Table I). Predictiveness of low hCG levels should be considered in combined multiparameter analyses because it may improve sensitivity and specificity of screening. Previous reports on SP1 levels in maternal serum indicate that SPl levels are elevated in pregnancies with fetal DS [Bartels and Lindemann, 1988; Wald et al., 19891. Our findings confirm this observation. However, the proportion of values above the 90th centile was suggested to be approximately 75% in the initial study [Bartels and Lindemann, 19881,but recently Wald et al. [19891 and Knight e t al. [1989] observed only 31% and 42%,respectively, of DS pregnancies with values above the 90th centile. In this study, we observed that 30% (Fig. 1)of DS pregnancies had SP1 values above the 90th centile. At this time it does not appear that SP1 evaluation substantially improves detection over evaluation of hCG alone. However, the wide variation of percentages of cases detected with SP1 in different studies points to the inadequacy of retrospective studies.
No (%) of all abnormal pregnancies (N = 63) 0 39 (62)** 19 (30)** 11 (18)* 7 (11)* 4 ( 6)*
The underlying cause for the elevated levels of hCG and SP1 is unknown. Wald et al. [19881 suggested that the high concentration of hCG may be due to placental immaturity. This suggests that trisomic trophoblasts produce the greater amounts of hCG characteristic of a n unaffected pregnancy a few weeks earlier in gestation. The same reasoning suggests that a delay in development of the placenta should cause low SP1 levels in affected pregnancies. However, SP1 levels are shown to be high in contrast to the expected lower values for less mature placentas. Further, there is no evidence of immaturity of placental hCG production during the first trimester LCuckle et al., 1988; Bogart et al., 19891 but rather a lack of appropriate decline in hCG levels from the normal first trimester range. Therefore, it is worth considering that alterations of the placental mass or secretory capabilities of the cytotrophoblast may cause the high maternal serum levels of hCG and SP1 in women carrying a DS fetus. In the present study, the distribution of normal control hCG values was discordant between the sera collected in our 2 centers despite the same criteria being used for patient selection. Variations of biochemical values among studies are well known for maternal serum AFP screening [Hook, 19881. Methodologic differences between the assay systems can be excluded as a n explanation for the discrepancies in our study because hCG was measured in all samples by a single investigator. Hook [ 19881 suggested that geographic variation might be a possible explanation €or different results obtained in different countries. Such “geographic” variation can be caused by environmental factors such as smoking and alcohol consumption, frequency of diabetes
264
Bartels et al.
and obesity, and ethnic differences. Detailed data for these possible factors was not available for our 2 study populations. Further studies are currently underway to more carefully evaluate the possibility of ethnic or other causes of variation in normal hCG values. The recent expansion of biochemical screening procedures in various countries should soon provide more data for establishing the diagnostic value of prenatal screening test. At this time, the most practical biochemical screeningprotocol for fetal aneuploidy appears to be a combination of age, hCG, and AFP evaluation [Petrocik et al., 19891.However, exact evaluation of the predictive value of screening test for the recognition of pregnancies with a DS fetus or other chromosome abnormality requires prospective studies on unselected pregnancies.
ACKNOWLEDGMENTS We thank Dr. Dati of Behringwerke AG for kindly providing SP1 assay kits and M. Krawczak for statistical help. This work was supported in part by the Bundesministerium fur Forschung und Technologie (grant No. 0706534). REFERENCES Arab H, Siegel-Bartelt J , Wong PY, Doran T (1988): Maternal serum beta human chorionic gonadotropin (MSHCG) combined with maternal serum alpha-fetoprotein (MSAFP) appears superior for prenatal screening for Down syndrome (DS)than either test alone. Am J Hum Genet 43:A225. Bartels I, Lindemann A (1988): Maternal levels of pregnancy-specific pl-glycoprotein (SP-1) are elevated in pregnancies affected by Down’s syndrome. Hum Genet 80:46-48.
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