Accurate dating sonogram
The BPD in third trimester is not reliable and be useless when the patient pass 30 weeks and the BPD has to be side with other measurements when we take it in later trimesters to emphasize the normal growth of fetus and avoid wrong measurement of ultrasound.Keywords: Ultrasonography, GA, BPD, Third Trimester1. Introduction The accurate dating of pregnancy is critically important for pregnancy management from the first trimester delivery, and is particularly necessary for determining viability in premature labour and in post-dates deliveries  .Women who participated in the study were selected on following criteria: Regular menstrual cycles, known date of last menstrual period and previous live normal neonates in multipara.
while 84.61% of pregnant women have variable different by weeks in gestational age between ultrasound measurements and last menstrual period calculation this variable could be due to either the patient is given sonographer the wrong day of last menstrual period or the patient doesn’t have any previous scan in the first or second trimester  .
From collected data, it was found that out of 53 (100%) patients, 44 (84.62%) pregnant woman have different gestational age from US and last menstrual period (LMP).
From this study we can conclude that the main method to follow fetus growth in third trimester not biparietal diameter (BPD) measurement only.
In 37.5% of the total cases, it was observed that FL increased gradually from a minimum of 50 mm at the 27.25 weeks GA to 76 mm in the 37.43 weeks (Table 1). Data were prepared for BPD in different weeks of gestation in form of Tables and Graphs.
The number of pregnant women have different in gestational age when examined by ultrasound “biparietal diameter” and calculated the last menstrual period were represented in Table 1, Figure 1.
Search for accurate dating sonogram:
Ultrasonic studies have proven useful in determination of gestational age (GA) in first and second trimester, but their accuracy in third trimester is not reliable because of biologic variations like racial differences in fetal biometric measurements and inter-population variations  .