Objective To establish reference ranges for fetal size by gestation in the first trimester as indicated by cephalic, abdominal and limb measurements using trans-vaginal sonography.
Design A prospective descriptive study.
Setting Two antenatal clinics in Milan, Italy.
Subjects 270 women with normal singleton pregnancies at 42–108 days gestation. All had regular cycles and certain dates.
Interventions Vaginal ultrasound examination including inspection of fetal structures and lasting 5–15 min.
Main outcome measures As many of the following fetal measurements as possible according to gestational age and ease: crown-rump length (CRL), biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur and foot lengths.
Results CRL was measured in 183 fetuses, BPD in 201, HC in 162, AC in 160, femur length in 133 and foot length in 62. The best description of the relation of CRL to gestational age was achieved by a quadratic function (y =−3.98 − 0.308×+ 0.0117×2). Mean values thus derived were virtually identical to those obtained by Robinson & Fleming in 1975 with transabdominal static scanning. Fetal BPD, HC, AC and femur length correlated more closely with CRL than gestational age. Reference ranges (mean and 95% data intervals) were constructed for each biometrical measurement in relation to gestational age and CRL. The HC/AC and femur/foot length ratios did not show any significant variation with gestational age or CRL.
Conclusions These data may be useful in the early detection of genetic disorders affecting the growth of fetal head, trunk and limbs. Genetic and/or environmental variables play an appreciable role in the modulation of early fetal growth.