Did you know that the moment you missed your period that your baby is already formed? During the first 4 weeks, there are already a lot of development going on with the embryo. Brain and spinal cord starting to develop. Size is about 2mm.
The heart, no larger than a poppy seed, has been beating since day 18 when your just four days late for your menstrual period, and by 21 days it is already pumping , through its own closed circulatory system, blood whose type is different from yours.
Four weeks after fertilization, the eyes, ears and respiratory systems begins to form and your baby may even be sucking its thumb!