Traditionally, doctors have measured pregnancy from a number of convenient points, including the day of last menstruation, ovulation, fertilization, implantation and chemical detection. In medicine, pregnancy is often defined as beginning when the developing embryo becomes implanted into the endometriallining of a woman's uterus. In some cases where complications may have arisen, the fertilized egg might implant itself in thefallopian tubes or the cervix, causing an ectopic pregnancy. Most pregnant women do not have any specific signs or symptoms of implantation, although it is not uncommon to experience minimal bleeding at implantation. Some women will also experience cramping during their first trimester. This is usually of no concern unless there is spotting or bleeding as well. After implantation the uterine endometrium is called the decidua.The placenta which is formed partly from the decidua and partly from outer layers of the embryo is responsible for transport of nutrients and oxygen to, and removal of waste products from the fetus. The umbilical cord is the connecting cord from the embryo or fetus to the placenta.The developing embryo undergoes tremendous growth and changes during the process of foetal development.
Morning sickness can occur in about seventy percent of all pregnant women and typically improves after the first trimester.
In the first 12 weeks of pregnancy the nipples and areolas darken due to a temporary increase in hormones.
Most miscarriages occur during this period.