Human development is a continuous procedure that begins when the ovum from a female is fertilised via sperm from a male to form the zygote. Growth and differentiation transform the zygote into a multicellular adult human being. Though, it is important to realise that development does not stop at birth. It is a continuous procedure. It is usual to divide human development into prenatal and postnatal periods. Prenatal period refers to the period before birth. Throughout this period for the first eight weeks the developing human being is called an embryo since the organ systems are forming. From the 9th week onwards the term foetus is used. The foetal period (9 weeks to birth) is charactrised by growth and elaboration of structures. The postnatal period starts at birth and ends at death. Important developmental changes in addition to growth take place after birth, for example, the development of teeth and the changes during puberty. The brain triples in weight among birth and 16 years of age. Though, most developmental changes are completed by the age of 25.
The 266 days among conception and birth are traditionally divided into about three month periods, each termed as a trimester. We deal with each trimester but more emphasis is given Lo the first trimester as more dramatic changes take place during this period. But before we discuss the development of the human embryo it is significant to recapitulate the process of gametogenesis and the general structure of the female reproductive tract as the entire prenatal period is spent inside the womb of mother.