Purpose: The authors aimed to determine some practical contributive biometry ratios of the first trimester screening in order to note more accurate measurements and recognize abnormal/mistaken measurements. Materials and Methods: All medical records of singleton pregnancies whose first-trimester screening that was performed between the years of 2004-2010, were evaluated retrospectively. Singleton pregnancies with detected/suspicious anatomical or genetic fetal anomalies, any systemic disease, and familial genetic diseases were excluded. The following ratios were calculated and compared: measurements of biparietal diameter [BPD], head circumference [HC], abdominal circumference [AC], femur length [FL], and crown rump length [CRL] of included fetuses, to each other were calculated. Mean and standard deviations of the ratios were determined for each gestational weeks of 11(0.6), 12(0-6), and 13(0.6). Results: A total of 1,615 singleton pregnancies were included in the data analyses according to exclusion and inclusion criteria. Mean maternal age was 29.5 +/- 4.6 years. Mean gestational age of the fetuses was 12.6 +/- 0.6 weeks. Mean and standard deviation of the ratios were as follows; CRL/BPD: 3.0 +/- 0.2; AC/BPD: 3.0 +/- 0.2; CRL/AC: 1.0 +/- 0.1; CRL/HC: 0.8 +/- 0.1; CRL/FL: 8.8 +/- 1.6; BPD/FL: 2.9 +/- 0.6; AC/FL: 8.9 +/- 1.6; HC/FL: 11.1 +/- 2.2, and HC/AC: 1.3 +/- 0.1. Among these ratios the standard deviation was small in the ratios of CRL/BPD, AC/BPD, CRL/AC, CRL/HC, and HC/AC. The equations of these ratios were derived from linear regression analyses. The AC/BPD, and CRL/AC ratios had lower R2 values than others, indicating a rather constant ratio. Conclusions: The ratios of CRL/BPD, AC/BPD, and CRL/AC seem more practical to be used in the first-trimester fetal ultrasonography practice.