Gender-Specific Reference Charts of Fetal Head Circumference in a Singaporean Population.

INTRODUCTION: With the global outbreak of Zika virus and its association with microcephaly, an up-to-date fetal head circumference (HC) nomogram is crucial to offer a reference standard in order to make an accurate diagnosis. This study was conducted to revise the local fetal HC nomogram. MATERIALS AND METHODS: In this retrospective study, ultrasound data was used for construction of the fetal HC nomogram from a total of 6155 pregnancies in the ethnic Chinese population with low risk profile at KK Women's and Children's Hospital over a 10-year period. Regression model was fitted to calculate the mean and standard deviation of HC at each gestational age (GA). Comparison of HC between ethnic groups (no significant differences) and genders were made. The revised chart was compared with another commonly used reference chart (Hadlock). In an independent test population, different reference charts were used to estimate number of cases with microcephaly. RESULTS: A statistically significant difference of HC between the genders was observed across all gestational ages. Gender-specific reference charts and equation were computed. Our revised fetal HC chart showed a different distribution from the Hadlock chart. Compared with the gender-specific charts, the Hadlock HC chart would significantly under-report microcephaly cases in male fetuses, and tend to over-report in female fetuses. CONCLUSION: This study provides a new set of gender-specific fetal HC charts in the Singaporean population for antenatal ultrasound surveillance of microcephaly.

Validation of first trimester screening for trisomy 21 in Singapore with reference to performance of nasal bone.

INTRODUCTION: The aim of this study is to describe the performance of first trimester screening (FTS) for trisomy 21 using maternal age, serum biochemistry and fetal nuchal translucency (NT) in a single center and to evaluate the effect of nasal bone on screening performance. MATERIAL AND METHODS: In 12,585 singleton pregnancies, the NT and nasal bone were examined. The majority of these mothers also had their serum biochemical markers analyzed. Risk was computed using different combinations of maternal age, biochemistry, NT and nasal bone. Down syndrome cases were confirmed by karyotyping. RESULTS: There were 12,519 normal pregnancies, 31 with trisomy 21 and 35 with other chromosomal abnormalities. Without considering the nasal bone, the combined FTS detected 87.1% of trisomy 21 fetuses (false positive rate 5.1%), using 1:300 as the risk threshold, and this was further improved to 96.8% with the policy that classifies all fetuses with an absent nasal bone as high risk. Subgroup analysis showed that the detection rate would be 90.9%, with a false positive rate of 3.7%, if nasal bone was incorporated in the risk algorithm, compared to 81.8% and a false positive rate of 5.4% if it was not used. DISCUSSION: FTS is very effective in early detection of trisomy 21 in Singapore. The nasal bone is a useful marker that can substantially improve the screening performance.