Week 9 is when your embryo turns into a fetus. Your little one is about the size of an olive or grape and weighs only a fraction of an ounce. The embryonic tail disappears completely, and the placenta can now take over, producing the hormones necessary to maintain pregnancy and continue the development of the fetus.
This week the fetus' heart will divide into four chambers and the valves they'll have for the remainder of their life. Other organs, nerves, and muscles are moving into high gear, getting ready to begin functioning in ays similar to their eventual jobs after birth. Even the organs that determine your baby's sex will start forming this week, but they won't be complete or ready to give up their secrets until later. The fetus has new tiny muscles developing, and the eyes and eyelids are completely formed now but will remain fused shut until around 27 weeks.
- American Association of Pregnancy - Week 9