Second-trimester levels of pregnancy-associated plasma protein-A and free beta-hCG in pregnancies with trisomy 13.

OBJECTIVE: To examine the levels of free beta-human chorionic gonadotrophin (free beta-hCG) and pregnancy-associated plasma protein-A (PAPP-A) in second-trimester maternal serum from pregnancies affected by trisomy 13 and compare these with the known reduced levels of these markers in first-trimester cases in an attempt to better understand the pathophysiology of changes in marker levels in chromosomally abnormal pregnancies between the first and second trimester.

METHODS: Using the Kryptor immunoassay system, we measured free beta-hCG and PAPP-A in 32 singleton pregnancies affected by trisomy 13 between 14 and 20 weeks of gestation. Using medians established in a previous study, these results were compared against 450 normal singleton pregnancies over the same gestational range. The data were combined with data from 82 cases of trisomy 13 previously examined in the first trimester (11-13 weeks) and an analysis of analyte trend was performed.

RESULTS: The median free beta-hCG in multiples of the appropriate gestational median (MoM) in the second-trimester samples was not significantly different from the controls (1.15 (95% CI 0.827-1.651) vs 1.00). The median PAPP-A MoM in the second-trimester samples was significantly lower (p<0.001) than in controls (0.25 (95% CI 0.164-0.373) vs 1.00). Seventy-eight percent of cases were below the 5th centile of normal for PAPP-A. The combined cases in the first trimester had a median free beta-hCG MoM of 0.58 (95% CI 0.454-0.668) and a median PAPP-A MoM of 0.26 (95% CI 0.218-0.320). For PAPP-A, there was no significant change in median across the gestational period of 11 to 20 weeks, whilst for free beta-hCG, there was a significant increase with gestation (r=0.458, p<0.001).

CONCLUSIONS: Although PAPP-A levels are reduced in trisomy 13 pregnancies in the second trimester, this isolated lower marker value is unlikely to be of value in screening for trisomy 13 in the second trimester. The aetiology of reduced levels of PAPP-A in cases with trisomy 13 may be similar to that in cases with trisomy 18, but different from that in cases with trisomy 21 since the temporal pattern in trisomies 13 and 18 are different from that in trisomy 21.