['To evaluate whether the cephalic index (CI) in trisomy 21 (T21) and trisomy 18 (T18) fetuses is different from that in euploid fetuses, and if so, is this difference of clinical utility.', 'Retrospective. Over an 18-month period, patients attending a single centre for routine advanced maternal age amniocentesis were recruited for a prospective study of ultrasound soft markers of aneuploidy. This prospective database was searched for cases with the following criteria: (1) occipitofrontal diameters (OFD) measured at least twice; (2) gestational age between 98 and 126 days either by ultrasound-confirmed menstrual dates or early second- trimester biometry; (3) no major central nervous system abnormalities detected on prenatal ultrasound, and (4) normal fetal karyotype. This constituted the control group. The study group consisted of all cases of prenatally diagnosed T18 and T21 identified in the same time period with criterion 2 as above. The fetuses in the study group had the OFD measured in a blinded fashion from the biparietal diameter images. CI (= mean biparietal diameter/mean OFD) was calculated for all fetuses. Pearson coefficient and regression analysis were used to determine independence of CI to gestational age in the control group. Standard descriptive statistics were used to describe interval data and two-tailed t test was used to compare means between the study and control groups. ROC curves were constructed to evaluate the clinical efficacy of CI for T18 and T21.', 'Five hundred and ninety-seven fetuses were available for analysis. There were 551 fetuses in the control group and 46 in the study group. Within the study group, there were 30 T21 and 16 T18 fetuses. Within the control group, CI was independent of gestational age (R = 0.026, p = 0.922). Mean CI for the control group was 0.802 (SD 0.040) and this was not statistical different from either the T21 group (mean 0.816, SD 0.042, p = 0.067) or the T18 group (mean 0.792, SD 0.057, p = 0.491). Area under the ROC curves was determined for both T18 and T21 and both had poor results (0.545 and 0.598, respectively). When CI was evaluated in the control group according to the two main ethnic groups in the study, there was a trend towards a statistical difference (p = 0.046) between the fetuses of Oriental and Caucasian mothers.'] Is cephalic index a useful sonographic marker for trisomy 21 and trisomy 18?