Grooves are starting to form on the paddle-shaped limb buds, and they will become the bends in your child's arm and eventually, little fingers and toes.

If you’ve had your first ultrasound, known as the dating scan this week, you might hear the first thuds of your child's heart beating. What was previously tube-shaped beginnings of a heart is now more defined. You may even see the 4 chambers, though they may not yet be fully developed. 

The yolk sac your baby has been dependent on for nutrients is now starting to shrink as its role is being taken over by your developing placenta.
and demands are greater than it can supply. It will disappear shortly.

On the ultrasound scan, your doctor may have pointed out a funny looking tail to you. It's true, your baby does have what we call an embryonic tail. Everyone does at this stage, but it's slowly disappearing and becoming shorter as the weeks go by.

Ref: M19