Accuracy of ultrasound dating
Crown/rump length and gestational age are closely compared with each other until around the end of the first trimester.The ideal time for a sonographer to assess the gestational age in the first trimester is between 7-10 weeks of pregnancy.An image of your fetus will appear on an accompanying computer screen and while looking at this image, the technician will take some standard measurements from different angles and listen for a heartbeat.Don't worry: This type of exam is painless and risk-free, and seeing your baby for the first time will likely be an enjoyable experience.
Fetal growth assessment, either clinically or by ultrasound evaluation, also relies on accurate assessment of gestational age.
Any calculation of a conception date is an estimate, even in a woman with regular 28-day menstrual cycles.
Except for in-vitro fertilization (IVF) and other assisted reproductive procedures, you cannot precisely calculate the exact day a sperm meets an egg and fertilization (conception) occurs.
Other reasons to have a 7 week ultrasound are to: The embryo will be measured from the top of its head, the “crown” to its bottom or “rump”.
This is because it is the longest portion of the embryo’s body and provides an ideal measurement baseline of growth and development.