Week four features increasing hormones and a wide range of related side effects from moodiness and fatigue to breast soreness and nausea. At this time, progesterone is slowing down your digestion to allow more nutrients to enter the bloodstream. You might start noticing complexion problems and a little dizziness when you get up quickly due to normal circulatory changes.
There are also changes going on in your body that aren't quite as visible. Your baby now is about the size of a poppy seed and too small to even see on an ultrasound. The ball of cells that will develop into your baby has completed its journey from the fallopian tube to your uterus. It will divide into two separate bodies, with one becoming your baby and the other the placenta, your baby's lifeline until delivery. Your body is starting to produce the human chorionic gonadotropin hormone, or hCG. This hormone helps maintain the uterus lining and causes your ovaries to stop releasing eggs and your periods to cease.
You're now at the beginning of the embryonic period. The embryo is beginning to develop into three layers. The endoderm (inner layer) develops into the digestive system, liver and lungs. The mesoderm (middle layer) forms the heart, sex organs, bones, kidneys and muscles. And the ectoderm (outer layer) forms the nervous system, hair, skin and eyes. Yes, all that in something you can hardly see! This is a critical time for baby and any exposure to harmful substances such as alcohol, tobacco, lead, mercury or pesticides could have adverse effects.