In the sixth week of pregnancy you may start to have morning sickness, feel a bit tired and not in your normal self. This is due to the development and growth the baby is experiencing. Development is so important these weeks, as the complex systems of the organism start their formation, like the circulatory system, for instance.
During this week, the primitive heart is formed, it has two blood vessels and starts beating.
The central nervous system has grown longitudinally, forming the prosencephalon, and making the embryo to fold from the head to the tail, like drawing back to itself.
Meawhile, in the midpiece, a rough form of the intestine appears, origin of the whole digestive system.
The embryo now looks a bit awkward, the head is huge, and has the primordium of eyes and ears. They are black stains that will eventually become the eyes and eras of the baby.
Two arm buds appear, the rough drawing of the arms, though they cannot be noticed yet, just like the legs.
The embryo is about 4mm long by the end of this week.