Ultrasound dating birth
The third trimester is the best time for evaluating fetal growth, since fetuses add most of their weight during the second half of pregnancy.Third trimester ultrasound can also evaluate amniotic fluid, blood flow through Doppler studies, the cervix, placenta, and fetal activity.It is not a good time for fetal screening but is an important time for following and clarifying suspected abnormalities.3D/ 4D ultrasound performed in the early third trimester usually provide images that look more like a "baby" and less like a fetus because soft tissues are beginning to fill out.At the same time, it's power may be limited and even dangerous in the hands of inexperienced or untrained personnel.
In general, the more time the physician spends doing ultrasound for a living, the better that person probably is.
This is presuming that the pregnancy is actually at this stage of development.
Sometimes a trans-vaginal examination shows that your pregnancy is less advanced than expected.
The most accurate time is between 8 and 11 weeks gestation.
This is because the fetus is growing so quickly that there is a big difference in size from week to week.
Usually the expected date of delivery (EDD or EDC) is calculated from your last menstrual period - if the early dating scan calculates the EDD to be within 5 days of the EDD from your last menstrual period.