During the 3rd week of pregnancy, the fertilized egg typically moves towards the uterus and implantation occurs. This process is considered the beginning of pregnancy. Here are some developments that occur during the 3rd week of pregnancy:

1. Fertilization and Journey of the Egg to the Uterus: The egg cell travels through the fallopian tubes towards the uterus. Fertilization can occur during this time, and the fertilized egg begins to form an embryo.
2. Implantation: The fertilized egg implants into the inner wall of the uterus (endometrium), initiating a process known as implantation. During implantation, the embryo becomes embedded within the uterine lining and begins to grow.
3. Hormonal Changes: Levels of human chorionic gonadotropin (hCG), known as the pregnancy hormone, start to rise. This hormone is responsible for producing positive results on pregnancy tests.
4. Development of the Embryo: The fertilized egg develops into an embryo and begins to rapidly divide and grow. The embryo separates into cell layers to form basic structures.
5. Preparation for Future Development: The embryo starts to produce cells necessary for the formation of the placenta. The placenta is an organ that provides nourishment and oxygen to the baby throughout pregnancy.

During the 3rd week of pregnancy, these early developments are usually not noticeable to the expectant mother, and pregnancy symptoms are not yet apparent. However, pregnancy tests may yield positive results during this stage.