A Thai Reference for Normal Fetal Nasal Bone Length at 15 to 23 Weeks' Gestation

Abstract

Objective. The purpose of this study was to establish a Thai reference for normal fetal nasal bone length at 15 to 23 weeks' gestation. Methods. The fetal nasal bone was measured by sonography in 295 pregnant women at 15 to 23 weeks' gestation from May to August 2005. One of the first 4 authors performed 3 measurements for each woman when the fetus was in the midsagittal plane and the nasal bone was close to a 45° or 135° angle to the ultrasound beam. All neonates were examined after delivery to confirm an absence of congenital abnormalities. Results. The median nasal bone length increased by gestational age from 3.6 mm at 15 weeks to 7.3 mm at 23 weeks in a linear relationship. The fifth percentile nasal bone lengths were 2. 5, 3.04, 3.28, 3.64, 4.21, 4.82, 5.69, and 6.13 mm at 15, 16, 17, 18, 19, 20, 21, and 22 weeks, respectively. The median time for measurement was 5 minutes. Conclusions. The nasal bone length in Thai fetuses at 15 to 23 weeks was found to be on average shorter than that in white and African American fetuses. Key words: fetal nasal bone; normal length; Thai reference. University, Hat Yai, Songkhla 90110, own syndrome is the most common chromosomal abnormality in neonates. 1 Invasive prenatal diagnosis has been proposed for populations at risk (mainly women >35 years or who have had a previous child with Down syndrome), but this strategy only detects 20% to 25% of fetuses with Down syndrome.2 Many new tests have been reported to improve the sensitivity of screening, such as maternal serum biochemical studies and sonographic markers.Flattening of the facial profile and a small nose are common in neonates with Down syndrome. 3 Absence or hypoplasia of the fetal nasal bone in children with Down syndrome has been detected by first-and secondtrimester sonography. [4][5][6] In the second trimester, using the fifth percentile of fetal nasal bone length as a cutoff value resulted in a Down syndrome detection rate of 77.7% and a false-positive rate of only 0.7%.

Fetal nasal bone length at 19–26 weeks’ gestation in Vietnam

“…Following the criteria of measurement, a normal Vietnamese reference for fetal NBL at 19–26 weeks’ gestation was identified. Fetal NBL increases significantly with gestational age and BPD, in accordance with previous monosonographer reports . The present 50th percentile of fetal NBL was longer than that noted by Narayani and Radhakrishnan in India, but it was closely matched by Kanagawa et al .…”

“…in Thailand and Chen et al . in China (Table ) . Similarly, for the 5th percentile of fetal NBL, the present findings differed markedly to data from Southern Asia, but corresponded with the data from Eastern and Southeastern Asia (Table ).…”

“…At the second trimester of gestation, there is a higher proportion of nasal bone in the total volume of osteo‐cartilaginous tissue in the fetus with normal karyotype than in Down syndrome fetuses (the ratio is 35–50% of bone tissue in fetal euploidy versus 10–20% in trisomy 21) . Several studies have proposed an association between delayed ossification of fetal nasal bone and Down syndrome, but delayed maturation has also been noted in fetuses without abnormalities, depending on maternal ethnicity …”

“…4 Racial and ethnic origins have previously been shown to influence fetal NBL: [5][6][7] different normal fetal NBL have been reported for Caucasian, African, Northern Asian and South-Eastern subjects. 5,8,9 In Vietnam, the screening program for Down syndrome based on fetal nuchal translucency and combined test at the first trimester of gestation has been implemented since 2007. This program is still considered to be valuable, but Down syndrome continues to be the most common chromosomal abnormality in Vietnamese neonates, 10 with more than 800 babies born each year.…”

Fetal facial sonographic markers for second trimester Down syndrome screening in a Thai population

“…Most studies that have evaluated the FMF angle for DS have been conducted in white populations [7]. Ethnic differences in facial structure between white and Thai populations have been reported: the nasal bone length (NBL) of Thai fetuses was found to be shorter compared with that of white and African American fetuses [8]. Therefore, the study using a white population may not be applicable to the Thai population.…”