I have been meaning to do this for sometime now (I am several weeks behind now so I will be updating more than once a week to catch up)
Your baby in week 3:
The moment you’ve been waiting for has finally arrived: You’ve conceived and your soon-to-be-baby has started on its miraculous transformation from single cell to baby boy or girl. This week, the fertilized egg — or zygote — divides several times over to become a tiny ball of microscopic cells smaller than the period at the end of this sentence. As it divides, the blastocyst, as it's now called (don't worry, you'll come up with a cuter name soon!), makes its way from your fallopian tube to your uterus, where it will implant itself in the uterine wall and grow for the next nine months.
Your body in week 3:
You probably won't notice as the fertilized egg divides and makes its way to your uterus. But your body is gearing up to play landlord to the blastocyst (and soon-to-be baby) making its way toward the uterus. What does that mean for you? You’re just beginning to produce progesterone and estrogen, the pregnancy hormones that will be responsible for a host of bodily changes (and symptoms like morning sickness!) to come. For now, though, and the next couple of weeks, it’ll be like nothing is happening — at least on the outside!
Your baby in week 4:
No bigger than a poppy seed, the ball of cells that will develop into your baby has completed its journey from your fallopian tube to your uterus. Once settled in its new home, it will undergo the great divide — splitting into two groups. Half will become your baby (or embryo), while the other half forms the placenta, your baby's lifeline until delivery. The multilayered embryo then begins to grow into specialized parts of your baby's body. The inner layer, known as the endoderm, will develop into your baby's digestive system, liver, and lungs. The middle layer, called the mesoderm, will soon be your baby's heart, sex organs, bones, kidneys, and muscles. The outer layer, or ectoderm, will eventually form your baby's nervous system, hair, skin, and eyes.
Your body in week 4:
While the tiny ball of cells that is your baby begins to undergo its magical transformation into a multilayered embryo, your body is going through a magical transformation of its own — from a reliable buddy to a weird and wacky science experiment. You may start exhibiting early pregnancy symptoms like bloating, cramping, and mood swings that feel similar to premenstrual symptoms. You might also experience implantation bleeding , a common (and harmless) light pink, red, or brown spotting that can occur when the egg settles into your uterus.You might feel a little pressure in your abdomen (nothing to worry about!) and your breasts may feel a little tender and become even bigger (but get ready for more growth spurts!).