An embryo is the early developmental stage of an organism after fertilization, typically characterized by the formation of basic organ structures but not yet fully developed. In humans, the embryo stage lasts from fertilization up until about the eighth week of pregnancy. During this time, the embryo undergoes rapid cell division and differentiation.
The measurements of an embryo/fetus are fairly accurate.. Especially within the first 20 weeks. It's probable that you estimated how far along you were wrong.