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Keywords:

  • fetal biometry;
  • fetal growth;
  • free beta-human chorionic gonadotropin;
  • prediction;
  • pregnancy-associated plasma protein-A

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Objective

To determine whether first-trimester maternal serum levels of pregnancy-associated plasma protein-A (PAPP-A) and free beta-human chorionic gonadotropin (fβ-hCG) are independent predictors of second-trimester fetal growth parameters.

Methods

This was a cohort study over a 1-year period involving 594 Chinese women who underwent both first-trimester combined screening for Down syndrome and a routine second-trimester ultrasound examination. Maternal PAPP-A and fβ-hCG levels (expressed in log10 of multiples of median (MoM)), crown–rump length (CRL) (expressed in standardized Z-score (Z-CRL)), and maternal height and weight, were correlated with the Z-score of biparietal diameter (Z-BPD), femur length (Z-FL) and abdominal circumference (Z-AC) measured in the second trimester, using the Pearson test, followed by multiple regression analysis.

Results

Z-BPD, Z-FL and Z-AC were positively correlated with log10 PAPP-A MoM, CRL and maternal height (all P < 0.05), while log10 fβ-hCG MoM was negatively correlated with Z-AC (P < 0.05). After controlling for the effects of CRL, maternal height and weight, log10 PAPP-A MoM was found to be an independent positive predictor of Z-FL (r = 0.797, P < 0.001) and Z-AC (r = 0.305, P = 0.049), and log10 fβ-hCG MoM was an independent negative predictor of Z-FL (r = −0.381, P = 0.023) and Z-AC (r = −0.418, P = 0.002). Neither hormonal level was related to Z-BPD.

Conclusions

First-trimester PAPP-A and fβ-hCG are independent factors that influence subsequent fetal growth. PAPP-A level is positively correlated with FL and AC in the second trimester, while fβ-hCG level is negatively correlated with them. However, BPD is not affected by either of the hormones. Copyright © 2006 ISUOG. Published by John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Intrauterine growth restriction (IUGR) continues to be an important determinant of perinatal mortality and morbidity in modern obstetrics1. Physical evidence of abnormal fetal growth becomes typically apparent in the second half of pregnancy2, although recent studies have suggested that indicators of aberrant fetal growth may be present as early as in the first trimester3. In particular, two recent large prospective studies have shown that reduced pregnancy-associated plasma protein-A (PAPP-A) levels are associated with an increase in incidence of low birth weight (LBW) or delivery of small-for-gestational age (SGA) infants4, 5. Crown–rump length (CRL) measured in the first trimester is also strongly related to both PAPP-A levels6 and birth weight7 but was not taken into account in these studies. Therefore, whether PAPP-A is an independent factor of fetal growth is unknown. Furthermore, PAPP-A is also known to be a protease of various insulin-like growth factor binding proteins (IGFBPs)8. Since the expression and action of these binding proteins may be tissue specific9, PAPP-A may therefore have differential effects on growth of different fetal tissues, as well as the overall fetal weight.

The ability to predict subsequent abnormal fetal growth in the first trimester could enable more appropriate fetal surveillance and management, which potentially might reduce perinatal complications due to fetal growth abnormalities. At present, there is no information concerning the effect of first-trimester biochemical markers on different fetal growth parameters in the mid-trimester of pregnancy. Therefore, the aim of the present study was to investigate the relationship between maternal levels of first-trimester biochemical markers and second-trimester fetal ultrasonographic biometry.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

This was a prospective observational study of subjects attending the first-trimester combined screening program for Down syndrome in a university hospital over a 1-year period. The protocols of the screening program from The Fetal Medicine Foundation (http://www.fetalmedicine.com), including case selection, measurement of nuchal translucency (NT) and biochemical analysis, were strictly followed. Briefly, the test was performed only in pregnancies between 11 and 13 + 6 weeks' gestation (77 to 97 gestational days). An ultrasound examination, using a HDI5000 or HDI3000 machine (Philips Medical System, Seattle, WA, USA), was performed to measure the fetal CRL and NT. A maternal blood sample was taken at the same visit, and the levels of free beta-human chorionic gonadotropin (fβ-hCG) and PAPP-A were measured immediately on site using the Kryptor analyzer (Brahms Diagnostica GmbH, Berlin). The serum levels of PAPP-A and fβ-hCG were converted to a multiple of the maternal weight-adjusted gestation-specific median for the local Chinese population (PAPP-A and fβ-hCG MoMs, respectively)10. Maternal weight and height were also recorded at the time of screening. A routine morphology scan was arranged at 17–23 weeks for all women who indicated that they were going to deliver at our unit. Ultrasonographic biometry including biparietal diameter (BPD), femur length (FL) and abdominal circumference (AC) was performed. The ultrasonographers did not review the first-trimester biochemical report when performing the scan. All data were recorded in a specially designed database.

