Test tube baby means a baby born when

The correct option is: "ovum is fertilized externally and thereafter implanted in uterus"

A "test tube baby" is a term commonly used to refer to a baby conceived through In Vitro Fertilization (IVF). The step-by-step process that explains this definition includes:

1. External Fertilization: The process begins by retrieving mature eggs (ova) from the female ovaries and a sperm sample from the male. The egg and sperm are then combined manually in a laboratory dish (historically, a test tube, though modern practice uses petri dishes) where fertilization takes place outside the human body.
2. Embryo Development: The fertilized egg (zygote) is monitored as it divides and grows into an embryo over a period of 2 to 6 days in a controlled environment.
3. Implantation in the Uterus: Once the embryo reaches the appropriate stage, it is transferred and implanted directly into the female's uterus (womb). If the embryo successfully attaches to the uterine lining, pregnancy begins, and the baby develops naturally inside the mother until birth.

Thus, the baby does not grow or develop inside a test tube; rather, only the initial fertilization happens externally, after which the development proceeds normally inside the uterus.