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

(N/A) Pregnancy: The duration of development of the foetus in the female reproductive system is called pregnancy.
In human beings,the duration of pregnancy (gestation period) is approximately $280$ days (or $40$ weeks) from the last menstrual cycle.
During the first $8$ weeks,the developing organism is called an embryo,and after that,it is known as a foetus.
Embryonic Development:
$1$. Immediately after implantation,the inner cell mass differentiates into three germinal layers: ectoderm (outer),endoderm (inner),and soon after,mesoderm (middle).
$2$. These three layers give rise to all tissues and organs in the adult.
$3$. The inner cell mass contains stem cells,which have the potency to give rise to all tissues and organs.
Timeline of Development:
- After $1$ month: The embryo's heart is formed. Heart sounds can be heard via a stethoscope.
- By the end of $2$ months: The foetus develops limbs and digits.
- By the end of $12$ weeks (first trimester): Most major organ systems are formed,including limbs and external genital organs.
- During the $5$th month: The first movements of the foetus and the appearance of hair on the head are observed.
- By the end of $24$ weeks (second trimester): The body is covered with fine hair,eyelids separate,and eyelashes are formed.
- By the end of $9$ months: The foetus is fully developed and ready for delivery.