How are non-identical twins made?

Answer

Nonidentical twins occur when a woman produces two separate eggs that are then fertilized by two different sperm cells. The resulting twins sometimes look alike, but have separate genetic identities, like children of the same parents born during different pregnancies. Another name for nonidentical twins is fraternal twins.

Nonidentical twins only form when two eggs release during the same ovulation. In some cases, the eggs release simultaneously. In others, the ovaries initially release one egg. After fertilization that egg begins moving to the uterus to attach. The ovaries then release a second egg, fertilized by a later act of sexual intercourse. This leads to the potential of a woman giving birth to nonidentical twins with two separate fathers. In this case, the genetic identity of the twins is the same as a half-sibling born from separate pregnancies.

The sex of fraternal twins has a chance of differing. Identical twins develop when a single egg fertilized by a single sperm splits to form two different babies. These twins have the same genetic identity. Identical twins are always the same sex.

While some women produce multiple eggs with every ovulation, others begin producing multiple eggs as they grow older. Genetics also plays a role in the chance of conceiving twins. Use of fertility drugs increases the chance of producing more than one egg during each cycle.

Q&A Related to "How are non-identical twins made?"
non identical twins are made when occasionally during the woman's period she lets two eggs out at the same time, which are fertilised by two sperms to form non identical twins.
http://wiki.answers.com/Q/How_are_non_identical_tw...
identical twins are "monozygotic" - the are from the same egg ("mono" one + zygote "egg". As the single-celled egg reproduces and divides into more cells
http://uk.answers.yahoo.com/question/index?qid=200...
Identical twins are formed when one egg after being fertilized by one sperm, divides into two halves.
http://www.chacha.com/question/how-are-identical-t...
Identical twins come about after the egg is fertilized by the sperm, and the cells start replicating, but in the process, some of the cells REALLY split away and start acting as a
http://answers.yahoo.com/question/index?qid=200809...
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