By around the end of week 4, the cluster of cells finishes implanting into the womb lining and is now known as an embryo. A membrane called the amniotic sac is forming around the embryo. It is filled with a clear liquid called amniotic fluid, in which the baby floats and is cushioned.
Increasing levels of the pregnancy hormone chorionic gonadotrophin (HCG) mean that you may be starting to experience some of the early signs and symptoms of your pregnancy at 4 weeks. Early signs can include feeling sick, larger or tender breasts, needing to urinate more often, and feeling more tired than usual.
At this time your tastes might start changing. You might start craving particular foods, or developing a dislike for certain things such as coffee or alcohol. These are sometimes things that you need to cut back on anyway before and during pregnancy (such as alcohol). Don't force yourself to eat foods that make you feel ill, but remember you still need to eat a healthy balanced diet.