Femur length and trisomy 21: impact of gestational age on screening efficiency

Citation
Rjm. Snijders et al., Femur length and trisomy 21: impact of gestational age on screening efficiency, ULTRASOUN O, 16(2), 2000, pp. 142-145
Citations number
29
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Reproductive Medicine
Journal title
ULTRASOUND IN OBSTETRICS & GYNECOLOGY
ISSN journal
0960-7692 → ACNP
Volume
16
Issue
2
Year of publication
2000
Pages
142 - 145
Database
ISI
SICI code
0960-7692(200008)16:2<142:FLAT2I>2.0.ZU;2-Q
Abstract
Objective This study assesses two methods used to define relatively short f emur in screening for trisomy 21 and examines changes in performance of scr eening with gestational age. Design Retrospective analysis of data on menstrual age, femur length (FL) a nd biparietal diameter (BPD) in 49 trisomy 21 pregnancies and 6069 normal c ontrols. Reference ranges were derived for BPD/FL versus menstrual age and for FL versus BPD. Two methods of defining short femur (BPD/FL) and observe d-to-expected FL ratio) were examined for false-positive rates and detectio n rates for trisomy 21 at different gestational ages. Results In the control group the BPD/FL ratio and its standard deviation de creased with menstrual age. Trisomy 21 was associated with a significantly higher BPD/FL ratio (P < 0.001) and the deviation increased significantly w ith menstrual age (P < 0.05). Eleven percent of 28 fetuses examined at 15-1 7 weeks had a BPD/FL above the 95th centile compared with 24% of 21 fetuses examined at 18-20 weeks (P = 0.40). The median observed-to-expected FL rat io in the control group was 1.0 throughout the gestational age range but th e standard deviation decreased significantly with menstrual age (P <0.01). Trisomy 21 was associated with a significantly reduced observed-to-expected FL ratio (P < 0.001) and the deviation increased significantly with menstr ual age (P <less than> 0.05). A fixed cut-off of 0.91 for observed-to-expec ted FL ratio provided a false-positive rate of 12% at 15-17 weeks compared with 6% at 18-20 weeks of gestation (P < 0.001) with detection rates of 29 and 38%, respectively (P = 0.73). Conclusion Irrespective of the definition used to define the condition, rel atively short femur is a poor marker for trisomy 21 particularly when the a ssessment takes place before 18 weeks of gestation.