Conceiving: fertilization
Fertilization happens when a sperm meets and then penetrates an egg. Most human cells contain 46 chromosomes - thread-like structures that carry their genetic information. But the sperm cells and egg cells each have only 23 chromosomes. When a sperm and an egg meet and fuse, the resulting fertilized cell has the full 46 chromosomes.
© DK
The new cell, which is called a zygote, splits first into two identical cells, each with 46 chromosomes. It continues to divide slowly as it travels down the Fallopian tube, until it reaches the uterus. By the time it reaches the uterus it is a hollow clump of about 100 cells called a blastocyst.
Conception and implantation
Implantation
A week after fertilization has taken place, the blastocyst produces a hormone that helps it to burrow its way into the lining of the uterus, where it is bathed in the mother's blood, allowing food and waste to pass to and fro. Implantation is usually in the upper one-third of the uterus. The pregnancy is now established and the placenta starts to form.
Twins
When a woman releases more than one egg at a time, non-identical (fraternal) twins may develop from two separate eggs, fertilized by two separate sperm. Each embryo then has its own placenta inside the mother's uterus.
Identical twins come from a single egg, fertilized by a single sperm. This egg divides into two, and each develops independently into a genetically identical twin sharing a single placenta. Other multiple pregnancies, such as triplets, start in the same ways as twins and, similarly, the siblings may be fraternal or identical.
Posted 30.06.2010
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