How Pregnancy Occurs

Written by : Dr.M.D.Mazumdar, MD

How does a pregnancy occur?

The simple answer as everyone knows is - pregnancy occurs when a sperm produced by a male, fuses with an ovum produced by a female.


Every month a mature ovum is released from either one of a woman's two ovaries. This ovum is round in shape and about 100 microns in diameter, slightly smaller than the period at the end of this sentence. It is the largest cell in the human body and the only one visible to the naked eye.

The ovum is picked up by the fallopian tube on the same side. (There are two fallopian tubes on either side of the uterus near the ovaries. See image ). The tubes have long fingerlike projections called fimbria which it uses, rather like hands, to pick up the ovum.

The ovum then moves through the tube, propelled along by long hairs growing from cells in the tubes. Like grass bending before the wind, the hairs bend towards the uterus in waves, pushing the ovum slowly towards the uterus.

How Fertilization and Pregnancy Occurs

The egg remains viable, that is, alive for about 72 hours, but is capable of being fertilized for only about 12 - 24 hours. If it remains unfertilized during this period, it disintegrates in the tube without leaving any trace. Its end products (mainly proteins) are absorbed into the bloodstream and excreted through the urine or stool.

The sperm however is viable for a longer period and has been found in the uterus 5 - 7 days after coitus. But they are capable of fertilizing an ovum for only 48 - 72 hours after being ejaculated. The time taken by the sperm to reach the tubes is between 6 - 12 hours but many authorities say it can be as early as 1 hour.

Intercourse has to take place within this narrow time frame (1-2 days before ovulation or immediately after ovulation), for a pregnancy to occur. At every intercourse a normal man deposits 2 - 5 mililitre of semen in the upper part of the vagina (see diagram). Each mililitre of semen normally contains about 50 - 200 million sperms.

Fertilization, Implantation and Pregnancy
Fertilization, Implantation and Pregnancy

After intercourse

The sperms look rather like tadpoles with somewhat triangular heads and a long thin tail. The heads are about 4 - 7 microns in length - compare this to the breadth of a spider's thread which is roughly 7 microns in breadth! The thin tail is about 55 microns long.

Whisking their tails madly, the sperms swim rapidly upwards into the uterus and from there into the two tubes on either side at the rate of 3mm per hour. It takes an average of 10 hours for the sperm to reach the tubes.

All the sperm deposited in the vagina cannot swim into the uterus. Some die off in the vagina, some get entangled in the cervical mucous and some manage to swim just into the cervix before dying. But it is believed that even these sperms help in causing pregnancy by changing the acidity ('ph') of the vagina or by acting on the cervical mucous so that other sperms can penetrate it and reach the ovum. Only about 1 % of the total number of sperms deposited in the vagina make the journey.

Thousands of sperms flood the uterus and both the tubes. Some tumble out of the opening of the tubes into the abdomen.


Hundreds of sperms (estimated to be around 300) surround the ovum in the tube. They press against the membrane of the ovum attempting to penetrate it and fertilize the ovum. Finally one sperm succeeds. At once a chemical reaction is triggered off in the wall of the ovum, making it impenetrable to any other sperm. No other sperm can enter the ovum now.

Fertilization of an ovum by a sperm



The unsuccessful sperms slowly degenerate. They break down and become indistinguishable from any other protein end-products in the female partner's body (the sperm is made up mainly of protein). These get absorbed into the blood stream and are carried away to be expelled from the body in the stool or the urine.

The sperm that manages to penetrate the ovum fuses with it to form a single celled organism called a zygote. The zygote starts to divide as it is propelled towards the uterus - dividing first into 2 cells, then into 4 then into 8 and so on. As soon as the zygote starts to divide it is called an embryo.

When the process of in-vitro fertilization (IVF) is carried out, the ovum and the sperm is allowed to fertilize in a laboratory dish (petri dish). The embryo is usually transferred into the mother's uterus at the 4 - 8 celled stage, usually on the third day after fertilization.

By the time the zygote reaches the uterus at about the 6th to 9th day after ovulation, it is a 16-celled cluster of cells called a morula. The morula looks rather like a bunch of grapes. Each of its cells is identical to each other.

Theoretically, it is from the 2-celled to the 16-celled stages that the cells can be separated from each other and allowed to develop into clones of each other (identical twins). In nature, separation occurs spontaneously - usually at the 2-celled stage - to form identical twins. Separation at later stages can lead to the potentially fatal condition of conjoint twins or Siamese twins.


When it reaches the uterus, the morula sticks to the inner lining of the uterus (called the 'endometrium'). By this time, the hormone progesterone released by the ovaries finishes preparing the endometrium to receive the morula.

The morula burrows deep into the endometrium and by the 9th - 12th day after ovulation, is fully buried in it. This process is known as 'Implantation' and can cause a little bleeding called 'implantation bleeding'. The buried embryo then begins to develop, its cells increasing in number and gradually becoming different from each other in the way they function.

The development of the embryo continues until at the end of 9th months of pregnancy (40 weeks or 280 days), a fully formed baby, capable of leading a life independent from its mother is ready to be born.

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