Write a note on pregnancy and embryonic development in human beings.

Pregnancy : The duration of development of foetus in female reproductive system is called pregnancy.

In human beings the duration of pregnancy is approximately $160$ days after ovulation and 280 days after last menstrual cycle ($40$ weeks).

Some children take birth $1$ or $2$ weeks before or after/early or later.

During first 8 weeks fertilised ovum is called an embryo and after that it is known as foetus.

Changes occur in embryo during pregnancy : Immediately after implantation, the inner cell mass differentiates into an outer layer called ectoderm and an inner layer called endoderm.

A mesoderm soon appears between the ectoderm and the endoderm.

These three layers give rise to all tissues in adults.

- The inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs.

Embryonic development is a continuous process. Its summary is given below :

In human beings, after one month of pregnancy, the embryo's heart is formed. The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope.

By the end of the second month of pregnancy, the foetus develops limbs and digits.

By the end of $12$ weeks, most of the major organ systems are formed. For example, the limbs and external genital organs are well developed.

The first movements of the foetus and appearance of hair on the head are usually observed during the fifth month.

By the end of $24$ weeks, the body is covered with fine hair, eye-lids separate, and eyelashes are formed.

By the end of nine months of pregnancy, the foetus is fully developed and is ready for the delivery.