In the first few weeks there is a lot of growth in your belly, before you may even know you are pregnant .
Three weeks from the first day of your last menstrual period ,the fertilized egg moves slowly along the Fallopian tube towards the womb. The egg begins one simple cell .This cell divides again and again .By the time the egg riches the womb it has become a mass of over 100 cells called an embryo ,and is still growing .
Once in the womb, the embryo settles into the womb lining .This is called implantation .In weeks four to five the embryo settles into the womb lining .the outer cells reach out like roots to link with her mother's blood supply .The inner cells from into two and then three layers .Each of these layers will grow to be different parts of the baby's body.One layer becomes the brain and nervous system ,the skin ,eyes and ears.Another layer becomes the lungs ,stomach and gut .The third layer becomes the heart ,blood,muscles and bones .
The fifth week is the time of the first missed period , when most women are only just beginning to think they may be pregnant .Yet already the baby's nervous system is starting to develop .A grooves froms in the top layer of cells.The cells fold up and round to make hollow tube called the neural tube .This will become the baby's brain and spinal cord ,so the tube has a 'head end' and 'tail end'.defects in this tube are the cause of spina bifida .At the same time the heart is forming and the baby already has some of its own blood vessels.A string of these blood vessels connects baby and mother and will become the umbilical cord .