For the current study, only data from Chinese women with a singleton pregnancy who had both first-trimester screening and second-trimester scans were identified and extracted from the database. Those pregnancies confirmed to have chromosomally or structurally abnormal fetuses were excluded. To minimize the potential effects of uncertain gestational age on the assessment of fetal growth, women with unsure last menstrual period, those without regular monthly (26–31 days) menstrual cycles, or those requiring an adjustment of estimated date of confinement, were excluded.

Statistical analysis

All fetal biometric parameters, including CRL, BPD, FL and AC, were converted to the gestation-specific standard scores (Z-score), based on locally-derived nomograms (Table 1), which have been used in the unit as well as in other units in Hong Kong for the last 5 years. The Z-scores of the ultrasound parameters (Z-CRL, Z-BPD, Z-FL and Z-AC, respectively) were used to correct biometric measurements for differences in gestational age at the time of the ultrasound scan. They were calculated by dividing the difference between the observed value and the gestational-specific mean by the SD ((x–mean)/SD). A Z-score of 1 and −1 indicates that the observed value is 1 SD larger or smaller than the expected value, respectively.

Table 1. Formulae for calculation of gestation-specific means and SDs (in cm) in a Chinese population
ParameterMeanSD
  1. Gest, gestation (weeks).

Crown–rump length−2.468 + 0.172 × Gest + 0.041 × Gest21.253 × (−0.099 + 0.050 × Gest)
Biparietal diameter−1.295 + 0.197 × Gest + 0.008 × Gest21.253 × (0.177 + 0.003 × Gest)
Femur length−4.445 + 0.492 × Gest − 0.007 × Gest21.253 × (0.103 + 0.0003 × Gest)
Abdominal circumference−6.182 + 0.884 × Gest + 0.013 × Gest21.253 × (0.057 + 0.031 × Gest)

The Z-scores and the log conversion of the gestation-specific MoM of both hormones (log10 PAPP-A and log10 fβ-hCG MoMs) were used for further analysis. Pearson's correlation test was used to study the potential relationship between maternal weight and height, the first-trimester parameters (Z-CRL, log10 of both PAPP-A and fβ-hCG MoMs) and the second-trimester growth parameters (Z-BPD, Z-FL, Z-AC). All the variables were then analyzed further using multiple regression analysis with the backward stepwise method to identify independent predictors of second-trimester fetal growth parameters. The Statistical Package for Social Sciences for Windows version 10.0 (SPSS, Inc, Illinois, USA) was used for statistical analysis of all data. P < 0.05 was considered statistically significant.

Power calculation using PASS 2002 (NCSS, Kaysville, USA), assuming an alpha error of 0.05 and power of 80%, showed that the current sample size of 590 was able to detect a correlation coefficient of 0.115 or more.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

During the study period, 594 women fulfilled the inclusion criteria. The mean age of the women was 32 years (SD, 4 years). Sixty-six percent were nulliparous. The mean gestational age at first-trimester screening was 12 + 4 weeks (SD, 4 days), and at second-trimester ultrasound was 19 + 5 weeks (SD, 7 days).

All Z-BPD, Z-FL and Z-AC were positively correlated with Z-CRL, log10 PAPP-A MoM and maternal height, while log10 fβ-hCG MoM was negatively correlated with Z-AC but not Z-BPD or Z-FL (Table 2). There was no correlation between maternal weight in the first trimester and any second-trimester biometric parameter. The relationships between Z-FL, Z-AC, log10 PAPP-A MoM and log10 fβ-hCG MoM are shown in Figures 1 to 4.

thumbnail image

Figure 1. Relationship between Z-FL and PAPP-A. The middle line represents the regression mean while the lower and upper lines represent 95% confidence intervals (2.5th and 97.5th centiles). FL, femur length; MoM, multiples of the median; PAPP-A, pregnancy-associated plasma protein-A; Z, Z-score.

