All pregnancies start when a sperm fertilises an egg

All pregnancies start when a sperm fertilises an egg. This fertilized egg is called a zygote.
Sometimes a woman’s ovaries release two eggs, and two separate sperm fertilise each egg. This forms twins. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.
During pregnancy the baby gets food and oxygen from the placenta. Fraternal twins have separate placents and umbilical chords. The name for this is dichorionic.
Fraternal twins can be the same or opposite sex and their genes are different. Same sex fraternal twins usually look different – they might have the same hair or eye colour. Sometimes they might look quite similar.
Fraternal twins happen in around 1 in 40 pregnancies in Australia.
Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins.


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