The first 4 weeks of pregnancy would just be characterized by morning sickness and other associated symptoms of pregnancy. The actual physical development begins by the 4th week of pregnancy, that is, after the first month of pregnancy. Since it still is part of the first trimester, it is also a critical part of the pregnancy.
This is when the embryo has fully formed. It is still quite small, not more than the size of a seed. The embryo is surrounded by the amniotic fluid that cushions and protects it. The placenta is also just beginning to develop. As time goes by, the placenta will provide the fetus with all the nutrition it needs. Until that time, the yolk sac provides the nutrition.
The embryo is formed in 3 layers. The top layer is where the neural tube will form. This is what will develop into the brain, spinal cord and nerves. The middle layer will contain muscles, the heart and circulatory system. The innermost layer will develop to form the intestines, urinary system etc.
Now your baby is still quite small, which is why you haven’t been able to see the bulge of pregnancy yet. The first organ to form is the heart, which by this time has only one chamber but is still beating. The growth spurts which will be arms and legs of the baby have started appearing. The area for the mouth is also being developed.
By this time, the heart has begun separating into its 4 chambers, and their heart rate is almost twice as much as yours. The head is beginning to form and it’s bent over the area where the chest will be. The cavities for the eyes and nostrils have also begun to develop. Their skin is so thin, it’s almost transparent. The vocal organs and the tongue have also begun to develop.
By week 7, your baby has started developing nerves and ligaments. Their ears and eyes have begun development, as well as the eyelids. They will have an extended tailbone, the coccyx, that will be gone soon. The limbs have begun extending. Now, even the teeth have begun to slowly form. Their main organs, the heart and brain, will become more complex.
By the second month, the embryonic stage or stage of early development is over. Their hands are long enough to reach across the chest but legs aren’t fully distinct yet. The tailbone has disappeared. Their sense of smell has begun to develop as well as the nerves in their brain. They are now roughly the size of a kidney bean.