In week 4, now safe in your womb, the ball of cells (blastocyst) splits in two, becoming the embryo and the placenta. The amniotic sac and fluid are forming around baby, and will act as a cushion for the next eight months.
week 5: appleseed
Your embryo (looking kind of like a tadpole) is starting to form major organs (heart, kidney, liver, stomach) and systems (nervous, circulatory, digestive). Baby's presence in your uterus triggers production of hCG (the hormone detected by pregnancy tests)... which triggers production of other hormones like estrogen and progesterone... which trigger all those great symptoms you've probably been noticing!
Source: the bump