Biparietal diameter (BPD) is one of the basic biometric parameters used to assess fetal size. BPD together with head circumference (HC), abdominal circumference (AC), and femur length (FL) are computed to produce an estimate of fetal weight. After 13 weeks of gestation, the fetal age may be estimated using th biparietal diameter (the transverse diameter of the head, across the two parietal bones), the head circumference, the length of the femur, the crown-heel length (head to heel), and other fetal parameters. The biparietal diameter (BPD) is among the most accurate 2nd trimester measures of gestational age. Measured from the beginning of the fetal skull to the inside aspect of the distal fetal skull (“outer to inner”) at the level of the cavum septum pellucidum, this is one of the basic fetal measurements. Gestational age is usually determined by the date of the woman’s last menstrual period, and assuming ovulation occurred on day fourteen of the menstrual cycle. Sometimes a woman may be uncertain of the date of her last menstrual period, or there may be reason to suspect ovulation occurred significantly earlier or later than the fourteenth day of her cycle. Ultrasound scans offer an alternative method of estimating gestational age. The most accurate measurement for dating is the crown-rump length of the fetus, which can be done between 7 and 13 weeks of gestation. After 13 weeks of gestation, the fetal age may be estimated using the biparietal diameter (the transverse diameter of the head, across the two parietal bones), the head circumference, the length of the femur, the crown-heel length (head to heel), and other fetal parameters [1]. In the present study week wise measurement of fetal biparietal diameter was taken in 282 women in second & third trimester. Then week wise mean values and standard deviation of fetal Biparietal Diameter were calculated. Simple linear regression analysis of the observations was done for estimating gestational age from the measurements of Biparietal Diameter, for total number of cases, in second and in third trimester separately. The regression equations derived for Biparietal Diameter for estimating gestational age in a normally developing fetus increase with gestational age & showed good correlation with gestational age.

Keywords: Biparietal Diameter; Second Trimester; Third Trimester; Fetal Growth; Gestational Age; Ultrasound Scan.