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thumbnail image

Figure 2. Relationship between Z-AC and PAPP-A. The middle line represents the regression mean while the lower and upper lines represent 95% confidence intervals (2.5th and 97.5th centiles). AC, abdominal circumference; MoM, multiples of the median; PAPP-A, pregnancy-associated plasma protein-A; Z, Z-score.

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thumbnail image

Figure 3. Relationship between Z-FL and fβ-hCG. The middle line represents the regression mean while the lower and upper lines represent 95% confidence intervals (2.5th and 97.5th centiles). fβ-hCG, free beta-human chorionic gonadotropin; FL, femur length; MoM, multiples of the median; Z, Z-score.

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thumbnail image

Figure 4. Relationship between Z-AC and fβ-hCG. The middle line represents the regression mean while the lower and upper lines represent 95% confidence intervals (2.5th and 97.5th centiles). AC, abdominal circumference; fβ-hCG, free beta-human chorionic gonadotropin; MoM, multiples of the median; Z, Z-score.

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Table 2. Correlation between second-trimester fetal biometry and first-trimester measurements
First-trimester variableZ-BPDZ-FLZ-AC
Pearson coefficientPPearson coefficientPPearson coefficientP
  1. AC, abdominal circumference; BPD, biparietal diameter; CRL, crown–rump length; FL, femur length; MoM, multiples of the median; PAPP-A, pregnancy-associated plasma protein-A; Z, Z-value.

Z-CRL0.504< 0.0010.560< 0.0010.534< 0.001
Log10 PAPP-A MoM0.212< 0.0010.333< 0.0010.260< 0.001
Log10 fβ-hCG MoM−0.0360.386−0.0590.152−0.0850.040
Maternal height0.1290.0020.1030.0150.0900.033
Maternal weight−0.0370.3760.0770.0630.0170.678

On multiple regression analysis, Z-AC continued to be significantly related to Z-CRL, maternal height, log10 PAPP-A MoM and log10 fβ-hCG MoM. Z-FL not only continued to be significantly related to Z-CRL, maternal height and log10 PAPP-A MoM, but also became negatively correlated to log10 fβ-hCG MoM. However, Z-BPD was only significantly related to Z-CRL and maternal height but not to log10 PAPP-A MoM or log10 fβ-hCG MoM (Table 3).

Table 3. Significant independent predictors of second-trimester fetal measurements as analyzed by multiple regression
Dependent variableSignificant predictorsRegression coefficient (CI)P
  1. AC, abdominal circumference; BPD, biparietal diameter; CRL, crown–rump length; fβ-hCG, free beta-human chorionic gonadotropin; FL, femur length; MoM, multiples of the median; PAPP-A; pregnancy-associated plasma protein-A; Z, Z-value.

Z-BPDZ-CRL0.480 (0.417–0.556)< 0.001
Maternal height0.019 (0.013–0.036)0.001
Z-FLZ-CRL0.722 (0.620–0.824)< 0.001
Maternal height0.017 (0.002–0.032)0.022
Log10 PAPP-A MoM0.797 (0.413–1.182)< 0.001
Log10 fβ-hCG MoM−0.381 (−0.711 to −0.052)0.023
Z-ACZ-CRL0.549 (0.468–0.630)< 0.001
Maternal height0.014 (0.002–0.025)0.024
Log10 PAPP-A MoM0.305 (0–0.609)0.049
Log10 fβ-hCG MoM−0.418 (−0.679 to −0.158)0.002

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Since the discovery of PAPP-A and fβ-hCG, there is increasing evidence that extreme levels of these hormones are associated with IUGR or LBW. However, many of the studies in which they have been investigated have been retrospective with potential bias in the definition of gestational age, one of the most common confounding factors in the prediction of birth weight3. Another potential confounding factor is the association of CRL with both hormonal levels and birth weight7. In our study, we have shown that all of the CRL, PAPP-A and fβ-hCG levels measured in the first trimester are significant independent predictive factors for subsequent fetal growth. We have also minimized the problem of bias due to date by selecting a subgroup of women with a certain date of conception and regular monthly cycles, with a dating scan showing a CRL corresponding to the menstrual dates.

