The basic structure of the baby is formed during the first three months .the developing embriyo in this stage is known as foetus. In this period the foetus is very sensitive to anything that interferes the development. The substance which disturb the development of foetus , are called teratogens. Almost all systems have been formed in first three months of pregnancy . Pregnancy involves number of hormones. The progesterone which is secreted initially from corpus luteum and later by placenta . progesterone , the hormone which require continuously during entire period of pregnancy . progesterone is called pregnancy hormone.

The preganancy period last for 280 days. The pregnancy terminated with birth process which is known as parturition . labour is the technical term for the process in which Once the labour starts the cyclic contractions that begin at the top of uterus and travel down its length force the contents of the uterus move towards the cervix. By the stimulation of abdominal muscles the foetus come out through the vagina. After the baby birth , the placenta and remains of the umbilical cord are expelled out , which is known as after birth.