Anything which occurs outside the body is called in vitro and when fertilization of egg with sperm occurs outside the body it is called "IN VITRO FERTILIZATION". The first In Vitro fertilization was done in the test tube and that is why it is popularly known as Test Tube Baby.
The process of fertilization which normally in fallopian tube of the woman and the fertilized egg than moves into the uterus of the woman. It an implants there in to the uterus and developed into the baby. When for some reason this process of fertilization cannot take place naturally or sperm and egg cannot be transported naturally to the tube or from the tube to the uterus or either is egg having some problem or the sperm.
Some problem in fertilization during test tube baby process we need to asses the fertilization to help fertilization we take out of the egg from the wife of egg from the woman's ovary and in control lab and environment we fertilized egg with the help of sperm of her male partner. After fertilization the fertilized egg are cultured into the control laboratory environment to form pre-embryos and this pre-embryos are then transferred back into the uterus of the woman. If they implant and grow become the pregnancy, and hence test tube baby is most convenient process.