In our study, we have shown that CRL is the strongest predictor of birth weight and is positively correlated with all second-trimester parameters. PAPP-A is positively correlated with FL and AC but not BPD. The specific correlation between PAPP-A and both FL and AC is consistent with that of Smith et al. and Krantz et al. who showed that a lower PAPP-A level is associated with LBW at delivery4, 5, and that of Tul et al. who found that a high PAPP-A is associated with the delivery of a large-for-gestational age baby11. Furthermore, our findings are consistent with the in-vitro studies on the biological function of PAPP-A on somatic growth, in particular, the growth of bone tissue and liver, which are reflected by FL and AC, respectively. PAPP-A has been identified as a protease for IGFBPs, in particular IGFBP-412, and IGFBP-213 and IGFBP-514. As IGFBPs bind insulin-like growth factors (IGF-I and IGF-II), and hence inhibit their interaction with cell surface receptors15, lower levels of PAPP-A would be expected to be associated with reduction in IGF activity and thus growth, and vice versa. Moreover, the location and action of these IGFBPs may be tissue specific9. For example, IGFBP-4 is found abundantly in fibroblasts and osteoblasts9, 15, while the highest level of IGFBP-2 is found in hepatic tissue9. These facts may explain the specific positive correlation between PAPPA-A, FL and AC. In fact, in vitro and in vivo experiments have shown the positive effects of PAPP-A on bone growth by inhibiting IGFBP-416–19. On the other hand, BPD reflects the growth of the brain, which may be regulated via different IGFBPs (e.g. IGFBP-6) that are not responsive to PAPP-A20.

The results of previous studies that investigated the relationship between fβ-hCG and fetal growth were inconclusive. Morssink et al. performed a small case-controlled study comparing 73 SGA pregnancies, 87 pregnancies with a preterm delivery and 292 normal matched controls. They found no association between either fβ-hCG or PAPP-A with subsequent fetal growth restriction or preterm delivery21. Ong et al. studied more than 5000 cases and found no difference in fβ-hCG levels in SGA pregnancies compared to controls22. Smith et al. showed that low fβ-hCG was associated with IUGR in a study population of over 8000. However, this relationship was lost after adjusting for PAPP-A4. Krantz et al., in another study of over 8000 subjects, found that an extremely low fβ-hCG level (< 1st percentile) was associated with a relative risk of fetal growth restriction of 2.7 but the effect was not significant after adjustment for PAPP-A5. A common feature of these studies was that other important determinants of normal fetal size, such as maternal characteristics, were not factored into the analysis. In our present study of a population with very accurate gestational date, we have demonstrated that first-trimester fβ-hCG levels have a significant negative correlation with FL and AC in the second trimester, even after adjustment for the effects of CRL and PAPP-A. This finding is consistent with previous studies that have shown a relationship between a high second-trimester total hCG level with poor fetal growth23–29. The mechanism for the negative correlation between fβ-hCG and growth requires further investigation.

In contrast to most of the previous studies that have examined only the effects of extreme levels of hormones on the incidence of fetal growth abnormalities, we have studied the relationship and effect of these hormones on fetal growth parameters over the whole range of hormonal levels. Indeed, the current study confirmed that the effect of hormones on fetal growth was not a dichotomized phenomenon, but a continuum. However, the regression coefficients between the hormones and the ultrasound parameters are small. For example, it is 0.305 between Z-AC and log10 PAPP-A MoM, i.e. a 10-fold difference in PAPP-A levels is associated with only a 0.305 difference in Z-AC. Therefore, neither hormone could be used individually as a clinical test for subsequent abnormal fetal growth. Nevertheless, this is an important finding because it may enable the development of an algorithm to estimate an individualized risk of future development of fetal growth restriction based on multiple factors, including the first-trimester biochemical markers.

Short femur and humerus lengths in the second trimester are well-known soft markers for Down syndrome30–32. Our finding of a significant association between low PAPP-A and short femoral length in the second trimester could at least partly explain the presence of a short femur in Down syndrome fetuses in the second trimester. Furthermore, adjustment must be made in the algorithm for risk calculation of fetal Down syndrome if first-trimester markers including PAPP-A are integrated with second-trimester markers that include bone length.

Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

CRL and maternal levels of PAPP-A and fβ-hCG measured during the first trimester are independent factors for subsequent fetal growth. PAPP-A is positively correlated with FL and AC, while fβ-hCG is negatively correlated with both of them. However, BPD is not affected by either of the hormones. The clinical value of using these hormonal levels to predict and manage fetal growth restriction requires further investigation. The association between PAPP-A levels and the length of fetal long bones must be adjusted for when both are combined in screening for Down syndrome.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References
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