Fetal age assessment based on femur length at 10-25 weeks of gestation, and reference ranges for femur length to head circumference ratios.
BACKGROUND
The aim of the present study is to establish new reference charts for gestational age assessment based on fetal femur length (FL), and new reference ranges for FL to head ratios at gestational weeks 10-25, and to determine the effect of maternal and fetal factors on these charts.


METHODS
Six hundred fifty low-risk women with regular menstrual periods and singleton pregnancies were recruited to a prospective cross-sectional study after obtaining written consent. FL, outer-outer biparietal diameter (BPD), and head circumference (HC) were measured at 10-25 weeks of gestation. We used regression analysis in order to construct mean curves and to assess the effect of maternal and fetal factors on age assessment.


RESULTS
The new chart for age assessment by means of FL was based on 636 measurements. The 95% CI of the mean corresponded to <1 day. The variation between the mean and the 90th percentile was 5, 6, and 7 days at 13, 18, and 23 weeks, respectively, similar to the results when using BPD or HC. Maternal age modestly influenced gestational age assessment (1.3 days/10 years, P = 0.005), whereas smoking, height, body mass index, multiparity, fetal sex, cephalic index, and breech presentation had no impact. Reference charts for FL to head ratios have been presented. Maternal age, fetal sex, and cephalic index influenced the FL/BPD ratio, whereas only fetal sex influenced FL/HC.


CONCLUSIONS
Fetal age assessment based on FL is an equally robust method as using HC. FL/HC is a more robust ratio to characterize fetal proportions than is FL/BPD.
Johnsen
Synnøve Lian
SL
Department of Obstetrics and Gynaecology, Haukeland University Hospital, N-5021 Bergen, Norway. synnove.johnsen@helse-bergen.no
Rasmussen
Svein
S
Sollien
Rita
R
Kiserud
Torvid
T
D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D003430:Cross-Sectional Studies; D018568:Crown-Rump Length; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging / Q000196:embryology* / Q000254:growth & development*; D005865:Gestational Age; D006801:Humans; D009664:Norway; D015588:Observer Variation; D011247:Pregnancy; D011262:Pregnancy Trimester, Second; D011336:Probability; D011446:Prospective Studies; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D016216:Ultrasonography, Prenatal