The accuracy is best from 12 weeks to 20 weeks although it can still be used to date the pregnancy up to 24 weeks.
When the BPD measures 40 mm, you can see that it predicts the age of the baby as between about 16 weeks and two days and 18 weeks and 4 days.
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.
You would normally date the pregnancy at the average point – 17½ weeks.
Research shows that routine obstetric ultrasound before 24 weeks gestational age can significantly reduce the risk of failing to recognize multiple gestations and can improve pregnancy dating to reduce the risk of labor induction for post-dates pregnancy.
Dating and Growth Scans There are numerous indications over the course of a pregnancy for obstetric ultrasound.
These include an estimation of gestational age - optimally by 12 weeks, evaluation of fetal growth, assessment of vaginal bleeding, determination of fetal presentation , evaluation of a suspected multiple gestation, evaluation of a suspected ectopic pregnancy, evaluation of a suspected gestational trophoblastic neoplasia (molar pregnancy), confirmation of cardiac activity, fetal echocardiography, amniotic fluid index, biophysical profile, and many more.
Other more subtle features such as low-set ears, facial dysmorphia or clubbing of feet can be better assessed.
The ability to obtain a good 3D picture is nevertheless still very much dependent on operator skill, the amount of amniotic fluid around the fetus, fetal position and the degree of maternal obesity.