Fetuses affected by homozygous α0-thalassemia are anemic after the first trimester. Due to severe anemia and hypoxia, the fetuses will develop placentomegaly, cardiomegaly, hepatomegaly, subcutaneous edema, ascites, pleural and pericardial effusions, and hydramnios.1–3 Affected fetuses are either stillborn or die soon after birth. Obstetric complications in mothers carrying affected pregnancies are common. Current prenatal diagnosis involves DNA analysis of chorionic villi or amniocytes obtained by chorionic villus sampling or amniocentesis. These procedures are invasive and are associated with a miscarriage rate of 0.15–1% (Eddleman K, Berkowitz R, Kharbutli Y, Malone F, Vidaver J, Flint Porter T, et al. Pregnancy loss rates after midtrimester amniocentesis: the FASTER Trial [abstract]. Am J Obstet Gynecol 2003;189:S111).4,5 Prenatal diagnosis, especially early noninvasive diagnosis, is thus indicated. Our center, which is a tertiary referral center for prenatal diagnosis of thalassemia, has reported the effectiveness of using the prenatal ultrasonic measurements of the placental thickness3 and the fetal cardiothoracic ratio2 to exclude homozygous α0-thalassemia.
Decreased first-trimester maternal levels of both free β human chorionic gonadotropin (β-hCG) and pregnancy associated plasma protein-A (PAPP-A) are associated with an increased risk of problems of placental development that include miscarriage, fetal growth restriction, and preeclampsia.6 These two first-trimester markers are used in Down syndrome screening, along with nuchal translucency measurement, with a sensitivity of up to 85% with a false-positive rate of 5%.7,8 Increased nuchal translucency measurement, although clinically insignificant, had been found in fetuses with homozygous α0-thalassemia.9 It is possible that altered maternal serum levels of PAPP-A and β-hCG would be evident in pregnancies affected by homozygous α0-thalassemia because this condition is associated with hydropic fetal and placental changes from the first trimester of pregnancy.1–3 The objective of this study was to measure the value of first-trimester maternal serum free β-hCG and PAPP-A as predictors of homozygous α0-thalassemia–affected pregnancies.
MATERIALS AND METHODS
The study was conducted at the prenatal diagnostic and counseling department of Tsan Yuk Hospital with approval from the hospital ethics committee. We offered prenatal diagnosis to couples who were α0-thalassemia carriers or had given birth to babies affected by homozygous α0-thalassemia. Multiple pregnancies, fetal anomalies, and chromosomal abnormalities were excluded. The enrollment period was from August 1997 to June 2001.
At the first visit, the nature of the project was explained in detail, and consent was obtained from each woman. The women were offered the current prenatal diagnosis options of chorionic villus sampling at 11–14 weeks or amniocentesis at 16–18 weeks, or undergoing serial ultrasound examination at 12–15, 18–20, and 25–30 weeks. Invasive prenatal testing and hemoglobin study would be done only in the cases with abnormal findings. Chromosomal study and molecular study on thalassemia status were performed on chorionic villi or amniotic cells, or at abortion when obtainable. Cord blood was collected after delivery for DNA studies from all babies of α0-thalassemia carrier couples to ensure that no affected pregnancy was missed. The gestational age was ascertained by ultrasound measurement of the fetal crown–rump length. Maternal blood was collected after consent at the time of ultrasound examination for nuchal examination at 11–14 weeks. Maternal serum was then stored at −30°C for retrospective analysis of maternal serum biochemical markers at a later date.
Free β-hCG and PAPP-A concentrations were measured in stored maternal serum samples obtained at 11–14 weeks of gestation from 22 women with fetuses affected by homozygous α-thalassemia and from 436 controls randomly selected, matching for maternal age, ethnicity, and weight, as well as gestation at blood sampling. The controls traced from the database consisted of healthy women with normal singleton pregnancies who delivered appropriately grown infants at term. Each case and control subject was measured only once during the gestation. Free β-hCG and PAPP-A serum concentrations were measured by AutoDelphia instruments (Perkin-Elmer Life Sciences, Turku, Finland) using time-resolved fluorometry with reagents supplied by the instrument manufacturer.
Data on maternal serum free β-hCG and PAPP-A concentration in the pregnant women whose babies are affected with homozygous α0-thalassemia are compared with those in the control group. In the control group, nonlinear regression was done to describe the relationship between free β-hCG and PAPP-A concentrations and gestational age in days. Each analyte value was then expressed as a multiple of the normal median for gestation. The Wilcoxon rank sum test was used to determine the significance of the difference between the groups on multiples of the median (MoM) values. P < .05 is considered statistically significant. All statistical analyses were performed using Microsoft Excel 97 (Microsoft Corporation, Redmond, WA) and SPSS 10.0 (SPSS Inc, Chicago, IL).
