How Pregnancy Occurs

When ovulation occurs, usually at the 14th – 16th day of the menstrual cycle, the egg moves through the fallopian tube propelled by long hairs growing on the walls of the tube. The hairs bend towards the uterus in waves, pushing the egg slowly towards the uterus. The egg is capable of being fertilized for only up to 24 hours; if it is not fertilized, it disintegrates and is shed during menstruation.
Female Reproductive System

Sexual intercourse must take place within 2-3 days immediately before or after ovulation for pregnancy to occur. Millions of sperms are ejaculated into the vagina at every act of sexual intercourse. Sperms stay alive for a longer period than the woman’s egg and are capable of fertilizing an egg for 48 - 72 hours after being ejaculated. The time taken by the sperm to reach the fallopian tubes is between 6 – 12 hours, but some researchers say it can be as soon as 1 hour.

All the sperms deposited in the vagina cannot swim into the uterus. Some die in the vagina, some get entangled in the cervical mucus, and some manage to swim just into the cervix before dying. Only a few hundreds of sperms are able to reach the fallopian tubes and surround the egg in the tube. They press against the wall of the egg attempting to penetrate it and fertilize the egg until finally one sperm succeeds. Immediately, a chemical reaction is triggered off in the wall of the egg making it impenetrable to any other sperm. No other sperm can enter the egg now.

The unsuccessful sperms slowly disintegrate; they break down and become just like any other proteins in the woman’s body (sperms are made up mainly of protein) which are absorbed in the blood stream and carried away to be expelled from the body in the stool or the urine.

The sperm that manages to penetrate the egg unites with it to form a single cell 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. The embryo is usually transferred into the uterus on the 3rd day after fertilization, where it attaches itself deep into the lining of the uterus in a process called Implantation. This process occurs 6-12 days after conception and may be accompanied by light spotting and cramps.

The implanted embryo then begins to develop, its cells increasing in number and gradually becoming different from each other in the way they function until the end of 9 months (40 weeks) when a fully formed baby is ready to be born.