Week 3

Conception—the meeting of sperm and egg—takes place this week, usually between days 14 and 17 of a 28-day menstrual cycle. Around the 4th day after conception, the united egg and sperm reach your uterus.

With no noticeable physical changes at this point, you probably won't yet be aware that you have conceived.
 

My baby in the 3rd week of pregnancy

The fertilized egg, called an embryo, beginning to grow inside you is tiny—about the size of the head of a pin. By the time it reaches your uterus, it is a solid ball of about 200 cells.