Maternal serum samples collected at a median of 12 weeks (range 11–14) of gestation were available from 22 women whose pregnancies were affected by homozygous α0-thalassemia and 436 matched controls. There were no significant differences between the groups in maternal age, maternal weight, or gestation at sampling by Wilcoxon rank sum test (Table 1). All the cases and controls were of Chinese ethnicity. In the control group, maternal serum free β-hCG concentration decreased with gestation (median free β-hCG = 1378.4 × exp[−0.0367 × gestation in days]), whereas maternal serum PAPP-A concentration increased with gestation (median PAPP-A = 62.25 × exp[0.0455 × gestation in days]). The individual values in the homozygous α0-thalassemia group are plotted on the reference range for gestation for free β-hCG in Figure 1 and for PAPP-A in Figure 2. The median MoM free β-hCG in the homozygous α0-thalassemia group was 1.41 MoM, which is significantly increased (P<.05) compared with the control group (0.92 MoM). The median MoM PAPP-A in the homozygous α0-thalassemia group was 1.16 MoM and in the control group was 1.04 MoM (Table 2). The sensitivities and false-positive rates (receiver operating characteristic curve) for the detection of homozygous α0-thalassemia by measurement of maternal serum free β-hCG and PAPP-A in the first trimester are shown in Figure 3. The detection rate for the condition by maternal serum free β-hCG is 40% at a false-positive rate of 10%.
The findings of this study suggest that maternal serum concentration of free β-hCG at 12 weeks of gestation in pregnancies affected by homozygous α0-thalassemia is significantly increased in comparison with normal pregnancies. Maternal serum concentration of PAPP-A at 12 weeks of gestation is not significantly different from the normal.
Fetuses affected by homozygous α0-thalassemia develop severe anemia in early fetal life. This leads to hydropic changes, including cardiomegaly,2 an increase in placental thickness3 and placental volume.10 The hydropic placental changes are apparent from 10 weeks of gestation before the onset of fetal cardiomegaly.2,3,11 The altered level of maternal serum free β-hCG at 12 weeks of gestation could be a consequence of such hydropic changes in the homozygous α0-thalassemia–affected fetus and placenta. Previous studies of third-trimester hydropic placenta12 and placenta of mothers with diabetes mellitus13 showed increased expression of β-hCG. These placentas12,13 showed concurrent histological and functional immaturity, which results in levels of placental hormones and proteins reminiscent of less mature placentas in the first trimester of pregnancy. There was also evidence to suggest that an increased second-trimester maternal serum hCG level is associated with nonimmune hydrops in euploid pregnancies.14,15 Studies also showed that higher maternal serum hCG or free β-hCG levels were found during second-trimester screening in pregnancies with hydrops fetalis that were affected by Turner syndrome16–18 and Down syndrome19 when compared with similarly affected pregnancies without hydrops fetalis. Because there is a good correlation in serum free β-hCG between the first and second trimesters of pregnancy,20,21 it is perhaps not surprising that first-trimester serum free β-hCG is increased in hydrops fetalis resulting from early onset anemia of homozygous α0-thalassemia. The unaltered level of maternal serum PAPP-A could be accounted for by the altered functional capacity of the hydropic placentas. The exact mechanisms involved for such altered levels of maternal serum free β-hCG and PAPP-A in pregnancies affected by homozygous α0-thalassemia remain unclear. Further studies would be desirable to ascertain the analyte levels at different gestational ages.
The measurement of maternal serum free β-hCG and PAPP-A, with or without nuchal translucency, in an attempt to screen for Down syndrome in pregnancies at risk of homozygous α0-thalassemia may result in a raised false-positive rate. This, in turn, will affect the individual patient's specific risk for fetal Down syndrome, but is relatively irrelevant because the fetuses affected by homozygous α0-thalassemia will die of the disease.
It is cost-effective to run a universal prenatal screening program in an area where both β-thalassemia and α-thalassemia are prevalent.22 Although maternal serum concentration of free β-hCG at 12 weeks of gestation in pregnancies affected by homozygous α0-thalassemia is significantly increased in comparison with normal pregnancies, the sensitivity for this complication is only about 40% for a false-positive rate of 10%. This serum analyte alone may not be clinically useful as a predictor of pregnancies affected by homozygous α0-thalassemia. However, the absence of ultrasound features of fetal anemia (increased cardiothoracic ratio and placental thickness) and hydropic changes, together with normal maternal serum free β-hCG and PAPP-A at 11–14 weeks of gestation, will be reassuring signs of normality for fetuses at risk of homozygous α0-thalassemia and, hence, enable patients to avoid invasive tests in unaffected pregnancies. Future prospective studies would be needed to better understand the findings of this study.
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