When we identify a baby who is growing too rapidly for stage in pregnancy, then we worry about mother having elevated blood sugars, or diabetes. If we identify a baby who is growing too slowly, then we can sometimes identify a means to help the baby grow better, such as control of a maternal condition like asthma, and we can also recommend means for assessing ongoing growth and good health of the baby for birth.
Ultrasound is very different from X-rays, which involve radiation and have hazards for the pregnancy.
An average length of the embryo at 7 weeks is anywhere between 5mm-12mm. Obviously, every pregnancy is unique and individual factors influence the size of the embryo at this early stage, and the embryo shows development week by week.
Crown/rump length and gestational age are closely compared with each other until around the end of the first trimester.
Ultrasound at this stage in the pregnancy is less accurate than earlier sonography for determining the due date.
Between 22 and 26 weeks, the ultrasound should match the due date within two weeks, and after 26 weeks, within three weeks.