Just a few weeks later, Kate and Michael were pleasantly surprised to find out that she was pregnant. Now that she understood how the fertilized egg had arrived at the uterus, Kate was excited to continue to research the development of her baby. She estimated that implantation had occurred 2 weeks prior to her finding out about her pregnancy and called her best friend to share the news. Kate excitedly described a fetus, but her friend corrected her and said that it was probably still an embryo. Confused, Kate asked what the different was.

What would be a good explanation as to the difference between an embryo and a fetus?