Fertilisation and Pregnancy

 

1. Fertilisation is the process whereby the sperm fuses with the ovum to form the zygote. It occurs in the upper part of the Fallopian tube.

 

2. Only one sperm is needed to fertilise the ovum.
                            

3. The rapid growth and development of a baby between the zygote stage and birth is called pregnancy.

 

4. The zygote undergoes cell division as it travels down the Fallopian tube towards the uterus.

 

5. The zygote forms a ball of cells and is referred to as an embryo.

 

6. About seven days after fertilisation, the embryo embeds in the tissues of the uterus lining. This process is called implantation.

7. During this early stage of pregnancy, the embryo obtains its nourishment from nutrients in the ovum and the uterus lining.

 

8. The surfaces of the embryo and the uterus lining bind firmly to form a large disc called the placenta.

 

9. The job of nourishing the embryo is taken over by the placenta.

 

10. By eight weeks after fertilisation, the developing embryo becomes clearly human and is called a foetus.
                             

11. At the placenta, the foetal and maternal blood vessels flow very close to each other but do not mix.

 

12. The foetus is attached to the placenta by the umbilical cord.

 

13. The umbilical cord has two blood vessels:

(a) an umbilical artery which carries carbon dioxide and other wastes from the foetus to the placenta.

(b) an umbilical vein which carries blood containing oxygen and food from the placenta to the foetus.

       

14. Oxygen, soluble food, water and salts pass from the mother's blood into the foetal blood.

 

15. Carbon dioxide and nitrogenous waste pass from the foetal blood into the mother's blood.

 

16. The foetus is completely surrounded by a membrane called the amnion. The amnion is filled with amniotic fluid.

 

17. The amniotic fluid protects the embryo and later the foetus by cushioning it from physical damage.

 

18. The time for the full development of a foetus is forty weeks. This is often referred to as the gestation period.

 

19. In human beings, birth occurs in three main stages:

(a) The expansion of the cervix so that the baby's head can pass into the vagina.

(b) The strong contractions of the uterus which push the baby out of the mother's body.

(c) Further contractions of the uterus to expel the umbilical cord and placenta from the mother's body.