Changes After Fertilization

Implantation: The fertilized egg (zygote) moves down the Fallopian tube and continuously undergoes cell division and forms 2, 6, 8 , 16 cells and so on. By the time it reaches the uterus it has already formed a small hollow ball of numerous cells (blastocyst). This is a kind of embryo which forms a pit in the wall of the uterus and gets fixed in it in about a week's time after ovulation. This is a kind The embryo gets embedded in the wall of the uterus, which is thick and has muscles, glands and a large number of blood capillaries. This process is called implantation.

Pregnancy: The developing embryo at first derives nourishment directly from the mother’s blood flowing in the vessels lining the uterine wall. In about three weeks, it starts absorbing food and oxygen through an organ called placenta. The placenta is a disc like organ in the lining of the uterine wall. It has numerous villi, which are in direct contact with the mother’s blood flowing in the uterine wall. These villi provide a large surface area for glucose and oxygen to pass from the mother to the embryo and for wastes produced by the embryo to be passed into the mother’s blood. The embryo is connected to the placenta by a tube called the umbilical cord. By the end of five weeks of pregnancy, the embryo is quite advanced. The heart and circulatory system have been formed. After two months limbs have also been formed. By eight weeks, the embryo starts showing human features and is referred to as foetus. The total period of embryonic development, from the time of fertilization to birth, is called gestation period. It is around 280 days, or 9 months, in humans.

Birth: The wall of the uterus develops a thick layer of muscles during pregnancy. At the time of birth, the uterine muscles contract rhythmically and powerfully, causing labour pains to the mother. At the time of birth, the baby is pushed out by powerful contractions.Finally, the baby is expelled by the contraction of the uterine muscles. This is called birth or parturition.

What happens when the egg is not fertilized: If the ovum is not fertilized in the upper part of the oviduct, it keeps on descending and is finally passed out through the vagina. It remains in the body for about 24 - 72 hours. As an egg is released for fertilization every month, the uterus also prepares itself every month for the implantation of a fertilized egg. The uterus becomes thick-walled and spongy in order to nourish the embryo. If no fertilization takes place, the thick uterine wall is no longer needed. So, it gradually begins to shrink. This shrinkage ruptures its blood vessels. As a result, blood and mucus ooze out of the vagina. This period, which lasts for 3—5 days, is called the menstrual period, and the process is called menstruation. If the ovum is fertilized, it gets implanted in the uterus wall and embryonic development starts. In this case, the uterus continues to develop in order to hold the embryo. And in this case, there is no question of its shrinkage resulting in menstruation.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Settling down of blastocyst on uterine wall is called ______________________
A. Blastulation		B. Implantation
C. Impregnation		D. Insemination
Right Option : A
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Question : 2

Number of cells present in egg is __________________
A. One	B. Two	C. Three	D. Millions
Right Option : A
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Question : 3

Connection between developing embryo and placenta is called ______________________
A. Umblical cord	B. Allantois	C. Uterus	D. Amnion
Right Option : A
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