The usefulness of hCG and unconjugated oestriol in prenatal diagnosis of trisomy 18.

OBJECTIVE

To evaluate the usefulness of the two maternal serum markers, human chorionic gonadotrophin (hCG) and unconjugated oestriol (uE3), in the prenatal diagnosis of trisomy 18.

DESIGN

Retrospective evaluation of uE3 and hCG levels at mid-trimester in cases ot trisomy 18 pregnancies identified from a series of women screened for Down's syndrome.

SETTING

From a series of 53,893 women screened in the antenatal centre of University Hospital of Caen (France), 22 cases of trisomy 18 were diagnosed either after amniocentesis for maternal age, elevated risk of Down's syndrome, or fetal abnormalities and/or growth retardation on ultrasound assessment, or after birth. In addition, ll cases of trisomy 18 identified prenatally in two other centres were included.

RESULTS

Individual hCG and uE3 levels for pregnancies with trisomy 18 were significantly lower than in unaffected pregnancies: mean hCG was 0.62 multiples of the median (MoM) and median hCG was 0.5 MoM. uE3 was a much more effective marker than hCG. Mean uE3 was 0.40 MoM and median uE3 was 0.37 MoM. It was observed that screening for trisomy 18 based on selection for amniocentesis with cut-off values of 0.55 for hCG and 0.60 for uE3 would lead to a detection rate of 48% for 0.8% false positive rate. Using cut-off values of 0.70 MoM for each one of the two markers would detect 79% of cases of trisomy 18 with 3% false positive rate.

CONCLUSIONS

Our results confirm that low hCG and uE3 levels observed in the mid-trimester are predictive of an increased risk for trisomy 18. Since most fetuses with trisomy 18 exhibit morphological abnormalities which should be detected following a careful ultrasonographic examination, biochemical screening could help in the detection of those anatomical defects in selecting for scanning a group of high risk women.