i. Sexual Intercourse the Union of the Two Sexes:
The sexual intercourse also known as coitus is the first step towards pregnancy. During intercourse, the parasympathetic nerves of penis are stimulated. This causes dilation of arteries and venous obstruction leading to retention of blood within the penis. This causes erection of the penis.
The penis is placed in the vagina. The stimulation of sensory organs in the penis sets off a reflex action and the semen containing sperms is ejaculated in the vagina. This is known as copulation. A single ejaculation may contain 2-4 hundred million sperms, only one will actually fertilize the egg.
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The spermatozoa are deposited high up in the vagina close to the cervix. They reach the top of the fallopian tube within 5 minutes of their release due to contractions in the walls of uterus and fallopian tube. Spermatozoa remain viable in the female genital tract for 24-72 hours.
ii. Fertilization:
If the ovum receives a sperm during this period, the two fuse to form a zygote. This act of fusion of male gamete (sperm) and female gamete (egg) to form zygote is called fertilization. 12-13 days after onset of menstruation are most favourable for conception. Fertilization occurs in the fallopian tube
iii. Implantation:
The zygote immediately begins to divide and forms a mass of cells called morula. It continues to re-divide and is called blastocyst. It passes down to the uterus and fixes itself to the endometrium wall of the uterus. This fixing of morula in the endometrium wall of the uterus is called implantation and the female is said to be pregnant or in the stage of pregnancy. Implantation takes place about a week after fertilization.
Placenta:
The developing embryo is attached to the uterus by an organ called placenta. Placenta is an association between maternal and foetal tissue meant for physiological exchange. Umbilical cord is a tough structure that serves as the blood vascular connection between the foetus and uterine wall.
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From the first few weeks of development, the embryo is enclosed in a sac called amnion which is filled with amniotic fluid. This fluid acts as a shock-absorber and helps to protect the embryo from damage. There is one more layer, chorion that helps in the formation of embryonic part of the placenta.
The placenta is formed of fine finger-like processes called villi. There are two sets of villi. One set of villi is given out by the uterine wall while the other set by an extension from the embryo. These two sets of villi are interlocked but they do not open into each other. Thus, although the blood of the mother and the embryo are in close contact with each other, the two do not get mixed.
Functions of Placenta:
i. Placenta serves as a tissue through which oxygen and food are supplied from the maternal blood to the foetus.
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ii. It also transports carbon dioxide and excretory waste from the foetal blood to the maternal blood.
iii. Oxygen and nutrients (glucose, amino acids and salts) from the mother’s blood vessels diffuse across into the embryonic blood vessels through placenta. On the other hand, carbon dioxide and nitrogenous waste from the embryo pass to the mother’s blood vessels.
iv. Placenta is permeable to respiratory gases, nutrients and antibodies. The membrane prevents harmful material from reaching the embryo. It does not allow the passage of germs from the mother to the embryo. However, if the mother is already infected with HIV then HIV can pass through the blood to the embryo.
v. Placenta also produces two hormones progesterone and oestrogen. Under the influence of these hormones neither ovulation nor menstruation takes place till pregnancy continues. However, these phenomena are resumed after childbirth. The period of complete development of the foetus till birth of the baby is called gestation period. It is of about 280 days.